Java Interview Questions - DOC by liuqingyan


									                            Java Interview Questions                                 1
                   Java Interview Questions

Question    which containers use a border Layout as their default layout? (AWT)
Answer      The window, Frame and Dialog classes use a border layout as their default layout

Question    What is the preferred size of a component? (AWT)
            The preferred size of a component is the minimum component size that will allow the
           component to display normally

Question    Which containers use a FlowLayout as their default layout? (AWT)
Answer      The Panel and Applet classes use the FlowLayout as their default layout

Question    What is the immediate superclass of the Applet class?
Answer      Panel

Question    Name three Component subclasses that support painting (AWT)
Answer      The Canvas, Frame, Panel, and Applet classes support painting

Question    What is the immediate superclass of the Dialog class?
Answer      Window

Question    What is clipping? (AWT)
Answer      Clipping is the process of confining paint operations to a limited area or shape

Question    What is the difference between a MenuItem and a CheckboxMenuItem? (AWT)
            The CheckboxMenuItem class extends the MenuItem class to support a menu item that may
           be checked or unchecked

Question    What class is the top of the AWT event hierarchy? (AWT)
Answer      The java.awt.AWTEvent class is the highest-level class in the AWT event-class hierarchy

             In which package are most of the AWT events that support the event-delegation model
           defined? (AWT)
             Most of the AWT-related events of the event-delegation model are defined in the
           java.awt.event package. The AWTEvent class is defined in the java.awt package.

Question    Which class is the immediate superclass of the MenuComponent class
Answer      Object

Question    Which containers may have a MenuBar?
Answer      Frame

Question    What is the relationship between the Canvas class and the Graphics class? (AWT)
Answer      A Canvas object provides access to a Graphics object via its paint() method.

Question    How are the elements of a BorderLayout organized? (AWT)
            The elements of a BorderLayout are organized at the borders (North, South, East, and West)
           and the center of a container.

Question    What is the difference between a Window and a Frame? (AWT)
            The Frame class extends Window to define a main application window that can have a menu
                             Java Interview Questions                                 2

Question    What is the difference between the Font and FontMetrics classes? (AWT)
            The FontMetrics class is used to define implementation-specific properties, such as ascent
           and descent, of a Font object.

Question    How are the elements of a CardLayout organized? (AWT)
Answer      The elements of a CardLayout are stacked, one on top of the other, like a deck of cards.

Question    What is the relationship between clipping and repainting? (AWT)
            When a window is repainted by the AWT painting thread, it sets the clipping regions to the
           area of the window that requires repainting.

            What is the relationship between an event-listener interface and an event-adapter class?
Question    (AWT)

            An event-listener interface defines the methods that must be implemented by an event
Answer     handler for a particular kind of event. An event adapter provides a default implementation of
           an event-listener interface.

Question     How can a GUI component handle its own events? (AWT)
             A component can handle its own events by implementing the required event-listener
           interface and adding itself as its own event listener.

Question    How are the elements of a GridBagLayout organized? (AWT)
            The elements of a GridBagLayout are organized according to a grid. However, the elements
Answer     are of different sizes and may occupy more than one row or column of the grid. In addition,
           the rows and columns may have different sizes.

            What advantage do Java's layout managers provide over traditional windowing systems?
Question    (AWT)

            Java uses layout managers to lay out components in a consistent manner across all
           windowing platforms. Since Java's layout managers aren't tied to absolute sizing and
           positioning, they are able to accomodate platform-specific differences among windowing

Question    What is the difference between the paint() and repaint() methods? (AWT)
            The paint() method supports painting via a Graphics object. The repaint() method is used to
           cause paint() to be invoked by the AWT painting thread.

Question    How can the Checkbox class be used to create a radio button?
Answer      By associating Checkbox objects with a CheckboxGroup

Question     What is the difference between a Choice and a List? (AWT)
             A Choice is displayed in a compact form that requires you to pull it down to see the list of
           available choices. Only one item may be selected from a Choice. A List may be displayed in
           such a way that several List items are visible. A List supports the selection of one or more List

Question    What interface is extended by AWT event listeners? (AWT)
Answer      All AWT event listeners extend the java.util.EventListener interface.

Question    What is a layout manager? (AWT)
Answer      A layout manager is an object that is used to organize components in a container

Question    Which Component subclass is used for drawing and painting?
Answer      Canvas
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Question    What are the problems faced by Java programmers who dont use layout managers? (AWT)
            Without layout managers, Java programmers are faced with determining how their GUI will
Answer     be displayed across multiple windowing systems and finding a common sizing and positioning
           that will work within the constraints imposed by each windowing system

Question    Can there be an abstract class with no abstract methods in it?
Answer      Yes

Question    Can an Interface be final?
Answer      No

Question    Can an Interface have an inner class? (CoreJava)
            Yes public interface abc { static int i=0; void dd(); class a1 { a1() { int j;
Answer     System.out.println("in interfia"); }; public static void main(String a1[]) {
           System.out.println("in interfia"); } } }

Question    Can we define private and protected modifiers for variables in interfaces?
Answer      No

Question    What is Externalizable? (CoreJava)
            Externalizable is an Interface that extends Serializable Interface. And sends data into
Answer     Streams in Compressed Format. It has two methods, writeExternal(ObjectOuput out) and
           readExternal(ObjectInput in)

Question    What modifiers are allowed for methods in an Interface? (CoreJava)
Answer      Only public and abstract modifiers are allowed for methods in interfaces.

Question     What is a local, member and a class variable? (CoreJava)
             Variables declared within a method are "local" variables.
           Variables declared within the class i.e not within any methods are "member" variables (global
Answer     variables).
           Variables declared within the class i.e not within any methods and are defined as "static" are
           class variables

            I made my class Cloneable but I still get 'Can't access protected method clone. Why?
Question    (CoreJava)

             Yeah, some of the Java books, in particular "The Java Programming Language", imply that all
           you have to do in order to have your class support clone() is implement the Cloneable
Answer     interface. Not so. Perhaps that was the intent at some point, but that's not the way it works
           currently. As it stands, you have to implement your own public clone() method, even if it
           doesn't do anything special and just calls super.clone().

Question    What are the different identifier states of a Thread?
            The different identifiers of a Thread are:
           R - Running or runnable thread
           S - Suspended thread
           CW - Thread waiting on a condition variable
           MW - Thread waiting on a monitor lock
           MS - Thread suspended waiting on a monitor lock

            can the Kawa or any another J-editor export a .EXE file and to be has an install shield
Question    (CoreJava)

Answer      I didn't know i need an answer.
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Question     What are some alternatives to inheritance? (CoreJava)
             Delegation is an alternative to inheritance. Delegation means that you include an instance of
           another class as an instance variable, and forward messages to the instance. It is often safer
           than inheritance because it forces you to think about each message you forward, because the
Answer     instance is of a known class, rather than a new class, and because it doesn't force you to
           accept all the methods of the super class: you can provide only the methods that really make
           sense. On the other hand, it makes you write more code, and it is harder to re-use (because it
           is not a subclass).

Question     Why isn't there operator overloading? (CoreJava)
             Because C++ has proven by example that operator overloading makes code almost
           impossible to maintain. In fact there very nearly wasn't even method overloading in Java, but
Answer     it was thought that this was too useful for some very basic methods like print(). Note that
           some of the classes like DataOutputStream have unoverloaded methods like writeInt() and

Question     What does it mean that a method or field is "static"? (CoreJava)
             Static variables and methods are instantiated only once per class. In other words they are
           class variables, not instance variables. If you change the value of a static variable in a
           particular object, the value of that variable changes for all instances of that class.
           Static methods can be referenced with the name of the class rather than the name of a
           particular object of the class (though that works too). That's how library methods like
           System.out.println() work. out is a static field in the java.lang.System class.

Question    Diffrence between JRE And JVM AND JDK

Question    Why do threads block on I/O? (CoreJava)
            Threads block on i/o (that is enters the waiting state) so that other threads may execute
           while the i/o Operation is performed.

Question    What is synchronization and why is it important? (CoreJava)
            With respect to multithreading, synchronization is the capability to control the access of
           multiple threads to shared resources. Without synchronization, it is possible for one thread to
           modify a shared object while another thread is in the process of using or updating that object's
           value. This often leads to significant errors.

Question    Is null a keyword? (CoreJava)
Answer      The null value is not a keyword.

            Which characters may be used as the second character of an identifier,but not as the first
           character of an identifier? (CoreJava)
            The digits 0 through 9 may not be used as the first character of an identifier but they may be
           used after the first character of an identifier.

Question    What modifiers may be used with an inner class that is a member of an outer class? (CoreJava)
            A (non-local) inner class may be declared as public, protected, private, static, final, or

            How many bits are used to represent Unicode, ASCII, UTF-16, and UTF-8 characters?
Question    (CoreJava)

            Unicode requires 16 bits and ASCII require 7 bits. Although the ASCII character set uses only
Answer     7 bits, it is usually represented as 8 bits. UTF-8 represents characters using 8, 16, and 18 bit
           patterns. UTF-16 uses 16-bit and larger bit patterns.
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Question    What are wrapped classes? (CoreJava)
Answer      Wrapped classes are classes that allow primitive types to be accessed as objects.

            What restrictions are placed on the location of a package statement within a source code file?
Question    (CoreJava)

            A package statement must appear as the first line in a source code file (excluding blank lines
           and comments).

Question     What is the difference between preemptive scheduling and time slicing? (CoreJava)
             Under preemptive scheduling, the highest priority task executes until it enters the waiting or
           dead states or a higher priority task comes into existence. Under time slicing, a task executes
           for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then
           determines which task should execute next, based on priority and other factors.

Question    What is a native method? (CoreJava)
Answer      A native method is a method that is implemented in a language other than Java.

Question    What are order of precedence and associativity, and how are they used? (CoreJava)
            Order of precedence determines the order in which operators are evaluated in expressions.
           Associatity determines whether an expression is evaluated left-to-right or right-to-left

Question    What is the catch or declare rule for method declarations? (CoreJava)
            If a checked exception may be thrown within the body of a method, the method must either
           catch the exception or declare it in its throws clause.

            Can an anonymous class be declared as implementing an interface and extending a class?
Question    (CoreJava)

            An anonymous class may implement an interface or extend a superclass, but may not be
           declared to do both.

Question    What is the range of the char type? (CoreJava)
Answer      The range of the char type is 0 to 2^16 - 1.

Question     What is the purpose of finalization? (CoreJava)
             The purpose of finalization is to give an unreachable object the opportunity to perform any
           cleanup processing before the object is garbage collected.

Question     What is the difference between the Boolean & operator and the && operator? (CoreJava)
             If an expression involving the Boolean & operator is evaluated, both operands are evaluated.
           Then the & operator is applied to the operand. When an expression involving the && operator
           is evaluated, the first operand is evaluated. If the first operand returns a value of true then
           the second operand is evaluated. The && operator is then applied to the first and second
           operands. If the first operand evaluates to false, the evaluation of the second operand is

            How many times may an object's finalize() method be invoked by the garbage collector?
Question    (CoreJava)

Answer      An object's finalize() method may only be invoked once by the garbage collector.

Question    What is the purpose of the finally clause of a try-catch-finally statement? (CoreJava)
            The finally clause is used to provide the capability to execute code no matter whether or not
           an exception is thrown or caught.

Question    What is the argument type of a program's main() method? (CoreJava)
Answer      A program's main() method takes an argument of the String[] type.
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Question    Which Java operator is right associative?
Answer      The = operator is right associative.

Question    Can a double value be cast to a byte? (CoreJava)
Answer      Yes, a double value can be cast to a byte.

Question    What is the difference between a break statement and a continue statement? (CoreJava)
            A break statement results in the termination of the statement to which it applies (switch, for,
Answer     do, or while). A continue statement is used to end the current loop iteration and return control
           to the loop statement.

Question     What must a class do to implement an interface? (CoreJava)
             It must provide all of the methods in the interface and identify the interface in its implements

             What is the advantage of the event-delegation model over the earlier event-inheritance
           model? (CoreJava)
             The event-delegation model has two advantages over the event-inheritance model. First, it
           enables event handling to be handled by objects other than the ones that generate the events
           (or their containers). This allows a clean separation between a component's design and its
Answer     use. The other advantage of the event-delegation model is that it performs much better in
           applications where many events are generated. This performance improvement is due to the
           fact that the event-delegation model does not have to repeatedly process unhandled events,
           as is the case of the event-inheritance model.

Question    How are commas used in the intialization and iteration parts of a for statement? (CoreJava)
            Commas are used to separate multiple statements within the initialization and iteration parts
           of a for statement.

Question    What is an abstract method? (CoreJava)
Answer      An abstract method is a method whose implementation is deferred to a subclass.

Question    What value does read() return when it has reached the end of a file?
Answer      The read() method returns -1 when it has reached the end of a file.

Question    Can a Byte object be cast to a double value? (CoreJava)
Answer      No, an object cannot be cast to a primitive value.

Question     What is the difference between a static and a non-static inner class? (CoreJava)
             A non-static inner class may have object instances that are associated with instances of the
           class's outer class. A static inner class does not have any object instances.

Question    If a variable is declared as private, where may the variable be accessed? (CoreJava)
Answer      A private variable may only be accessed within the class in which it is declared.

Question    What is an object's lock and which object's have locks? (CoreJava)
            An object's lock is a mechanism that is used by multiple threads to obtain synchronized
           access to the object. A thread may execute a synchronized method of an object only after it
           has acquired the object's lock. All objects and classes have locks. A class's lock is acquired on
           the class's Class object.

Question     What is the % operator? (CoreJava)
             It is referred to as the modulo or remainder operator. It returns the remainder of dividing the
           first operand by the second operand.
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Question    When can an object reference be cast to an interface reference?
            An object reference be cast to an interface reference when the object implements the
           referenced interface.

Question    Which class is extended by all other classes? (CoreJava)
Answer      The Object class is extended by all other classes.

Question    Can an object be garbage collected while it is still reachable? (CoreJava)
            A reachable object cannot be garbage collected. Only unreachable objects may be garbage

Question    Is the ternary operator written x : y ? z or x ? y : z ?
Answer      It is written x ? y : z.

Question    How is rounding performed under integer division? (CoreJava)
Answer      The fractional part of the result is truncated. This is known as rounding toward zero.

             What is the difference between the Reader/Writer class hierarchy and the
           InputStream/OutputStream class hierarchy? (CoreJava)
             The Reader/Writer class hierarchy is character-oriented, and the InputStream/OutputStream
           class hierarchy is byte-oriented.

Question     What classes of exceptions may be caught by a catch clause? (CoreJava)
             A catch clause can catch any exception that may be assigned to the Throwable type. This
           includes the Error and Exception types.

            If a class is declared without any access modifiers, where may the class be accessed?
Question    (CoreJava)

            A class that is declared without any access modifiers is said to have package access. This
Answer     means that the class can only be accessed by other classes and interfaces that are defined
           within the same package.

Question    Does a class inherit the constructors of its superclass? (CoreJava)
Answer      A class does not inherit constructors from any of its superclasses.

Question    What is the purpose of the System class? (CoreJava)
Answer      The purpose of the System class is to provide access to system resources.

Question    Name the eight primitive Java types. (CoreJava)
Answer      The eight primitive types are byte, char, short, int, long, float, double, and boolean.

Question    Which class should you use to obtain design information about an object?
Answer      The Class class is used to obtain information about an object's design.

Question    Is "abc" a primitive value? (CoreJava)
Answer      The String literal "abc" is not a primitive value. It is a String object.

Question    What restrictions are placed on the values of each case of a switch statement? (CoreJava)
            During compilation, the values of each case of a switch statement must evaluate to a value
           that can be promoted to an int value.

Question    What modifiers may be used with an interface declaration?
Answer      An interface may be declared as public or abstract.

Question    Is a class a subclass of itself?
                              Java Interview Questions                                 8
Answer      A class is a subclass of itself.

Question     What is the difference between a while statement and a do statement? (CoreJava)
             A while statement checks at the beginning of a loop to see whether the next loop iteration
Answer     should occur. A do statement checks at the end of a loop to see whether the next iteration of a
           loop should occur. The do statement will always execute the body of a loop at least once.

Question    What modifiers can be used with a local inner class?
Answer      A local inner class may be final or abstract.

Question     What is the purpose of the File class? (CoreJava)
             The File class is used to create objects that provide access to the files and directories of a
           local file system.

Question    Can an exception be rethrown? (CoreJava)
Answer      Yes, an exception can be rethrown.

Question    When does the compiler supply a default constructor for a class? (CoreJava)
Answer      The compiler supplies a default constructor for a class if no other constructors are provided.

Question    If a method is declared as protected, where may the method be accessed? (CoreJava)
            A protected method may only be accessed by classes or interfaces of the same package or by
           subclasses of the class in which it is declared.

            Which non-Unicode letter characters may be used as the first character of an identifier?
Question    (CoreJava)

Answer      The non-Unicode letter characters $ and _ may appear as the first character of an identifier

Question    What restrictions are placed on method overloading? (CoreJava)
Answer      Two methods may not have the same name and argument list but different return types.

Question    What is casting? (CoreJava)
            There are two types of casting, casting between primitive numeric types and casting between
           object references. Casting between numeric types is used to convert larger values, such as
           double values, to smaller values, such as byte values. Casting between object references is
           used to refer to an object by a compatible class, interface, or array type reference.

Question    What is the return type of a program's main() method?
Answer      A program's main() method has a void return type.

Question    What class of exceptions are generated by the Java run-time system? (CoreJava)
Answer      The Java runtime system generates RuntimeException and Error exceptions.

Question    What class allows you to read objects directly from a stream? (CoreJava)
Answer      The ObjectInputStream class supports the reading of objects from input streams.

Question    What is the difference between a field variable and a local variable? (CoreJava)
            A field variable is a variable that is declared as a member of a class. A local variable is a
           variable that is declared local to a method.

Question    How are this() and super() used with constructors? (CoreJava)
            this() is used to invoke a constructor of the same class. super() is used to invoke a
           superclass constructor.

Question    What is the relationship between a method's throws clause and the exceptions that can be
                             Java Interview Questions                                 9
           thrown during the method's execution?
            A method's throws clause must declare any checked exceptions that are not caught within
           the body of the method.

Question    Why are the methods of the Math class static? (CoreJava)
Answer      So they can be invoked as if they are a mathematical code library.

Question     What are the legal operands of the instanceof operator? (CoreJava)
             The left operand is an object reference or null value and the right operand is a class,
           interface, or array type.

Question    What an I/O filter? (CoreJava)
            An I/O filter is an object that reads from one stream and writes to another, usually altering
           the data in some way as it is passed from one stream to another.

Question    If an object is garbage collected, can it become reachable again? (CoreJava)
            Once an object is garbage collected, it ceases to exist. It can no longer become reachable

Question    What are E and PI? (CoreJava)
Answer      E is the base of the natural logarithm and PI is mathematical value pi.

Question    Are true and false keywords? (CoreJava)
Answer      The values true and false are not keywords.

Question    What is the difference between the File and RandomAccessFile classes? (CoreJava)
            The File class encapsulates the files and directories of the local file system. The
Answer     RandomAccessFile class provides the methods needed to directly access data contained in any
           part of a file.

Question    What happens when you add a double value to a String?
Answer      The result is a String object.

Question    What is your platform's default character encoding? (CoreJava)
            If you are running Java on English Windows platforms, it is probably Cp1252. If you are
           running Java on English Solaris platforms, it is most likely 8859_1.

Question    Which package is always imported by default? (CoreJava)
Answer      The java.lang package is always imported by default.

            What interface must an object implement before it can be written to a stream as an object?
Question    (CoreJava)

            An object must implement the Serializable or Externalizable interface before it can be written
           to a stream as an object.

Question    Whats the difference between notify() and notifyAll()? (CoreJava)
            notify() is used to unblock one waiting thread; notifyAll() is used to unblock all of them.
           Using notify() is preferable (for efficiency) when only one blocked thread can benefit from the
Answer     change (for example, when freeing a buffer back into a pool). notifyAll() is necessary (for
           correctness) if multiple threads should resume (for example, when releasing a "writer" lock on
           a file might permit all "readers" to resume).

Question    Why can't I say just abs() or sin() instead of Math.abs() and Math.sin()? (CoreJava)
            The import statement does not bring methods into your local name space. It lets you
           abbreviate class names, but not get rid of them altogether. That's just the way it works, you'll
                              Java Interview Questions                                  10
           get used to it. It's really a lot safer this way. <br> However, there is actually a little trick you
           can use in some cases that gets you what you want. If your top-level class doesn't need to
           inherit from anything else, make it inherit from java.lang.Math. That *does* bring all the
           methods into your local name space. But you can't use this trick in an applet, because you
           have to inherit from java.awt.Applet. And actually, you can't use it on java.lang.Math at all,
           because Math is a "final" class which means it can't be extended.

Question    Wha is the output from System.out.println("Hello"+null);
Answer      Hellonull

Question     Why are there no global variables in Java? (CoreJava)
             Global variables are considered bad form for a variety of reasons:
           · Adding state variables breaks referential transparency (you no longer can understand a
           statement or expression on its own: you need to understand it in the context of the settings of
           the global variables).
           · State variables lessen the cohesion of a program: you need to know more to understand how
Answer     something works. A major point of Object-Oriented programming is to break up global state
           into more easily understood collections of local state.
           · When you add one variable, you limit the use of your program to one instance. What you
           thought was global, someone else might think of as local: they may want to run two copies of
           your program at once.
           For these reasons, Java decided to ban global variables.

Question     What does it mean that a class or member is final? (CoreJava)
             A final class can no longer be subclassed. Mostly this is done for security reasons with basic
           classes like String and Integer. It also allows the compiler to make some optimizations, and
           makes thread safety a little easier to achieve.
           Methods may be declared final as well. This means they may not be overridden in a subclass.
           Fields can be declared final, too. However, this has a completely different meaning. A final
Answer     field cannot be changed after it's initialized, and it must include an initializer statement where
           it's declared. For example,
           public final double c = 2.998;
           It's also possible to make a static field final to get the effect of C++'s const statement or some
           uses of C's #define, e.g.
           public static final double c = 2.998;

Question     What does it mean that a method or class is abstract? (CoreJava)
             An abstract class cannot be instantiated. Only its subclasses can be instantiated. You indicate
           that a class is abstract with the abstract keyword like this:
           public abstract class Container extends Component {
           Abstract classes may contain abstract methods. A method declared abstract is not actually
           implemented in the current class. It exists only to be overridden in subclasses. It has no body.
Answer     For example,
           public abstract float price();
           Abstract methods may only be included in abstract classes. However, an abstract class is not
           required to have any abstract methods, though most of them do.
           Each subclass of an abstract class must override the abstract methods of its superclasses or
           itself be declared abstract.

Question    what is a transient variable? (CoreJava)
Answer      transient variable is a variable that may not be serialized.

Question    How are Observer and Observable used? (CoreJava)
            Objects that subclass the Observable class maintain a list of observers. When an Observable
           object is updated it invokes the update() method of each of its observers to notify the
           observers that it has changed state. The Observer interface is implemented by objects that
           observe Observable objects.
                             Java Interview Questions                                11
Question    Can a lock be acquired on a class?
Answer      Yes, a lock can be acquired on a class. This lock is acquired on the class's Class object.

Question    What state does a thread enter when it terminates its processing?
Answer      When a thread terminates its processing, it enters the dead state.

Question    How does Java handle integer overflows and underflows? (CoreJava)
            It uses those low order bytes of the result that can fit into the size of the type allowed by the

Question    What is the difference between the >> and >>> operators? (CoreJava)
            The >> operator carries the sign bit when shifting right. The >>> zero-fills bits that have
           been shifted out.

Question    Is sizeof a keyword? (CoreJava)
Answer      The sizeof operator is not a keyword.

Question    Does garbage collection guarantee that a program will not run out of memory? (CoreJava)
            Garbage collection does not guarantee that a program will not run out of memory. It is
Answer     possible for programs to use up memory resources faster than they are garbage collected. It is
           also possible for programs to create objects that are not subject to garbage collection

Question    Can an object's finalize() method be invoked while it is reachable? (CoreJava)
            An object's finalize() method cannot be invoked by the garbage collector while the object is
           still reachable. However, an object's finalize() method may be invoked by other objects.

Question    What value does readLine() return when it has reached the end of a file?
Answer      The readLine() method returns null when it has reached the end of a file.

Question    Can a for statement loop indefinitely? (CoreJava)
Answer      Yes, a for statement can loop indefinitely. For example, consider the following: for(;;) ;

Question    To what value is a variable of the String type automatically initialized?
Answer      The default value of an String type is null.

Question    What is a task's priority and how is it used in scheduling? (CoreJava)
            A task's priority is an integer value that identifies the relative order in which it should be
Answer     executed with respect to other tasks. The scheduler attempts to schedule higher priority tasks
           before lower priority tasks.

Question    What is the range of the short type? (CoreJava)
Answer      The range of the short type is -(2^15) to 2^15 - 1.

Question    What is the purpose of garbage collection? (CoreJava)
            The purpose of garbage collection is to identify and discard objects that are no longer needed
           by a program so that their resources may be reclaimed and reused.

Question    What do you understand by private, protected and public? (CoreJava)
            These are accessibility modifiers. Private is the most restrictive, while public is the least
           restrictive. There is no real difference between protected and the default type (also known as
           package protected) within the context of the same package, however the protected keyword
           allows visibility to a derived class in a different package.

Question    What is Downcasting ? (CoreJava)
Answer      Downcasting is the casting from a general to a more specific type, i.e. casting down the
                              Java Interview Questions                               12

            Can a method be overloaded based on different return type but same argument type ?
Question    (CoreJava)

            No, because the methods can be called without using their return type in which case there is
           ambiquity for the compiler

Question    What happens to a static var that is defined within a method of a class ?
Answer      Can't do it. You'll get a compilation error

Question     How many static init can you have ? (CoreJava)
             As many as you want, but the static initializers and class variable initializers are executed in
Answer     textual order and may not refer to class variables declared in the class whose declarations
           appear textually after the use, even though these class variables are in scope.

Question    What is the difference amongst JVM Spec, JVM Implementation, JVM Runtime ? (CoreJava)
            The JVM spec is the blueprint for the JVM generated and owned by Sun. The JVM
Answer     implementation is the actual implementation of the spec by a vendor and the JVM runtime is
           the actual running instance of a JVM implementation

Question     Describe what happens when an object is created in Java? (CoreJava)
             Several things happen in a particular order to ensure the object is constructed properly:
           1. Memory is allocated from heap to hold all instance variables and implementation-specific
           data of the object and its superclasses. Implemenation-specific data includes pointers to class
           and method data.
           2. The instance variables of the objects are initialized to their default values.
           3. The constructor for the most derived class is invoked. The first thing a constructor does is
           call the consctructor for its superclasses. This process continues until the constrcutor for
           java.lang.Object is called, as java.lang.Object is the base class for all objects in java.
           4. Before the body of the constructor is executed, all instance variable initializers and
           initialization blocks are executed. Then the body of the constructor is executed. Thus, the
           constructor for the base class completes first and constructor for the most derived class
           completes last.

Question    What does the "final" keyword mean in front of a variable? A method? A class?
            FINAL for a variable : value is constant
Answer     FINAL for a method : cannot be overridden
           FINAL for a class : cannot be derived

Question     What is the difference between instanceof and isInstance? (CoreJava)
             instanceof is used to check to see if an object can be cast into a specified type without
           throwing a cast class exception.
           Determines if the specified Object is assignment-compatible with the object represented by
           this Class. This method is the dynamic equivalent of the Java language instanceof operator.
           The method returns true if the specified Object argument is non-null and can be cast to the
           reference type represented by this Class object without raising a ClassCastException. It
           returns false otherwise.

Question     Why is not recommended to have instance variables in Interface (CoreJava)
             By Default, All data members and methods in an Interface are public. Having public variables
           in a class that will be implementing it will be violation of the Encapsulation principal. I hope
           that's pretty ok.. If anybody has a better framed answer. U r welcome at

Question    What is the diffrence between inner class and nested class? (CoreJava)
Answer      When a class is defined within a scope od another class, then it becomes inner class.
                              Java Interview Questions                                 13
           If the access modifier of the inner class is static, then it becomes nested class.

Question    Diffrence between JRE And JVM AND JDK (CoreJava) (CoreJava)
            The "JDK" is the Java Development Kit. I.e., the JDK is bundle of software that you can use
           to develop Java based software. The "JRE" is the Java Runtime Environment. I.e., the JRE is
           an implementation of the Java Virtual Machine which actually executes Java programs.
           Typically, each JDK contains one (or more) JRE's along with the various development tools like
           the Java source compilers, bundling and deployment tools, debuggers, development libraries,

Question     is it necessary to initialize a final variable at the time of declaretion ? (CoreJava)
             NO, it's not necessary.
           Many text books say like this but thats not true. Value of a final variable can be instance
           specific also, but in this case we have to initialise the variable in all the constructors.
           If we want to have a common final value of a variable for all the instances then there are two
Answer     ways.
           1. Initialise the variable at class level (at the time of declaration) or 2. just declare variable at
           class level and initialise it in any one of the instance blocks i.e.
           A. class A { final int a; {a=5;}}
           B. class A { final int a = 5;}

             What is the output of the following line.
           System.out.println(-5<<-2); (CoreJava)
             At This point of time I am not sure about the answer. If anybody knows, please tell me at
Answer OR
           I shall be highly thankful to you.

Question    What is a compilation unit? (CoreJava)
Answer      A compilation unit is a Java source code file.

Question    What restrictions are placed on method overriding? (CoreJava)
            Overridden methods must have the same name, argument list, and return type.
           The overriding method may not limit the access of the method it overrides.
           The overriding method may not throw any exceptions that may not be thrown by the
           overridden method.

Question    How can a dead thread be restarted?
Answer      A dead thread cannot be restarted.

Question    What happens if an exception is not caught? (CoreJava)
            An uncaught exception results in the uncaughtException() method of the thread's
Answer     ThreadGroup being invoked, which eventually results in the termination of the program in
           which it is thrown.

Question    Which arithmetic operations can result in the throwing of an ArithmeticException?
Answer      Integer / and % can result in the throwing of an ArithmeticException

Question    Can an abstract class be final? (CoreJava)
Answer      An abstract class may not be declared as final

             What happens if a try-catch-finally statement does not have a catch clause to handle an
           exception that is thrown within the body of the try statement? (CoreJava)
             The exception propagates up to the next higher level try-catch statement (if any) or results
           in the program's termination

Question    What is numeric promotion?
                             Java Interview Questions                                         14
             Numeric promotion is the conversion of a smaller numeric type to a larger numeric type, so
           that integer and floating-point operations may take place. In numerical promotion, byte, char,
           and short values are converted to int values. The int values are also converted to long values,
           if necessary. The long and float values are converted to double values, as required

Question    What is the difference between a public and a non-public class? (CoreJava)
            A public class may be accessed outside of its package. A non-public class may not be
           accessed outside of its package.

Question    To what value is a variable of the boolean type automatically initialized?
Answer      The default value of the boolean type is false

Question    Can try statements be nested?
Answer      Yes

Question    What is the difference between the prefix and postfix forms of the ++ operator? (CoreJava)
            The prefix form performs the increment operation and returns the value of the increment
Answer     operation. The postfix form returns the current value all of the expression and then performs
           the increment operation on that value.

Question    What is the purpose of a statement block? (CoreJava)
Answer      A statement block is used to organize a sequence of statements as a single statement group

Question    What is a Java package and how is it used? (CoreJava)
            A Java package is a naming context for classes and interfaces. A package is used to create a
           separate name space for groups of classes and interfaces. Packages are also used to organize
           related classes and interfaces into a single API unit and to control accessibility to these classes
           and interfaces.

Question    What modifiers may be used with a top-level class?
Answer      A top-level class may be public, abstract, or final.

Question    What are the Object and Class classes used for? (CoreJava)
            The Object class is the highest-level class in the Java class hierarchy. The Class class is used
           to represent the classes and interfaces that are loaded by a Java program.

            How does a try statement determine which catch clause should be used to handle an
           exception? (CoreJava)
            When an exception is thrown within the body of a try statement, the catch clauses of the try
Answer     statement are examined in the order in which they appear. The first catch clause that is
           capable of handling the exception is executed. The remaining catch clauses are ignored.

Question     What are synchronized methods and synchronized statements? (CoreJava)
             Synchronized methods are methods that are used to control access to an object. A thread
           only executes a synchronized method after it has acquired the lock for the method's object or
Answer     class. Synchronized statements are similar to synchronized methods. A synchronized
           statement can only be executed after a thread has acquired the lock for the object or class
           referenced in the synchronized statement.

Question    What is the difference between an if statement and a switch statement? (CoreJava)
            The if statement is used to select among two alternatives. It uses a boolean expression to
           decide which alternative should be executed. The switch statement is used to select among
           multiple alternatives. It uses an int expression to determine which alternative should be

Question    What gives java it's "write once and run anywhere" nature?
                              Java Interview Questions                                15
            Java is compiled to be a byte code which is the intermediate language between source code
           and machine code. This byte code is not platorm specific and hence can be fed to any
           platform. After being fed to the JVM, which is specific to a particular operating system, the
           code platform specific machine code is generated thus making java platform independent.

Question    What are the four corner stones of OOP ? (CoreJava)
Answer      Abstraction, Encapsulation, Polymorphism and Inheritance

Question    Difference between a Class and an Object ? (CoreJava)
            A class is a definition or prototype whereas an object is an instance or living representation
           of the prototype

Question    What is the difference between method overriding and overloading? (CoreJava)
            Overriding is a method with the same name and arguments as in a parent, whereas
           overloading is the same method name but different arguments

Question    What is a "stateless" protocol ? (CoreJava)
            Without getting into lengthy debates, it is generally accepted that protocols like HTTP are
Answer     stateless i.e. there is no retention of state between a transaction which is a single request
           response combination

Question     What is constructor chaining and how is it achieved in Java ? (CoreJava)
             A child object constructor always first needs to construct its parent (which in turn calls its
Answer     parent constructor.). In Java it is done via an implicit call to the no-args constructor as the
           first statement.

Question    What is passed by ref and what by value ? (CoreJava)
            All Java method arguments are passed by value. However, Java does manipulate objects by
           reference, and all object variables themselves are references

             You can create a String object as String str = "abc"; Why cant a button object be created as
           Button bt = "abc";? Explain (CoreJava)
             The main reason you cannot create a button by Button bt1= "abc"; is because "abc" is a
           literal string (something slightly different than a String object, by-the-way) and bt1 is a Button
Answer     object. The only object in Java that can be assigned a literal String is java.lang.String.
           Important to note that you are NOT calling a java.lang.String constuctor when you type String
           s = "abc";

Question     What does the "abstract" keyword mean in front of a method? A class? (CoreJava)
             Abstract keyword declares either a method or a class. If a method has a abstract keyword in
           front of it,it is called abstract method.Abstract method hs no body.It has only arguments and
           return type.Abstract methods act as placeholder methods that are implemented in the
           Abstract classes can't be instantiated.If a class is declared as abstract,no objects of that class
           can be created.If a class contains any abstract method it must be declared as abstract

Question     How many methods do u implement if implement the Serializable Interface? (CoreJava)
             The Serializable interface is just a "marker" interface, with no methods of its own to
           implement. Other 'marker' interfaces are

             What are the practical benefits, if any, of importing a specific class rather than an entire
           package (e.g. import* versus import (CoreJava)
             It makes no difference in the generated class files since only the classes that are actually
Answer     used are referenced by the generated class file. There is another practical benefit to importing
           single classes, and this arises when two (or more) packages have classes with the same name.
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           Take java.util.Timer and javax.swing.Timer, for example. If I import java.util.* and
           javax.swing.* and then try to use "Timer", I get an error while compiling (the class name is
           ambiguous between both packages). Let's say what you really wanted was the
           javax.swing.Timer class, and the only classes you plan on using in java.util are Collection and
           HashMap. In this case, some people will prefer to import java.util.Collection and import
           java.util.HashMap instead of importing java.util.*. This will now allow them to use Timer,
           Collection, HashMap, and other javax.swing classes without using fully qualified class names

            What is the difference between logical data independence and physical data independence?
Question    (CoreJava)

            Logical Data Independence - meaning immunity of external schemas to changeds in
Answer     conceptual schema. Physical Data Independence - meaning immunity of conceptual schema to
           changes in the internal schema.

Question    What is user defined exception ? (CoreJava)
            Apart from the exceptions already defined in Java package libraries, user can define his own
           exception classes by extending Exception class.

Question    Difference Between Abstraction and Encapsulation (CoreJava)
            Abstraction is removing some distinctions between objects, so as to show their
           Encapsulation is hiding the details of the implementation of an object so that there are no
           external dependencies on the particular implementation.

Question    What are Checked and Un-Checked Exceptions? Explain. (CoreJava)
            Throwable extends Object (checked)
           Exception extends Throwable (checked)
           RuntimeException extends Exception (un-checked)
           Error extends Throwable (un-checked)
           So anything that extends Throwable or Exception (except RuntimeException) will be checked.
           Anything that extends Error or RuntimeException will be un-checked
           Checked exceptions are problems that arise in correct code and may be due to technical
           problems such as IO problems or user mistakes such as opening a socket when the remote
           machine does not exist. Because these problems can occur at anytime, say due to network
           outage, you must have code that can handle and recover from these. In fact, the Java
           compiler checks that you have trapped them, hence checked exceptions.
           Runtime exceptions are typically bugs in the program. Errors are severe problems such as out
           of memory and sufficiently rare, that you are not required to handle them as they are usually

Question    Can we sort an Hashtable? (CoreJava)
            Yes. Here is an example

Question    What is the exact difference between Abstract classes and Interfaces? (CoreJava)
            Interfaces provide a form of multiple inheritance -- any number of interfaces can be
           implemented A class can extend only one other class. Interfaces are limited to public methods
           and constants with no implementation. Abstract classes can have a partial implementation,
           protected parts, static methods, etc.

Question    What is the difference between ArrayList and Vector
Answer      Methods in Vectors are synchronized

Question    What is the best way to implement structures in java without losing its properties?
Answer      I want a clear answer for this please. If possible a example. Thank u
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Question    Which is garbage collected first : Normal variable or Static variable?
Answer      Please give me the answer

Question    How the Private Consturctor is called in the main java program
Answer      i need answer

Question    Can we overload main method in java?

Question    difference between jsp and servlet
Answer      i don't know

Question    Can we have inner class in the interface ? (CoreJava)
Answer      Yes.We can have inner class in the interface.

Question    Explain about Singleton Class (CoreJava)
            Refer this link for an example
           In general, you use a singleton to enforce the notion that there will be only one instance of a
           given class. Singletons should be used in situations where creating more than one of
           something would be a logical error.

           For example, a ConnectionPool would be a good place to use a singleton. If clients could
           arbitrarily create ConnectionPools without regard to what already exists, you would have a
           waste of resources. So you limit the possible number of connection pools to 1 (per JVM), and
Answer     you then know that all clients are getting their connections from a single source.

           Another example of Singleton use is for Object Factories. Say you have a class called
           FooFactory that is responsible for fetching/saving Foo objects to/from a database. You want to
           ensure that for each Foo record in the db, there is only one corresponding Foo object floating
           around your application. By centralizing all the creation logic in a single class, and making that
           class a Singleton, you eliminate the possibility fo duplicate objects.

           The code that uses a connection obtained from the connection pool is another matter. If all it
           does is do a getData() type operation, there is no harm in having more than one of them.

Question    What is the exact difference between Abstract classes and Interfaces?
Answer      Im new 2 java . this is frequetly asked pls give the answer

Question    we know that c++ is developed using c, so using what language java is developed
Answer      I don't know the answer if someone answers it will be fine

Question     what is j2EE? (CoreJava)
             J2EE menas Java 2 Enterprise Edition.. it follows certain rules and regulations to develop web
Answer     technologies. and certain technologies are under comes i this category like
           EJB/JSP/Servlets/JMS/Web Servers/App Servers/Struts and Oracle-9.0i

Question    Are enterprise beans allowed to use Thread.sleep()? (EJB)
            Enterprise beans make use of the services provided by the EJB container, such as life-cycle
Answer     management. To avoid conflicts with these services, enterprise beans are restricted from
           performing certain operations: Managing or synchronizing threads

            Is it possible to write two EJB's that share the same Remote and Home interfaces, and have
           different bean classes? if so, what are the advantages/disadvantages? (EJB)
            It's certainly possible. In fact, there's an example that ships with the Inprise Application
           Server of an Account interface with separate implementations for CheckingAccount and
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           SavingsAccount, one of which was CMP and one of which was BMP.

Question    Is it possible to specify multiple JNDI names when deploying an EJB? (EJB)
            No. To achieve this you have to deploy your EJB multiple times each specifying a different
           JNDI name.

             Is there any way to force an Entity Bean to store itself to the db? I don't wanna wait for the
           container to update the db, I want to do it NOW! Is it possible? (EJB)
             Specify the transaction attribute of the bean as RequiresNew. Then as per section of
           the EJB v 1.1 spec EJB container automatically starts a new transaction before the method
           call. The container also performs the commit protocol before the method result is sent to the

           I am developing a BMP Entity bean. I have noticed that whenever the create method is
         invoked, the ejbLoad() and the ejbStore() methods are also invoked. I feel that once my
Question database insert is done, having to do a select and update SQL queries is major overhead. is
         this behavior typical of all EJB containers? Is there any way to suppress these invocations?

            This is the default behaviour for EJB. The specification states that ejbLoad() will be called
Answer     before every transaction and ejbStore() after every transaction. Each Vendor has
           optimizations, which are proprietary for this scenario.

Question     Can an EJB send asynchronous notifications to its clients? (EJB)
             Asynchronous notification is a known hole in the first versions of the EJB spec. The
           recommended solution to this is to use JMS, which is becoming available in J2EE-compliant
           servers. The other option, of course, is to use client-side threads and polling. This is not an
           ideal solution, but it's workable for many scenarios.

Question    How can I access EJB from ASP? (EJB)
            You can use the Java 2 Platform, Enterprise Edition Client Access Services (J2EETM CAS)
Answer     COM Bridge 1.0, currently downloadable from

Question    Is there a guarantee of uniqueness for entity beans? (EJB)
            There is no such guarantee. The server (or servers) can instantiate as many instances of the
           same underlying Entity Bean (with the same PK) as it wants. However, each instance is
Answer     guaranteed to have up-to-date data values, and be transactionally consistent, so uniqueness is
           not required. This allows the server to scale the system to support multiple threads, multiple
           concurrent requests, and multiple hosts.

             How do the six transaction attributes map to isolation levels like "dirty read"? Will an
           attribute like "Required" lock out other readers until I'm finished updating? (EJB)
             The Transaction Attributes in EJB do not map to the Transaction Isolation levels used in
           JDBC. This is a common misconception. Transaction Attributes specify to the container when a
Answer     Transaction should be started, suspended(paused) and committed between method
           invocations on Enterprise JavaBeans. For more details and a summary of Transaction
           Attributes refer to section 11.6 of the EJB 1.1 specification.

            I have created a remote reference to an EJB in FirstServlet. Can I put the reference in a
           servlet session and use that in SecondServlet? (EJB)
            Yes. The EJB client (in this case your servlet) acquires a remote reference to an EJB from the
           Home Interface; that reference is serializable and can be passed from servlet to servlet. If it is
           a session bean, then the EJB server will consider your web client's servlet session to
           correspond to a single EJB session, which is usually (but not always) what you want.

Question    Can the primary key in the entity bean be a Java primitive type such as int? (EJB)
Answer      The primary key can't be a primitive type--use the primitive wrapper classes, instead. For
                             Java Interview Questions                               19
           example, you can use java.lang.Integer as the primary key class, but not int (it has to be a
           class, not a primitive)

Question    What's new in the EJB 2.0 specification? (EJB)
            Following are the main features supported in EJB 2.0 * Integration of EJB with JMS *
Answer     Message Driven Beans * Implement additional Business methods in Home interface which are
           not specific for bean instance. * EJB QL.

Question    What is the need of Remote and Home interface. Why cant it be in one? (EJB)
            In a few words, I would say that the main reason is because there is a clear division of roles
           and responsabilities between the two interfaces.
           The home interface is your way to communicate with the container, that is who is responsable
           of creating, locating even removing one or more beans.
           The remote interface is your link to the bean, that will allow you to remotely access to all its
           methods and members.
           As you can see there are two distinct elements (the container and the beans) and you need
           two different interfaces for accessing to both of them.

Question    What is the difference between Java Beans and EJB?s? (EJB)
            Java Beans are client-side objects and EJBs are server side object, and they have completely
           different development, lifecycle, purpose.

             Question With regard to Entity Beans, what happens if both my EJB Server and Database
           crash, what will happen to unsaved changes? Is there any transactional log file used? (EJB)
             Actually, if your EJB server crashes, you will not even be able to make a connection to the
           server to perform a bean lookup, as the server will no longer be listening on the port for
           incoming JNDI lookup requests. You will lose any data that wasn't committed prior to the
           crash. This is where you should start looking into clustering your EJB server.
           Another Answer
           Hi, Any unsaved and uncommited changes are lost the moment your EJB Server crashes. If
Answer     your database also crashes, then all the saved changes are also lost unless you have some
           backup or some recovery mechanism to retrieve the data. So consider database replication
           and EJB Clustering for such scenarios, though the occurence of such a thing is very very rare.
           Thx, Uma All databse have the concept of log files(for exampe oracle have redo log files
           concept). So if data bases crashes then on starting up they fill look up the log files to perform
           all pending jobs. But is EJB crashes, It depend upon the container how frequenlty it passivates
           or how frequesntly it refreshes the data with Database.

Question     Question Can you control when passivation occurs? (EJB)
             The developer, according to the specification, cannot directly control when passivation
           occurs. Although for Stateful Session Beans, the container cannot passivate an instance that is
           inside a transaction. So using transactions can be a a strategy to control passivation.
           The ejbPassivate() method is called during passivation, so the developer has control over what
           to do during this exercise and can implement the require optimized logic.
           Some EJB containers, such as BEA WebLogic, provide the ability to tune the container to
           minimize passivation calls.
           Taken from the WebLogic 6.0 DTD -
           "The passivation-strategy can be either "default" or "transaction". With the default setting the
           container will attempt to keep a working set of beans in the cache. With the "transaction"
           setting, the container will passivate the bean after every transaction (or method call for a non-
           transactional invocation)."

          Does EJB 1.1 support mandate the support for RMI-IIOP ? What is the meaning of "the client
Question API must support the Java RMI-IIOP programming model for portability, but the underlying
         protocol can be anything" ? (EJB)
          EJB1.1 does mandate the support of RMI-IIOP.
Answer   OK, to answer the second question:
         There are 2 types of implementations that an EJB Server might provide: CORBA-based EJB
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           Servers and Proprietry EJB Servers. Both support the RMI-IIOP API but how that API is
           implemented is a different story. (NB: By API we mean the interface provided to the client by
           the stub or proxy).
           A CORBA-based EJB Server actually implements its EJB Objects as CORBA Objects (it therefore
           encorporates an ORB and this means that EJB's can be contacted by CORBA clients (as well as
           RMI-IIOP clients)
           A proprietry EJB still implements the RMI-IIOP API (in the client's stub) but the underlying
           protocol can be anything. Therefore your EJB's CANNOT be contacted by CORBA clients. The
           difference is that in both cases, your clients see the same API (hence, your client portability)
           BUT how the stubs communicate with the server is different.

            The EJB specification says that we cannot use Bean Managed Transaction in Entity Beans.
           Why? (EJB)
            The short, practical answer is... because it makes your entity beans useless as a reusable
           component. Also, transaction management is best left to the application server - that's what
           they're there for. It's all about atomic operations on your data. If an operation updates more
           than one entity then you want the whole thing to succeed or the whole thing to fail, nothing in
           between. If you put commits in the entity beans then it's very difficult to rollback if an error
           occurs at some point late in the operation.

Question    Can I invoke Runtime.gc() in an EJB? (EJB)
            You shouldn't.
           What will happen depends on the implementation, but the call will most likely be ignored. You
           should leave system level management like garbage collection for the container to deal with.
           After all, that's part of the benefit of using EJBs, you don't have to manage resources yourself.

Question     What is clustering? What are the different algorithms used for clustering? (EJB)
             Clustering is grouping machines together to transparantly provide enterprise services.The
           client does not now the difference between approaching one server or approaching a cluster of
           servers.Clusters provide two benefits: scalability and high availability. Further information can
           be found in the JavaWorld article J2EE Clustering.

             What is the advantage of using Entity bean for database operations, over directly using JDBC
           API to do database operations? When would I use one over the other? (EJB)
             Entity Beans actually represents the data in a database. It is not that Entity Beans replaces
           JDBC API. There are two types of Entity Beans Container Managed and Bean Mananged. In
           Container Managed Entity Bean - Whenever the instance of the bean is created the container
           automatically retrieves the data from the DB/Persistance storage and assigns to the object
           variables in bean for user to manipulate or use them. For this the developer needs to map the
           fields in the database to the variables in deployment descriptor files (which varies for each
           In the Bean Managed Entity Bean - The developer has to specifically make connection, retrive
           values, assign them to the objects in the ejbLoad() which will be called by the container when
           it instatiates a bean object. Similarly in the ejbStore() the container saves the object values
           back the the persistance storage. ejbLoad and ejbStore are callback methods and can be only
           invoked by the container. Apart from this, when you use Entity beans you dont need to worry
           about database transaction handling, database connection pooling etc. which are taken care
           by the ejb container. But in case of JDBC you have to explicitly do the above features. what
           suresh told is exactly perfect. ofcourse, this comes under the database transations, but i want
           to add this. the great thing about the entity beans of container managed, whenever the
           connection is failed during the transaction processing, the database consistancy is mantained
           automatically. the container writes the data stored at persistant storage of the entity beans to
           the database again to provide the database consistancy. where as in jdbc api, we, developers
           has to do manually.

Question    What is the role of serialization in EJB? (EJB)
            A big part of EJB is that it is a framework for underlying RMI: remote method invocation.
           You're invoking methods remotely from JVM space 'A' on objects which are in JVM space 'B' --
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           possibly running on another machine on the network.
           To make this happen, all arguments of each method call must have their current state plucked
           out of JVM 'A' memory, flattened into a byte stream which can be sent over a TCP/IP network
           connection, and then deserialized for reincarnation on the other end in JVM 'B' where the
           actual method call takes place.
           If the method has a return value, it is serialized up for streaming back to JVM A. Thus the
           requirement that all EJB methods arguments and return values must be serializable. The
           easiest way to do this is to make sure all your classes implement

Question     What is EJB QL? (EJB)
             EJB QL is a Query Language provided for navigation across a network of enterprise beans
           and dependent objects defined by means of container managed persistence. EJB QL is
           introduced in the EJB 2.0 specification. The EJB QL query language defines finder methods for
           entity beans with container managed persistenceand is portable across containers and
Answer     persistence managers. EJB QL is used for queries of two types of finder methods: Finder
           methods that are defined in the home interface of an entity bean and which return entity
           objects. Select methods, which are not exposed to the client, but which are used by the Bean
           Provider to select persistent values that are maintained by the Persistence Manager or to
           select entity objects that are related to the entity bean on which the query is defined.

Question    Why bean must not implement the remote interface XARB
Answer      Asking

Question     Is is possible for an EJB client to marshall an object of class java.lang.Class to an EJB? (EJB)
            Technically yes, spec. compliant NO! - The enterprise bean must not attempt to query a class
Answer     to obtain information about the declared members that are not otherwise accessible to the
           enterprise bean because of the security rules of the Java language.

Question    Is it legal to have static initializer blocks in EJB? (EJB)
            Although technically it is legal, static initializer blocks are used to execute some piece of code
           before executing any constructor or method while instantiating a class. Static initializer blocks
Answer     are also typically used to initialize static fields - which may be illegal in EJB if they are
           read/write - In EJB this can be achieved by including the code in either the ejbCreate(),
           setSessionContext() or setEntityContext() methods.

Question    Is it possible to stop the execution of a method before completion in a SessionBean? (EJB)
            Stopping the execution of a method inside a Session Bean is not possible without writing
Answer     code inside the Session Bean. This is because you are not allowed to access Threads inside an

Question    What is the default transaction attribute for an EJB? (EJB)
            There is no default transaction attribute for an EJB. Section 11.5 of EJB v1.1 spec says that
           the deployer must specify a value for the transaction attribute for those methods having
           container managed transaction. In weblogic, the default transaction attribute for EJB is

            What is the difference between session and entity beans? When should I use one or the
           other? (EJB)
            An entity bean represents persistent global data from the database; a session bean
           represents transient user-specific data that will die when the user disconnects (ends his
           session). Generally, the session beans implement business methods (e.g. Bank.transferFunds)
           that call entity beans (e.g. Account.deposit, Account.withdraw)

             Is there any default cache management system with Entity beans ? In other words whether a
           cache of the data in database will be maintained in EJB ? (EJB)
             Caching data from a database inside the Application Server are what Entity EJB's are used
           for.The ejbLoad() and ejbStore() methods are used to synchronize the Entity Bean state with
                             Java Interview Questions                               22
           the persistent storage(database). Transactions also play an important role in this scenario. If
           data is removed from the database, via an external application - your Entity Bean can still be
           "alive" the EJB container. When the transaction commits, ejbStore() is called and the row will
           not be found, and the transcation rolled back.

Question    Why is ejbFindByPrimaryKey mandatory? (EJB)
            An Entity Bean represents persistent data that is stored outside of the EJB Container/Server.
           The ejbFindByPrimaryKey is a method used to locate and load an Entity Bean into the
           container, similar to a SELECT statement in SQL. By making this method mandatory, the client
           programmer can be assured that if they have the primary key of the Entity Bean, then they
           can retrieve the bean without having to create a new bean each time - which would mean
           creating duplications of persistent data and break the integrity of EJB.

Question     Why do we have a remove method in both EJBHome and EJBObject? (EJB)
             With the EJBHome version of the remove, you are able to delete an entity bean without first
           instantiating it (you can provide a PrimaryKey object as a parameter to the remove method).
           The home version only works for entity beans. On the other hand, the Remote interface
           version works on an entity bean that you have already instantiated. In addition, the remote
           version also works on session beans (stateless and statefull) to inform the container of your
           loss of interest in this bean.

Question    How can I call one EJB from inside of another EJB? (EJB)
            EJBs can be clients of other EJBs. It just works. Use JNDI to locate the Home Interface of the
           other bean, then acquire an instance reference, and so forth.

Question     What is the difference between a Server, a Container, and a Connector? (EJB)
             An EJB server is an application, usually a product such as BEA WebLogic, that provides (or
           should provide) for concurrent client connections and manages system resources such as
           threads, processes, memory, database connections, network connections, etc.
           An EJB container runs inside (or within) an EJB server, and provides deployed EJB beans with
           transaction and security management, etc. The EJB container insulates an EJB bean from the
           specifics of an underlying EJB server by providing a simple, standard API between the EJB
           bean and its container.
           A Connector provides the ability for any Enterprise Information System (EIS) to plug into any
           EJB server which supports the Connector architecture. See
  for more indepth information on Connectors.

Question     How is persistence implemented in enterprise beans? (EJB)
             Persistence in EJB is taken care of in two ways, depending on how you implement your
           beans: container managed persistence (CMP) or bean managed persistence (BMP)
           For CMP, the EJB container which your beans run under takes care of the persistence of the
           fields you have declared to be persisted with the database - this declaration is in the
           deployment descriptor. So, anytime you modify a field in a CMP bean, as soon as the method
           you have executed is finished, the new data is persisted to the database by the container.
Answer     For BMP, the EJB bean developer is responsible for defining the persistence routines in the
           proper places in the bean, for instance, the ejbCreate(), ejbStore(), ejbRemove() methods
           would be developed by the bean developer to make calls to the database. The container is
           responsible, in BMP, to call the appropriate method on the bean. So, if the bean is being
           looked up, when the create() method is called on the Home interface, then the container is
           responsible for calling the ejbCreate() method in the bean, which should have functionality
           inside for going to the database and looking up the data.

Question    What is an EJB Context? (EJB)
            EJBContext is an interface that is implemented by the container, and it is also a part of the
           bean-container contract. Entity beans use a subclass of EJBContext called EntityContext.
Answer     Session beans use a subclass called SessionContext. These EJBContext objects provide the
           bean class with information about its container, the client using the bean and the bean itself.
           They also provide other functions. See the API docs and the spec for more details.
                              Java Interview Questions                                   23
Question     Is method overloading allowed in EJB?
Answer       Yes you can overload methods

Question     Should synchronization primitives be used on bean methods? (EJB)
             No. The EJB specification specifically states that the enterprise bean is not allowed to use
Answer      thread primitives. The container is responsible for managing concurrent access to beans at

              Question Are we allowed to change the transaction isolation property in middle of a
            transaction? (EJB)
Answer        No. You cannot change the transaction isolation level in the middle of transaction.

                                          Question For Entity Beans, What happens to an instance field not
Question                                 mapped to any persistent storage,when the bean is passivated?

                                           The specification infers that the container never serializes an
                                         instance of an Entity bean (unlike stateful session beans). Thus
                                         passivation simply involves moving the bean from the "ready" to the
                                         "pooled" bin. So what happens to the contents of an instance
                                         variable is controlled by the programmer. Remember that when an
                                         entity bean is passivated the instance gets logically disassociated
                                         from it's remote object.
                                         Be careful here, as the functionality of passivation/activation for
                                         Stateless Session, Stateful Session and Entity beans is completely
                                         different. For entity beans the ejbPassivate method notifies the
                                         entity bean that it is being disassociated with a particular entity prior
                                         to reuse or for dereferenc.
          Question What is a
         Message Driven Bean,
         What functions does a
Question message driven bean have
         and how do they work in
         collaboration with JMS?

            Message driven beans
           are the latest addition to
           the family of component
           bean types defined by the
           EJB specification. The
           original bean types include
           session beans, which
           contain business logic and
           maintain a state
           associated with client
           sessions, and entity
Answer     beans, which map objects
           to persistent data.
           Message driven beans will
           provide asynchrony to EJB
           based applications by
           acting as JMS message
           consumers. A message
           bean is associated with a
           JMS topic or queue and
           receives JMS messages
           sent by EJB clients or
           other beans.
                             Java Interview Questions                          24
          Unlike entity beans and
          session beans, message
          beans do not have home
          or remote interfaces.
          Instead, message driven
          beans are instantiated by
          the container as required.
          Like stateless session
          beans, message beans
          maintain no client-specific
          state, allowing the
          container to optimally
          manage a pool of
          message-bean instances.
          Clients send JMS
          messages to message
          beans in exactly the same
          manner as they would
          send messages to any
          other JMS destination.
          This similarity is a
          fundamental design goal
          of the JMS capabilities of
          the new specification.
          To receive JMS messages,
          message driven beans
          implement the
          interface, which defines a
          single "onMessage()"
          When a message arrives,
          the container ensures that
          a message bean
          corresponding to the
          message topic/queue
          exists (instantiating it if
          necessary), and calls its
          onMessage method
          passing the client's
          message as the single
          argument. The message
          bean's implementation of
          this method contains the
          business logic required to
          process the message.
          Note that session beans
          and entity beans are not
          allowed to function as
          message beans.

          The EJB container implements the EJBHome and EJBObject classes. For every request from a
Question unique client, does the container create a separate instance of the generated EJBHome and
         EJBObject classes? (EJB)
          The EJB container maintains an instance pool. The container uses these instances for the EJB
         Home reference irrespective of the client request. while refering the EJB Object classes the
Answer   container creates a separate instance for each client request.
         Another Answer
         The instance pool maintainence is up to the implementation of the container. If the container
                             Java Interview Questions                                25
           provides one, it is available otherwise it is not mandatory for the provider to implement it.
           Having said that, yes most of the container providers implement the pooling functionality to
           increase the performance of the app server. How it is implemented, it is again up to the

             What is the advantage of puttting an Entity Bean instance from the "Ready State" to "Pooled
           state"? (EJB)
             The idea of the "Pooled State" is to allow a container to maintain a pool of entity beans that
           has been created, but has not been yet "synchronized" or assigned to an EJBObject. This
           mean that the instances do represent entity beans, but they can be used only for serving
           Home methods (create or findBy), since those methods do not relay on the specific values of
           the bean. All these instances are, in fact, exactly the same, so, they do not have meaningful
           state. Jon Thorarinsson has also added: It can be looked at it this way:
Answer     If no client is using an entity bean of a particular type there is no need for cachig it (the data
           is persisted in the database).
           Therefore, in such cases, the container will, after some time, move the entity bean from the
           "Ready State" to the "Pooled state" to save memory.
           Then, to save additional memory, the container may begin moving entity beans from the
           "Pooled State" to the "Does Not Exist State", because even though the bean's cache has been
           cleared, the bean still takes up some memory just being in the "Pooled State".

Question     Can a Session Bean be defined without ejbCreate() method? (EJB)
             The ejbCreate() methods is part of the bean's lifecycle, so, the compiler will not return an
           error because there is no ejbCreate() method.
           However, the J2EE spec is explicit:
           the home interface of a Stateless Session Bean must have a single create() method with no
           arguments, while the session bean class must contain exactly one ejbCreate() method, also
Answer     without arguments.
           Stateful Session Beans can have arguments (more than one create method) stateful beans can
           contain multiple ejbCreate() as long as they match with the home interface definition
           You need a reference to your EJBObject to startwith. For that Sun insists on putting a method
           for creating that reference (create method in the home interface). The EJBObject does matter
           here. Not the actual bean.

             Is it possible to share an HttpSession between a JSP and EJB? What happens when I change
           a value in the HttpSession from inside an EJB? (EJB)
             You can pass the HttpSession as parameter to an EJB method, only if all objects in session
           are serializable.This has to be consider as "passed-by-value", that means that it's read-only in
           the EJB. If anything is altered from inside the EJB, it won't be reflected back to the
           HttpSession of the Servlet Container.The "pass-by-reference" can be used between EJBs
           Remote Interfaces, as they are remote references. While it IS possible to pass an HttpSession
           as a parameter to an EJB object, it is considered to be "bad practice (1)" in terms of object
Answer     oriented design. This is because you are creating an unnecessary coupling between back-end
           objects (ejbs) and front-end objects (HttpSession). Create a higher-level of abstraction for
           your ejb's api. Rather than passing the whole, fat, HttpSession (which carries with it a bunch
           of http semantics), create a class that acts as a value object (or structure) that holds all the
           data you need to pass back and forth between front-end/back-end. Consider the case where
           your ejb needs to support a non-http-based client. This higher level of abstraction will be
           flexible enough to support it. (1) Core J2EE design patterns (2001)

           Is there any way to read values from an entity bean without locking it for the rest of the
         transaction (e.g. read-only transactions)? We have a key-value map bean which deadlocks
         during some concurrent reads. Isolation levels seem to affect the database only, and we need
         to work within a transaction. (EJB)
           The only thing that comes to (my) mind is that you could write a 'group accessor' - a method
         that returns a single object containing all of your entity bean's attributes (or all interesting
         attributes). This method could then be placed in a 'Requires New' transaction. This way, the
         current transaction would be suspended for the duration of the call to the entity bean and the
                             Java Interview Questions                               26
           entity bean's fetch/operate/commit cycle will be in a separate transaction and any locks should
           be released immediately. Depending on the granularity of what you need to pull out of the
           map, the group accessor might be overkill.

             What is the difference between a "Coarse Grained" Entity Bean and a "Fine Grained" Entity
           Bean? (EJB)
             A 'fine grained' entity bean is pretty much directly mapped to one relational table, in third
           normal form.
           A 'coarse grained' entity bean is larger and more complex, either because its attributes include
           values or lists from other tables, or because it 'owns' one or more sets of dependent objects.
           Note that the coarse grained bean might be mapped to a single table or flat file, but that
           single table is going to be pretty ugly, with data copied from other tables, repeated field
           groups, columns that are dependent on non-key fields, etc.
           Fine grained entities are generally considered a liability in large systems because they will tend
           to increase the load on several of the EJB server's subsystems (there will be more objects
           exported through the distribution layer, more objects participating in transactions, more
           skeletons in memory, more EJB Objects in memory, etc.) The other side of the coin is that the
           1.1 spec doesn't mandate CMP Error! No index entries for dependent objects (or
           even indicate how they should be supported), which makes it more difficult to do coarse
           grained objects with CMP. The EJB 2.0 specification improves this in a huge way.

Question     What is EJBDoclet? (EJB)
             EJBDoclet is an open source JavaDoc doclet that generates a lot of the EJB related source
           files from custom JavaDoc comments tags embedded in the EJB source file.

Question     What are the main benefits of J2EE? (EJB)
             J2EE provides the following:
           Faster solutions delivery time to market. J2EE uses "containers" to simplify development. J2EE
           containers provide for the separation of business logic from resource and lifecycle
           management, which means that developers can focus on writing business logic -- their value
           add -- rather than writing enterprise infrastructure. For example, the Enterprise JavaBeansTM
           (EJBTM) container (implemented by J2EE technology vendors) handles distributed
           communication, threading, scaling, transaction management, etc. Similarly, Java Servlets
           simplify web development by providing infrastructure for component, communication, and
           session management in a web container that is integrated with a web server.
           Freedom of choice. J2EE technology is a set of standards that many vendors can implement.
           The vendors are free to compete on implementations but not on standards or APIs. Sun
           supplies a comprehensive J2EE Compatibility Test Suite (CTS) to J2EE licensees. The J2EE CTS
Answer     helps ensure compatibility among the application vendors which helps ensure portability for
           the applications and components written for J2EE. J2EE brings Write Once, Run AnywhereTM
           (WORATM) to the server.
           Simplified connectivity. J2EE technology makes it easier to connect the applications and
           systems you already have and bring those capabilities to the web, to cell phones, and to
           devices. J2EE offers Java Message Service for integrating diverse applications in a loosely
           coupled, asynchronous way. J2EE also offers CORBA support for tightly linking systems
           through remote method calls. In addition, J2EE 1.3 adds J2EE Connectors for linking to
           enterprise information systems such as ERP systems, packaged financial applications, and CRM
           By offering one platform with faster solution delivery time to market, freedom of choice, and
           simplified connectivity, J2EE helps IT by reducing TCO and simultaneously avoiding single-
           source for their enterprise software needs.

Question    Name a few Design Patterns used in J2ee applications (EJB)
Answer      MVC, Front Controller, Session Facade, Data Access Object

Question    What is the deployment order for the deployed server components in WebLogic server?
            § JDBC Connection Pools
           § JDBC Multi Pools
                              Java Interview Questions                              27
           § JDCB Data Sources
           § JDBC Tx Data Sources
           § JMS Connection Factories
           § JMS Servers
           § Connector Components
           § EJB Components
           § Web App Components
           --- An examination of the log file .,.,

            Why do the create() or find() method return the remote reference or a primary key only?
Question    (EJB)

             The EJB Specification prohibits this behavior, and the weblogic.ejbc compiler checks for this
           behavior and prohibits any polymorphic type of response from a create() or find() method.
           The reason the create() and find() methods cannot return any object or primitive type is
           similar to the reason that regular constructors can be cast into the class itself or any of it?s
Answer     super classes.
           For example
           A a = new A() or
           A b = new B() where B is a child of A.
           You cannot do, for example Vector v = new A();

Question    Which XML parser comes with WebLogic Server 6.1? (EJB)
            We bundle a parser, based on Apache's Xerces 1.3.1 parser, in WebLogic Server 6.1. In
           addition, we include a WebLogic proprietary high-performance non-validating parser that you
           can use for small to medium sized XML documents. The WebLogic XML Registry allows you to
           configure the parser you want to use for specific document types.

             Can I use the getAttribute() and setAttribute() methods of Version 2.2 of the Java Servlet
           API to parse XML documents? (EJB)
             Yes. Use the setAttribute() method for SAX mode parsing and the getAttribute() method for
           DOM mode parsing. Using these methods in a Servlet, however, is a WebLogic-specific feature.
           This means that the Servlet may not be fully portable to other Servlet engines, so use the
           feature with caution.

             How can I run multiple instances of the same servlet class in the same WebLogic Server
           instance? (EJB)
             If you want to run multiple instances, your servlet will have to implement the
           SingleThreadModel interface. An instance of a class that implements the SingleThreadModel
           interface is guaranteed not to be invoked by multiple threads simultaneously. Multiple
           instances of a SingleThreadModel interface are used to service simultaneous requests, each
           running in a single thread.
           When designing your servlet, consider how you use shared resources outside of the servlet
           class such as file and database access. Because there are multiple instances of servlets that
           are identical, and may use exactly the same resources, there are still synchronization and
           sharing issues that must be resolved, even if you do implement the SingleThreadModel

Question    What technologies are included in J2EE? (EJB)
            The primary technologies in J2EE are: Enterprise JavaBeansTM (EJBsTM), JavaServer
Answer     PagesTM (JSPsTM), Java Servlets, the Java Naming and Directory InterfaceTM (JNDITM), the
           Java Transaction API (JTA), CORBA, and the JDBCTM data access API.

Question     What is the Java Authentication and Authorization Service (JAAS) 1.0? (EJB)
             The Java Authentication and Authorization Service (JAAS) provides a way for a J2EE
           application to authenticate and authorize a specific user or group of users to run it. JAAS is a
Answer     Java programing language version of the standard Pluggable Authentication Module (PAM)
           framework that extends the Java 2 platform security architecture to support user-based
                             Java Interview Questions                              28

Question    Must my bean-managed persistence mechanism use the WebLogic JTS driver? (EJB)
Answer      BEA recommend that you use the TxDataSource for bean-managed persistence.

Question    Must EJBs be homogeneously deployed across a cluster? Why? (EJB)
            Yes. Beginning with WebLogic Server version 6.0, EJBs must be homogeneously deployed
           across a cluster for the following reasons:
           n To keep clustering EJBs simple
           n To avoid cross server calls which results in more efficiency. If EJBs are not deployed on all
           servers, cross server calls are much more likely.
Answer     n To ensure that every EJB is available locally
           n To ensure that all classes are loaded in an undeployable way
           Every server must have access to each EJB's classes so that it can be bound into the local
           JNDI tree. If only a subset of the servers deploys the bean, the other servers will have to load
           the bean's classes in their respective system classpaths which makes it impossible to undeploy
           the beans.

Question    Is an XSLT processor bundled in WebLogic Server? (EJB)
            Yes, we bundle an XSLT processor, based on Apache's Xalan 2.0.1 processor, in WebLogic
           Server 6.1.

             I plugged in a version of Apache Xalan that I downloaded from the Apache Web site, and now
           I get errors when I try to transform documents. What is the problem? (EJB)
             You must ensure that the version of Apache Xalan you download from the Apache Web site is
           compatible with Apache Xerces version 1.3.1. Because you cannot plug in a different version
           of Apache Xerces (see the preceding question), the only version of Apache Xerces that is
           compatible with WebLogic Server 6.1 is 1.3.1.
           The built-in parser (based on version 1.3.1 of Apache Xerces) and transformer (based on
           version 2.0.1 of Apache Xalan) have been modified by BEA to be compatible with each other.

Question     How do I increase WebLogic Server memory? (EJB)
             Increase the allocation of Java heap memory for WebLogic Server. (Set the minimum and the
           maximum to the same size.) Start WebLogic Server with the -ms32m option to increase the
           allocation, as in this example:
Answer     $ java ... -ms32m -mx32m ...
           This allocates 32 megabytes of Java heap memory to WebLogic Server, which improves
           performance and allows WebLogic Server to handle more simultaneous connections. You can
           increase this value if necessary.

Question     What causes exceptions in the log file of WebLogic Server? (EJB)
             You may see messages like these in the log file:
           (Windows NT)
  Connection Reset by Peer
  Connection Reset by Peer
  Broken pipe
Answer     These messages occur when you are using servlets. A client initiates an HTTP request, and
           then performs a series of actions on the browser:
           1. Click Stop or enter equivalent command or keystrokes
           2. Click Refresh or enter equivalent command or keystrokes
           3. Send a new HTTP request.
           The messages indicate that WebLogic Server has detected and recovered from an interrupted
           HTTP request.

Question    What is the function of T3 in WebLogic Server? (EJB)
            T3 provides a framework for WebLogic Server messages that support for enhancements.
Answer     These enhancements include abbreviations and features, such as object replacement, that
           work in the context of WebLogic Server clusters and HTTP and other product tunneling.
                             Java Interview Questions                               29
           T3 predates Java Object Serialization and RMI, while closely tracking and leveraging these
           specifications. T3 is a superset of Java Object. Serialization or RMI; anything you can do in
           Java Object Serialization and RMI can be done over T3.
           T3 is mandated between WebLogic Servers and between programmatic clients and a WebLogic
           Server cluster. HTTP and IIOP are optional protocols that can be used to communicate
           between other processes and WebLogic Server. It depends on what you want to do. For
           example, when you want to communicate between
           n A browser and WebLogic Server-use HTTP
           n An ORB and WebLogic Server-IIOP.

             What are the enhancements in EJB 2.0 specification with respect to Asynchronous
           communication? (EJB)
             EJB 2.0 mandates integration between JMS and EJB.
           We have specified the integration of Enterprise JavaBeans with the Java Message Service, and
           have introduced message-driven beans. A message-driven bean is a stateless component that
Answer     is invoked by the container as a result of the arrival of a JMS message. The goal of the
           message-driven bean model is to make developing an enterprise bean that is asynchronously
           invoked to handle the processing of incoming JMS messages as simple as developing the same
           functionality in any other JMS MessageListener.

Question     What are the enhancements in EJB 2.0 with respect to CMP? (EJB)
             EJB 2.0 extends CMP to include far more robust modeling capability, with support for
           declarative management of relationships between entity EJBs. Developers no longer need to
           re-establish relationships between the various beans that make up their application -- the
           container will restore the connections automatically as beans are loaded, allowing bean
Answer     developers to navigate between beans much as they would between any standard Java
           EJB 2.0 also introduces for the first time a portable query language, based on the abstract
           schema, not on the more complex database schema. This provides a database and vendor-
           independent way to find entity beans at run time, based on a wide variety of search criteria.

Question    Can you briefly describe about local interfaces? (EJB)
            EJB was originally designed around remote invocation using the Java Remote Method
           Invocation (RMI) mechanism, and later extended to support to standard CORBA transport for
           these calls using RMI/IIOP. This design allowed for maximum flexibility in developing
           applications without consideration for the deployment scenario, and was a strong feature in
           support of a goal of component reuse in J2EE.
           Many developers are using EJBs locally -- that is, some or all of their EJB calls are between
           beans in a single container.
           With this feedback in mind, the EJB 2.0 expert group has created a local interface mechanism.
           The local interface may be defined for a bean during development, to allow streamlined calls
           to the bean if a caller is in the same container. This does not involve the overhead involved
           with RMI like marshalling etc. This facility will thus improve the performance of applications in
           which co-location is planned.
           Local interfaces also provide the foundation for container-managed relationships among entity
           beans with container-managed persistence.

            What are the special design care that must be taken when you work with local interfaces?
Question    (EJB)

            It is important to understand that the calling semantics of local interfaces are different from
           those of remote interfaces. For example, remote interfaces pass parameters using call-by-
           value semantics, while local interfaces use call-by-reference.
           This means that in order to use local interfaces safely, application developers need to carefully
Answer     consider potential deployment scenarios up front, then decide which interfaces can be local
           and which remote, and finally, develop the application code with these choices in mind.
           While EJB 2.0 local interfaces are extremely useful in some situations, the long-term costs of
           these choices, especially when changing requirements and component reuse are taken into
           account, need to be factored into the design decision.
                             Java Interview Questions                               30

Question    What happens if remove( ) is never invoked on a session bean? (EJB)
            In case of a stateless session bean it may not matter if we call or not as in both cases
           nothing is done. The number of beans in cache is managed by the container.
Answer     In case of stateful session bean, the bean may be kept in cache till either the session times
           out, in which case the bean is removed or when there is a requirement for memory in which
           case the data is cached and the bean is sent to free pool.

            What is the difference between creating a distributed application using RMI and using a EJB
           architecture? (EJB)
            It is possible to create the same application using RMI and EJB. But in case of EJB the
           container provides the requisite services to the component if we use the proper syntax. It thus
           helps in easier development and lesser error and use of proven code and methodology. But
           the investment on application server is mandatory in that case. But this investment is
           warranted because it results in less complex and maintainable code to the client, which is what
           the end client wants. Almost all the leading application servers provide load balancing and
           performance tuning techniques. In case of RMI we have to code the services and include in the
           program the way to invoke these services.

Question    Can the bean class implement the EJBObject class directly? If not why? (EJB)
            It is better not to do it will make the Bean class a remote object and its methods can be
Answer     accessed without the containers? security, and transaction implementations if our code by
           mistake passed it in one of its parameters. Its just a good design practice.

Question    What does isIdentical() method return in case of different type of beans? (EJB)
            Stateless ? true always
Answer     Stateful ? depends whether the references point to the same session object
           Entity ? Depends whether the primary key is the same and the home is same

Question     How should you type cast a remote object? Why? (EJB)
             A client program that is intended to be interoperable with all compliant EJB Container
           implementations must use the javax.rmi.PortableRemoteObject.narrow(...) method to perform
           type-narrowing of the client-side representations of the remote home and remote interfaces.
           Programs using the cast operator for narrowing the remote and remote home interfaces are
           likely to fail if the Container implementation uses RMI-IIOP as the underlying communication

Question     What should you do in a passivate method? (EJB)
             You try to make all nontransient variables, which are not one of the following to null. For the
           given list the container takes care of serializing and restoring the object when activated.
           Serializable objects, null, UserTransaction, SessionContext, JNDI contexts in the beans
           context, reference to other beans, references to connection pools.
           Things that must be handled explicitly are like a open database connection etc. These must be
           closed and set to null and retrieved back in the activate method.

Question     What is the relationship between local interfaces and container-managed relationships? (EJB)
             Entity beans that have container-managed relationships with other entity beans, must be
           accessed in the same local scope as those related beans, and therefore typically provide a
           local client view. In order to be the target of a container-managed relationship, an entity bean
           with container-managed persistence must provide a local interface.

Question    What does a remove method do for different cases of beans? (EJB)
            Stateless Session : Does not do anything to the bean as moving the bean from free pool to
           cache are managed by the container depending on load.
           Stateful Session: Removes the bean from the cache.
           Entity Bean: Deletes the bean (data) from persistent storage
                             Java Interview Questions                               31
Question     How does a container-managed relationship work?
             An entity bean accesses related entity beans by means of the accessor methods for its
           container-managed relationship fields, which are specified by the cmr-field elements of its
           abstract persistence schema defined in the deployment descriptor. Entity bean relationships
Answer     are defined in terms of the local interfaces of the related beans, and the view an entity bean
           presents to its related beans is defined by its local home and local interfaces. Thus, an entity
           bean can be the target of a relationship from another entity bean only if it has a local

            What is the new basic requirement for a CMP entity bean class in 2.0 from that of ejb 1.1?
Question    (EJB)

            It must be abstract class. The container extends it and implements methods which are
           required for managing the relationships

Question    What are the basic classes required in the client for invoking an EJB? (EJB)
            The home and the remote interfaces, the implementation of the Naming Context Factory, the
           stubs and skeletons.
           In some App servers the stubs and the skeletons can be dynamically downloaded from the

Question     What is the difference between Message Driven Beans and Stateless Session beans? (EJB)
             In several ways, the dynamic creation and allocation of message-driven bean instances
           mimics the behavior of stateless session EJB instances, which exist only for the duration of a
           particular method call. However, message-driven beans are different from stateless session
           EJBs (and other types of EJBs) in several significant ways:
           § Message-driven beans process multiple JMS messages asynchronously, rather than
           processing a serialized sequence of method calls.
Answer     § Message-driven beans have no home or remote interface, and therefore cannot be directly
           accessed by internal or external clients. Clients interact with message-driven beans only
           indirectly, by sending a message to a JMS Queue or Topic.
           Note: Only the container directly interacts with a message-driven bean by creating bean
           instances and passing JMS messages to those instances as necessary.
           § The Container maintains the entire lifecycle of a message-driven bean; instances cannot be
           created or removed as a result of client requests or other API calls.

Question    What is the need for Clustering?
            To scale the application so that it
Answer     1. Is Highly Available
           2. Has High Throughput.

Question    What are the benefits of Clustering (Workload Management)? (EJB)
            They are
           1. It balances client processing requests, allowing incoming work requests to be distributed
           according to a configured Workload Management selection policy.
           2. It provides fail over capability by redirecting client requests to a running server when one or
           more servers are unavailable. This improves the availability of applications and administrative
           3. It enables systems to be scaled up to serve a higher client load than provided by the basic
           configuration. With server groups and clones additional instances of servers can easily be
           added to the configuration.
           4. It enables servers to be transparently maintained and upgraded while applications remain
           available for users.
           5. It centralizes administration of application servers and other objects.

Question    What are the types of Scaling?
            There are two types of scaling
Answer     1. Vertical Scaling
           2. Horizontal Scaling.
                             Java Interview Questions                               32

Question     What is Vertical Scaling? (EJB)
             When multiple server clones of an application server are defined on the same physical m/c, it
Answer     is called Vertical Scaling. The objective is to use the processing power of that m/c more

Question    What is Horizontal Scaling? (EJB)
            When Clones of an application server are defined on multiple physical m/c, it is called
           Horizontal Scaling. The objective is to use more than one less powerful m/c more efficiently.

Question    What is a Server Group? (EJB)
            A server group is a template of an Application Server(and its contents) i.e, it is a logical
           representation of the application server. It has the same structure and attributes as the real
           Application Server, but it is not associated with any node, and does not correspond to any real
           server process running on any node.

Question    What is a Clone? (EJB)
            The copies of a server group are called Clones. But unlike a Server Group Clones are
           associated with a node and are real server process running in that node.

Question    What is Ripple Effect? (EJB)
            The process of propagating the changes in the properties of a server group during runtime to
           all the associated clones is called Ripple Effect.

Question    What level of Load Balancing is possible with EJBs? (EJB)
            The workload management service provides load balancing for the following types of
           enterprise beans
Answer     1. Homes of entity or session beans
           2. Instances of entity beans
           3. Instances of stateless session beans

Question    What is the basic requirement for in-memory replication in Weblogic?
            1. The data in session should consist only of Serialized objects.
           2. Only setAttribute function should be used to set objects in session

Question    How JDBC services can be used in clustered environment? (EJB)
            Identical DataSource has to be created in each clustered server instances and configure to
           use different connection pools.

Question    What are the services that should not be used in a Clustered Environment?
            Non-clustered services:
           1. File Services
Answer     2. Time services
           3. Weblogic events
           4. Weblogic Workspaces (In WebLogic 5.1)

Question    Mention some tools to cluster Web Servers?
            Web Servers can be clustered using
Answer     1. Edge Server.
           2. DNS

Question    What is in-memory replication? (EJB)
            The process by which the contents in the memory of one physical m/c are replicated in all
           the m/c in the cluster is called in-memory replication.

Question    Ho w can an EJB implement remote interface like normally we implement interfaces in java.
                             Java Interview Questions                               33

Answer      any body explain

Question    Transaction control in ejbs
Answer      .

Question     What cannot be or is recommended not to be done using EJB? (EJB)
             · Enterprise Bean must not use read/write static fields. Using read-only static fields is
           allowed. Therefore, it is recommended that all static fields in enterprise bean class be declared
           as final.
           · Enterprise Bean must not use thread synchronization primitives to synchronize execution of
           multiple instances.
           · Enterprise bean must not attempt to manage threads. Enterprise bean must not attempt to
           start, stop, suspend, or resume a thread; or to change a thread's priority or name. The
           enterprise bean must not attempt to manage thread groups.
           · Enterprise Bean must not use the AWT functionality to attempt to output information to a
           display, or to input information from a keyboard.
           · Enterprise bean must not use package to attempt to access files and directories in file
           · Enterprise bean must not attempt to directly read or write a file descriptor.
           · Enterprise bean must not attempt to listen on a socket, accept connections on a socket, or
           use a socket for multicast.
           · Enterprise bean must not attempt to set the socket factory used by ServerSocket, Socket, or
           the stream handler factory used by URL.
           · Enterprise bean must not attempt to query a class to obtain information about the declared
           members that are not otherwise accessible to the enterprise bean because of the security
           rules of the Java language.
           · The enterprise bean must not attempt to use the Reflection API to access information that
           the security rules of the Java programming language make unavailable.
           · Enterprise bean must not attempt to create a class loader; obtain the current class loader;
           set the context class loader; set security manager; create a new security manager; stop the
           JVM; or change the input, output, and error streams.
           · Enterprise bean must not attempt to gain access to packages and classes that the usual rules
           of the Java programming language make unavailable to the enterprise bean.
           · Enterprise bean must not attempt to define a class in a package.
           · Enterprise bean must not attempt to use the subclass and object substitution features of the
           Java Serialization Protocol.
           · Enterprise bean must not attempt to obtain the security policy information for a particular
           code source.
           · Enterprise bean must not attempt to access or modify the security configuration objects
           (Policy, Security, Provider, Signer, and Identity).
           · Enterprise bean must not attempt to pass this as an argument or method result. The
           enterprise bean must pass the result of SessionContext.getEJBObject() or EntityContext.
           getEJBObject() instead.
           Enterprise bean must not attempt to load a native library.

Question    What is t3 protocol in weblogic? (EJB)
            T3 is a physical layer (layer 1) protocol, and definitely you don't expect any browser to
Answer     recognize it (e.g. TCP is a layer 4 protocol, and HTTP is running on top of TCP port 80). It's
           main use is for EJB's communicating.

Question    Describe the visitor design pattern (General)
            Represents an operation to be performed on the elements of an object structure. Visitor lets
           you define a new operation without changing the classes of the elements on which it operates.
Answer     The root of a class hierarchy defines an abstract method to accept a visitor. Subclasses
           implement this method with visitor.visit(this). The Visitor interface has visit methods for all
           subclasses of the baseclass in the hierarchy.
                             Java Interview Questions                                 34
Question    What is a design pattern?
            A design pattern systematically names, motivates, and explains a general design that
           addresses a recurring design problem.

Question    Describe the visitor design pattern (General)
            Represents an operation to be performed on the elements of an object structure. Visitor lets
           you define a new operation without changing the classes of the elements on which it operates.
Answer     The root of a class hierarchy defines an abstract method to accept a visitor. Subclasses
           implement this method with visitor.visit(this). The Visitor interface has visit methods for all
           subclasses of the baseclass in the hierarchy.

Question    what is the difference between weblogic 5,weblogic 6, and weblogic 7
Answer      what is the difference between weblogic 5,weblogic 6, and weblogic 7

Question    What is the Collections API? (JavaUtil)
            The Collections API is a set of classes and interfaces that support operations on collections of

Question    What is the List interface? (JavaUtil)
Answer      The List interface provides support for ordered collections of objects.

Question    What is the Vector class? (JavaUtil)
Answer      The Vector class provides the capability to implement a growable array of objects

Question    What is an Iterator interface? (JavaUtil)
Answer      The Iterator interface is used to step through the elements of a Collection

Question    Which java.util classes and interfaces support event handling? (JavaUtil)
Answer      The EventObject class and the EventListener interface support event processing

Question    What is the GregorianCalendar class? (JavaUtil)
Answer      The GregorianCalendar provides support for traditional Western calendars

Question    What is the Locale class? (JavaUtil)
            The Locale class is used to tailor program output to the conventions of a particular
           geographic, political, or cultural region

Question    What is the SimpleTimeZone class? (JavaUtil)
Answer      The SimpleTimeZone class provides support for a Gregorian calendar

Question    What is the Map interface? (JavaUtil)
            The Map interface replaces the JDK 1.1 Dictionary class and is used associate keys with

Question    What is the highest-level event class of the event-delegation model? (JavaUtil)
            The java.util.EventObject class is the highest-level class in the event-delegation class

Question    What is the Collection interface? (JavaUtil)
            The Collection interface provides support for the implementation of a mathematical bag - an
           unordered collection of objects that may contain duplicates

Question    What is the Set interface? (JavaUtil)
            The Set interface provides methods for accessing the elements of a finite mathematical set.
           Sets do not allow duplicate elements
                             Java Interview Questions                               35

Question    What is the purpose of the enableEvents() method? (JavaUtil)
            The enableEvents() method is used to enable an event for a particular object. Normally, an
           event is enabled when a listener is added to an object for a particular event. The
           enableEvents() method is used by objects that handle events by overriding their event-
           dispatch methods.

Question    What is the ResourceBundle class? (JavaUtil)
            The ResourceBundle class is used to store locale-specific resources that can be loaded by a
           program to tailor the program's appearance to the particular locale in which it is being run.

Question    What is the default size of the vector? (JavaUtil)
            Vector v = new Vector();
Answer     Constructs an empty vector so that its internal data array has size 10 and its standard
           capacity increment is zero.

Question     How does an included file in jsp look in the compiled .java file? (JavaUtil)
             If you are using jsp action include as <jsp:include page="a.jsp"/> then you see the following
           line of code in the service method of the .java file (servlet)
           org.apache.jasper.runtime.JspRuntimeLibrary.include(request, response, "a.jsp", out, false);

           If you are using directive include as *lt;%@ include file="a.jsp" %> then you see the following
           line of code within the static block as
           static {
           _jspx_dependants = new java.util.Vector(1);

Question    What is the difference between ArrayList and Vector? (JavaUtil)
            a.Internally, both the ArrayList and Vector hold onto their contents using an Array. A Vector
           defaults to doubling the size of its array, while the ArrayList increases its array size by 50
           b.ArrayList doesn't have a constructor for specifying the incremental capacity, where as Vector
           has a constructor to specify the initial capacity and incremental capacity.
           c.Vector is synchronized where as ArrayList is not synchronized

Question     What is the difference between Iterator and Enumeration? (JavaUtil)
             Iterator takes the place of Enumeration in the Java collections framework. Iterators differ
           from enumerations in two ways: Iterators allow the caller to remove elements from the
           underlying collection during the iteration with well-defined semantics. Method names have
           been improved.

Question    What is the query used to display all tables names in SQL Server (Query analyzer)?
Answer      select * from information_schema.tables

Question    How many types of JDBC Drivers are present and what are they?
            There are 4 types of JDBC Drivers
           Type 1: JDBC-ODBC Bridge Driver
Answer     Type 2: Native API Partly Java Driver
           Type 3: Network protocol Driver
           Type 4: JDBC Net pure Java Driver

Question    What is the fastest type of JDBC driver? (JDBC)
            JDBC driver performance will depend on a number of issues:
           (a) the quality of the driver code,
           (b) the size of the driver code,
           (c) the database server and its load,
                             Java Interview Questions                             36
           (d) network topology,
           (e) the number of times your request is translated to a different API.
           In general, all things being equal, you can assume that the more your request and response
           change hands, the slower it will be. This means that Type 1 and Type 3 drivers will be slower
           than Type 2 drivers (the database calls are make at least three translations versus two), and
           Type 4 drivers are the fastest (only one translation).

Question    What Class.forName will do while loading drivers? (JDBC)
            It is used to create an instance of a driver and register it with the DriverManager. When you
           have loaded a driver, it is available for making a connection with a DBMS.

Question     How to Retrieve Warnings? (JDBC)
             SQLWarning objects are a subclass of SQLException that deal with database access warnings.
           Warnings do not stop the execution of an application, as exceptions do; they simply alert the
           user that something did not happen as planned. A warning can be reported on a Connection
           object, a Statement object (including PreparedStatement and CallableStatement objects), or a
           ResultSet object. Each of these classes has a getWarnings method, which you must invoke in
           order to see the first warning reported on the calling object
           SQLWarning warning = stmt.getWarnings();
Answer     if (warning != null) {
           while (warning != null) {
           System.out.println("Message: " + warning.getMessage());
           System.out.println("SQLState: " + warning.getSQLState());
           System.out.print("Vendor error code: ");
           warning = warning.getNextWarning();

Question    what are stored procedures? How is it useful? (JDBC)
            A stored procedure is a set of statements/commands which reside in the database. The
           stored procedure is precompiled and saves the database the effort of parsing and compiling sql
           statements everytime a query is run. Each Database has it's own stored procedure language,
           usually a variant of C with a SQL preproceesor. Newer versions of db's support writing stored
           procs in Java and Perl too.
Answer     Before the advent of 3-tier/n-tier architecture it was pretty common for stored procs to
           implement the business logic( A lot of systems still do it). The biggest advantage is of course
           speed. Also certain kind of data manipulations are not achieved in SQL. Stored procs provide a
           mechanism to do these manipulations. Stored procs are also useful when you want to do
           Batch updates/exports/houseKeeping kind of stuff on the db. The overhead of a JDBC
           Connection may be significant in these cases.

Question    How to call a Stored Procedure from JDBC? (JDBC)
            The first step is to create a CallableStatement object. As with Statement an and
           PreparedStatement objects, this is done with an open Connection object. A CallableStatement
           object contains a call to a stored procedure.
           CallableStatement cs = con.prepareCall("{call SHOW_SUPPLIERS}");
           ResultSet rs = cs.executeQuery();

Question    Is the JDBC-ODBC Bridge multi-threaded? (JDBC)
            No. The JDBC-ODBC Bridge does not support concurrent access from different threads. The
           JDBC-ODBC Bridge uses synchronized methods to serialize all of the calls that it makes to
           ODBC. Multi-threaded Java programs may use the Bridge, but they won't get the advantages
           of multi-threading.

Question    Does the JDBC-ODBC Bridge support multiple concurrent open statements per connection?
                             Java Interview Questions                              37

            No. You can open only one Statement object per connection when you are using the JDBC-
           ODBC Bridge.

Question    What is cold backup, hot backup, warm backup recovery? (JDBC)
            a. cold backup - All these files must be backed up at the same time, before the databaseis
           b. hot backup - official name is 'online backup' ? is a backup taken of each tablespace while
           the database is running and is being accessed by the users.

Question    When we will Denormalize data? (JDBC)
            Data denormalization is reverse procedure, carried out purely for reasons of improving
           performance.It maybe efficient for a high-throughput system to replicate data for certain data.

Question    What is the advantage of using PreparedStatement? (JDBC)
            If we are using PreparedStatement the execution time will be less.The PreparedStatement
           object contains not just an SQL statement,but the SQL statement that has been
           precompiled.This means that when the PreparedStatement is executed,the RDBMS can just
           run the PreparedStatement's Sql statement without having to compile it first.

Question     What is a "dirty read"? (JDBC)
             Quite often in database processing, we come across the situation wherein one transaction
           can change a value, and a second transaction can read this value before the original change
           has been committed or rolled back. This is known as a dirty read scenario because there is
           always the possibility that the first transaction may rollback the change, resulting in the
           second transaction having read an invalid value. While you can easily command a database to
           disallow dirty reads, this usually degrades the performance of your application due to the
           increased locking overhead. Disallowing dirty reads also leads to decreased system

Question     What is Metadata and why should I use it? (JDBC)
             Metadata ('data about data') is information about one of two things: Database information
           (java.sql.DatabaseMetaData), or Information about a specific ResultSet
           Use DatabaseMetaData to find information about your database, such as its capabilities and
           structure. Use ResultSetMetaData to find information about the results of an SQL query, such
           as size and types of columns

Question     Different types of Transaction Isolation Levels? (JDBC)
             The isolation level describes the degree to which the data being updated is visible to other
           transactions. This is important when two transactions are trying to read the same row of a
           table. Imagine two transactions A & B.
           Three types of inconsistencies can occur:
           · Dirty-read: A has changed a row, but has not committed the changes. B reads the
           uncommitted data but his view of the data may be wrong if A rolls back his changes and
           updates his own changes to the database.
           · Non-repeatable read: B performs a read, but A modifies or deletes that data later. If B reads
           the same row again, he will get different data.
Answer     · Phantoms: A does a query on a set of rows to perform an operation. B modifies the table
           such that a query of A would have given a different result. The table may be inconsistent.
                             Java Interview Questions                             38

Question    What is 2 phase commit? (JDBC)
            A 2-phase commit is an algorithm used to ensure the integrity of a committing transactionIn
           Phase 1, the transaction coordinator contacts potential participants in the transaction. The
           participants all agree to make the results of the transaction permanent but do not do so
           immediately. The participants log information to disk to ensure they can complete Phase 2. If
           all the participants agree to commit, the coordinator logs that agreement and the outcome is
           decided. The recording of this agreement in the log ends Phase
           In Phase 2, the coordinator informs each participant of the decision, and they permanently
           update their resources.

Question     How do you handle your own transaction ? (JDBC)
             Connection Object has a method called setAutocommit ( Boolean istrue)
Answer     - Default is true
           Set the Parameter to false , and begin your transaction

Question     What is the normal procedure followed by a java client to access the db.? (JDBC)
             The database connection is created in 3 steps:
           1.Find a proper database URL (see FAQ on JDBC URL)
           2.Load the database driver
           3.Ask the Java DriverManager class to open a connection to your database
           In java code, the steps are realized in code as follows:
Answer     1.Create a properly formatted JDBR URL for your database. (See FAQ on JDBC URL for more
           information). A JDBC URL has the form
           jdbc:someSubProtocol://myDatabaseServer/theDatabaseName 2.
           3 . Connection conn = DriverManager.getConnection("a.JDBC.URL",

Question     What is a data source? (JDBC)
             A DataSource class brings another level of abstraction than directly using a connection
Answer     object. Data source can be referenced by JNDI. Data Source may point to RDBMS, file System
           , any DBMS etc.

Question    What are collection pools? What are the advantages? (JDBC)
            A connection pool is a cache of database connections that is maintained in memory, so that
           the connections may be reused

Question    How do you get Column names only for a table (SQL Server)? Write the Query. (JDBC)
            select name from syscolumns where id=(select id from sysobjects where name='user_hdr')
           order by colid --user_hdr is the table name

Question     What is the difference between cached rowset, jdbrowset and webrowset? (JDBC)
             A CachedRowSet is a disconnected, serializable, scrollable container for tabular data. A
           primary purpose of the CachedRowSet class is to provide a representation of a JDBC ResultSet
           that can be passed between different components of a distributed application. For example, a
           CachedResultSet can be used to send the result of a query executed by an Enterprise
           JavaBeans component running in a server environment over a network to a client running in a
           web browser. A second use for CachedRowSets is to provide scrolling and updating for
           ResultSets that don't provide these capabilities themselves. A CachedRowSet can be used to
Answer     augment the capabilities of a JDBC driver that doesn't have full support for scrolling and
           updating. Finally, a CachedRowSet can be used to provide Java applications with access to
           tabular data in an environment such as a thin client or PDA, where it would be inappropriate to
           use a JDBC driver due to resource limitations or security considerations. The CachedRowSet
           class provides a means to "get rows in" and "get changed rows out" without the need to
           implement the full JDBC API.
           A JdbcRowSet is a connected rowset that wraps a ResultSet object. The main use of
           JdbcRowSet is to wrap a ResultSet and make it appear as a JavaBeans(tm) component.
                             Java Interview Questions                              39
           The WebRowSet class extends CachedRowSet with the ability to write out the state of the the
           RowSet as an XML document. The format of the XML document is described by the DTD

Question    How do I view the complete prepared statement after setting values to it?
Answer      Please answer the Q

Question    Is thin driver provided by Oracle a type 4 driver?
Answer      YES

Question    How may messaging models do JMS provide for and what are they? (JMS)
Answer      JMS provide for two messaging models, publish-and-subscribe and point-to-point queuing

Question    What is messaging? (JMS)
            Messaging is a mechanism by which data can be passed from one application to another

Question     What is point-to-point messaging? (JMS)
             With point-to-point message passing the sending application/client establishes a named
           message queue in the JMS broker/server and sends messages to this queue. The receiving
           client registers with the broker to receive messages posted to this queue. There is a one-to-
           one relationship between the sending and receiving clients.

          Can two different JMS services talk to each other? For instance, if A and B are two different
Question JMS providers, can Provider A send messages directly to Provider B? If not, then can a
         subscriber to Provider A act as a publisher to Provider B? (JMS)
          The answers are no to the first question and yes to the second. The JMS specification does
         not require that one JMS provider be able to send messages directly to another provider.
         However, the specification does require that a JMS client must be able to accept a message
         created by a different JMS provider, so a message received by a subscriber to Provider A can
         then be published to Provider B. One caveat is that the publisher to Provider B is not required
         to handle a JMSReplyTo header that refers to a destination that is specific to Provider A.

            What is the advantage of persistent message delivery compared to nonpersistent delivery?
Question    (JMS)

             If the JMS server experiences a failure, for example, a power outage, any message that it is
           holding in primary storage potentially could be lost. With persistent storage, the JMS server
           logs every message to secondary storage. (The logging occurs on the front end, that is, as
           part of handling the send operation from the message producing client.) The logged message
           is removed from secondary storage only after it has been successfully delivered to all
           consuming clients

Question     How is a java object message delivered to a non-java Client? (JMS)
             It is according to the specification that the message sent should be received in the same
           format. A non-java client cannot receive a message in the form of java object. The provider in
           between handles the conversion of the data type and the message is transferred to the other

Question     What is MDB and What is the special feature of that? (JMS)
             MDB is Message driven bean, which very much resembles the Stateless session bean. The
Answer     incoming and out going messages can be handled by the Message driven bean. The ability to
           communicate asynchronously is the special feature about the Message driven bean.

Question    Give an example of using the publish/subscribe model. (JMS)
            JMS can be used to broadcast shutdown messages to clients connected to the Weblogic
Answer     server on a module wise basis. If an application has six modules, each module behaves like a
           subscriber to a named topic on the server.
                             Java Interview Questions                              40

Question    What is the difference between the Mailing and Messaging? (JMS)
            Java Mailing is the set of APIs that primarily concerns with the sending of Mail messages
           through the standard mail protocols. Messaging is the way of communicating to the remote
           machines using Message Oriented Middlewares. Message Oriented Middlewares do not use
           mailing internally for communication. They create their own channels for communication.

Question    What are the types of messaging? (JMS)
            There are two kinds of Messaging.
           Synchronous Messaging:
           Synchronous messaging involves a client that waits for the server to respond to a message.
           Asynchronous Messaging:
           Asynchronous messaging involves a client that does not wait for a message from the server.
           An event is used to trigger a message from a server.

Question     What is publish/subscribe messaging? (JMS)
             With publish/subscribe message passing the sending application/client establishes a named
           topic in the JMS broker/server and publishes messages to this queue. The receiving clients
Answer     register (specifically, subscribe) via the broker to messages by topic; every subscriber to a
           topic receives each message published to that topic. There is a one-to-many relationship
           between the publishing client and the subscribing clients.

            Why doesn't the JMS API provide end-to-end synchronous message delivery and notification
           of delivery? (JMS)
            Some messaging systems provide synchronous delivery to destinations as a mechanism for
           implementing reliable applications. Some systems provide clients with various forms of
           delivery notification so that the clients can detect dropped or ignored messages. This is not the
           model defined by the JMS API.
           JMS API messaging provides guaranteed delivery via the once-and-only-once delivery
           semantics of PERSISTENT messages. In addition, message consumers can insure reliable
           processing of messages by using either CLIENT_ACKNOWLEDGE mode or transacted sessions.
           This achieves reliable delivery with minimum synchronization and is the enterprise messaging
           model most vendors and developers prefer.
           The JMS API does not define a schema of systems messages (such as delivery notifications). If
           an application requires acknowledgment of message receipt, it can define an application-level
           acknowledgment message.

Question     What are the core JMS-related objects required for each JMS-enabled application? (JMS)
             Each JMS-enabled client must establish the following:
           · A connection object provided by the JMS server (the message broker)
           · Within a connection, one or more sessions, which provide a context for message sending and
           · Within a session, either a queue or topic object representing the destination (the message
           staging area) within the message broker
           · Within a session, the appropriate sender or publisher or receiver or subscriber object
           (depending on whether the client is a message producer or consumer and uses a point-to-
           point or publish/subscribe strategy, respectively)
           Within a session, a message object (to send or to receive)

Question    What are the various message types supported by JMS? (JMS)
            Stream Messages ? Group of Java Primitives
           Map Messages ? Name Value Pairs.
           Name being a string
           Value being a java primitive
           Text Messages ? String messages (since being widely used a separate messaging Type has
           been supported)
           Object Messages ? Group of serialize able java object
           Bytes Message ? Stream of uninterrupted bytes
                              Java Interview Questions                                 41

Question     What is the Role of the JMS Provider? (JMS)
             The JMS provider handles security of the messages, data conversion and the client
Answer     triggering. The JMS provider specifies the level of encryption and the security level of the
           message, the best data type for the non-JMS client.

Question    How does a typical client perform the communication? (JMS)
            1. Use JNDI to locate administrative objects.
           1a. Locate a single ConnectionFactory object.
           1b. Locate one or more Destination objects.
           2. Use the ConnectionFactory to create a JMS Connection.
           3. Use the Connection to create one or more Session(s).
           4. Use a Session and the Destinations to create the MessageProducers and MessageConsumers
           5. Perform your communication.

Question    Give an example of using the point-to-point model. (JMS)
            The point-to-point model is used when the information is specific to a single client. For
Answer     example, a client can send a message for a print out, and the server can send information
           back to this client after completion of the print job.

Question     How does the Application server handle the JMS Connection? (JMS)
             - App server creates the server session and stores them in a pool
           - Connection consumer uses the server session to put messages in the session of the JMS.
           - Server session is the one that spawns the JMS session.
           - Applications written by Application programmers creates the message listener

Question    What protocols does JNDI provide an interface to? (JNDI)
            JNDI itself is independent of any specific directory access protocol. Individual service
           providers determine the protocols to support. JNDI supports popular protocols, such as LDAP
           (Light weight Directory Access Protocol) , NDS(Netscape Directory Service), DNS(Domain
           Naming Service), and NIS(Network Information Service), supplied by different vendors.

Question    What is Context and InitialContext? (JNDI)
            A context represents a set of bindings within a naming service. A Context object provides the
           methods for binding names to objects and unbinding names from objects, for renaming
           objects, and for listing the bindings. JNDI performs all naming operations relative to a context.
           The JNDI specification defines an InitialContext class. This class is instantiated with properties
           that define the type of naming service in use (such as provider URL, security, ID and password
           to use when connecting).

Question     What's the difference between JNDI lookup(), list(), listBindings(), and search()? (JNDI)
             lookup() attempts to find the specified object in the given context. I.e., it looks for a single,
           specific object and either finds it in the current context or it fails.
           list() attempts to return an enumeration of all of the NameClassPair's of all of the objects in
           the current context. I.e., it's a listing of all of the objects in the current context but only
           returns the object's name and the name of the class to which the object belongs.
           listBindings() attempts to return an enumeration of the Binding's of all of the objects in the
           current context. I.e., it's a listing of all of the objects in the current context with the object's
           name, its class name, and a reference to the object itself.
           search() attempts to return an enumeration of all of the objects matching a given set of
           search criteria. It can search across multiple contexts (or not). It can return whatever
           attributes of the objects that you desire. Etc. It's by far the most complex and powerful of
           these options but is also the most expensive.

Question    Components of JNDI (JNDI)
Answer      Naming Interface- The naming interface organizes information hierarchically and maps
                              Java Interview Questions                            42
           human-friendly names to addresses or objects that are machine-friendly. It allows access to
           named objects through multiple namespaces.
           Directory Interface- JNDI includes a directory service interface that provides access to
           directory objects, which can contain attributes, thereby providing attribute-based searching
           and schema support
           Service Provider Interface- JNDI comes with the SPI, which supports the protocols provided by
           third parties.

Question    How to use jndi

Question    Is JNDI a protocol? Where is it used? (JNDI)
            Yes.HotJava Views 1.1 is using JNDI to access LDAP. Enterprise APIs such as Enterprise
           JavaBeans, Java Message Service, JDBC 2.0 make use of JNDI to for their naming and
           directory needs. RMI over IIOP applications can use JNDI to access the CORBA (COS) naming

Question    JNDI

Question    Can we implement an interface in a JSP?
Answer      No

Question    What is the difference between ServletContext and PageContext?
            ServletContext: Gives the information about the container
           PageContext: Gives the information about the Request

            What is the difference in using request.getRequestDispatcher() and
           context.getRequestDispatcher()? (JSP)
            request.getRequestDispatcher(path): In order to create it we need to give the relative path
           of the resource
           context.getRequestDispatcher(path): In order to create it we need to give the absolute path of
           the resource.

Question    How to pass information from JSP to included JSP?
Answer      Using <%jsp:param> tag.

Question     What is the difference between directive include and jsp include? (JSP)
             <%@ include> : Used to include static resources during translation time.
           : Used to include dynamic content or static content during runtime.

Question    What is the difference between RequestDispatcher and sendRedirect? (JSP)
            RequestDispatcher: server-side redirect with request and response objects.
           sendRedirect : Client-side redirect with new request and response objects.

Question    How does JSP handle runtime exceptions? (JSP)
            Using errorPage attribute of page directive and also we need to specify isErrorPage=true if
           the current page is intended to URL redirecting of a JSP.

Question    How do you delete a Cookie within a JSP?

            Cookie mycook = new Cookie("name","value"); response.addCookie(mycook); Cookie
Answer     killmycook = new Cookie("mycook","value"); killmycook.setMaxAge(0);
           killmycook.setPath("/"); killmycook.addCookie(killmycook);

Question    How do I mix JSP and SSI #include?
                             Java Interview Questions                                   43
             If you're just including raw HTML, use the #include directive as usual inside your .jsp file.
           <!--#include file=""-->
           But it's a little trickier if you want the server to evaluate any JSP code that's inside the
Answer     included file. Ronel Sumibcay (ronel@LIVESOFTWARE.COM) says: If your file contains
           jsp code you will have to use
           <%@ vinclude="" %>
           The <!--#include file=""--> is used for including non-JSP files.

Question    How can my application get to know when a HttpSession is removed? (JSP)
            Define a Class HttpSessionNotifier which implements HttpSessionBindingListener and
Answer     implement the functionality what you need in valueUnbound() method.
           Create an instance of that class and put that instance in HttpSession.

Question     How can I implement a thread-safe JSP page? (JSP)
             You can make your JSPs thread-safe by having them implement the SingleThreadModel
Answer     interface. This is done by adding the directive <%@ page isThreadSafe="false" % > within
           your JSP page.

Question    How many JSP scripting elements are there and what are they?
            There are three scripting language elements:

Question    In the Servlet 2.4 specification SingleThreadModel has been deprecates, why? (JSP)
            Because it is not practical to have such model. Whether you set isThreadSafe to true or false,
Answer     you should take care of concurrent client requests to the JSP page by synchronizing access to
           any shared objects defined at the page level.

Question     How do I include static files within a JSP page? (JSP)
             Static resources should always be included using the JSP include directive. This way, the
           inclusion is performed just once during the translation phase. The following example shows the
Answer     syntax: Do note that you should always supply a relative URL for the file attribute. Although
           you can also include static resources using the action, this is not advisable as the inclusion is
           then performed for each and every request.

Question     How do I mix JSP and SSI #include? (JSP)
             If you're just including raw HTML, use the #include directive as usual inside your .jsp file.
           <!--#include file=""-->
           But it's a little trickier if you want the server to evaluate any JSP code that's inside the
           included file. If your file contains jsp code you will have to use
           <%@ vinclude="" %>
           The <!--#include file=""--> is used for including non-JSP files.

Question     Can a JSP page process HTML FORM data? (JSP)
             Yes. However, unlike servlets, you are not required to implement HTTP-protocol specific
           methods like doGet() or doPost() within your JSP page. You can obtain the data for the FORM
           input elements via the request implicit object within a scriptlet or expression as:
Answer     String item = request.getParameter("item");
           int howMany = new Integer(request.getParameter("units")).intValue();
           <%= request.getParameter("item") %>

Question    What JSP lifecycle methods can I override? (JSP)
Answer      You cannot override the _jspService() method within a JSP page. You can however, override
                             Java Interview Questions                              44
           the jspInit() and jspDestroy() methods within a JSP page. jspInit() can be useful for allocating
           resources like database connections, network connections, and so forth for the JSP page. It is
           good programming practice to free any allocated resources within jspDestroy().
           The jspInit() and jspDestroy() methods are each executed just once during the lifecycle of a
           JSP page and are typically declared as JSP declarations:
           public void jspInit() {

           public void jspDestroy() {

Question     How do I include static files within a JSP page? (JSP)
             Static resources should always be included using the JSP include directive. This way, the
           inclusion is performed just once during the translation phase. The following example shows the
Answer     <%@ include file="copyright.html" %>
           Do note that you should always supply a relative URL for the file attribute. Although you can
           also include static resources using the action, this is not advisable as the inclusion is then
           performed for each and every request.

Question     How do I perform browser redirection from a JSP page? (JSP)
             You can use the response implicit object to redirect the browser to a different resource, as:
           You can also physically alter the Location HTTP header attribute, as shown below:
           String newLocn = "/newpath/index.html";
Answer     response.setHeader("Location",newLocn);
           You can also use the: <jsp:forward page="/newpage.jsp" /> Also note that you can only use
           this before any output has been sent to the client. I beleve this is the case with the
           response.sendRedirect() method as well.
           If you want to pass any paramateres then you can pass using <jsp:forward
           page="/servlet/login"> <jsp:param name="username" value="jsmith" /> </jsp:forward>>

Question     Can a JSP page instantiate a serialized bean? (JSP)
             No problem! The useBean action specifies the beanName attribute, which can be used for
           indicating a serialized bean. For example:
           <jsp:useBean id="shop" type="shopping.CD" beanName="CD" /> <jsp:getProperty
           name="shop" property="album" />
           A couple of important points to note. Although you would have to name your serialized file
           "filename.ser", you only indicate "filename" as the value for the beanName attribute. Also, you
           will have to place your serialized file within the WEB-INF\jsp\beans directory for it to be
           located by the JSP engine.

Question    Can you make use of a ServletOutputStream object from within a JSP page? (JSP)
            No. You are supposed to make use of only a JSPWriter object (given to you in the form of the
           implicit object out) for replying to clients. A JSPWriter can be viewed as a buffered version of
           the stream object returned by response.getWriter(), although from an implementational
           perspective, it is not. A page author can always disable the default buffering for any page
           using a page directive as:
           <%@ page buffer="none" %>
                             Java Interview Questions                               45

             What's a better approach for enabling thread-safe servlets and JSPs? SingleThreadModel
           Interface or Synchronization? (JSP)
             Although the SingleThreadModel technique is easy to use, and works well for low volume
           sites, it does not scale well. If you anticipate your users to increase in the future, you may be
           better off implementing explicit synchronization for your shared data. The key however, is to
           effectively minimize the amount of code that is synchronzied so that you take maximum
           advantage of multithreading.
           Also, note that SingleThreadModel is pretty resource intensive from the server's perspective.
           The most serious issue however is when the number of concurrent requests exhaust the
           servlet instance pool. In that case, all the unserviced requests are queued until something
           becomes free - which results in poor performance. Since the usage is non-deterministic, it may
           not help much even if you did add more memory and increased the size of the instance pool.

Question     Can I stop JSP execution while in the midst of processing a request? (JSP)
             Yes. Preemptive termination of request processing on an error condition is a good way to
           maximize the throughput of a high-volume JSP engine. The trick (asuming Java is your
           scripting language) is to use the return statement when you want to terminate further
           processing. For example, consider:
           <% if (request.getParameter("foo") != null) {
           // generate some html or update bean property
           } else {
           /* output some error message or provide redirection back to the input form after creating a
           memento bean updated with the 'valid' form elements that were input. this bean can now be
           used by the previous form to initialize the input elements that were valid then, return from the
           body of the _jspService() method to terminate further processing */

             How can I get to view any compilation/parsing errors at the client while developing JSP
           pages? (JSP)
             With JSWDK 1.0, set the following servlet initialization property within the \WEB-
           INF\ file for your application:
           This will cause any compilation/parsing errors to be sent as part of the response to the client.

Question    Is there a way to reference the "this" variable within a JSP page? (JSP)
            Yes, there is. Under JSP 1.0, the page implicit object is equivalent to "this", and returns a
           reference to the servlet generated by the JSP page.

            How do I instantiate a bean whose constructor accepts parameters using the useBean tag?
Question    (JSP)

             Consider the following bean: package bar;
           public class FooBean {
           public FooBean(SomeObj arg) {
           //getters and setters here
Answer     The only way you can instantiate this bean within your JSP page is to use a scriptlet. For
           example, the following snippet creates the bean with session scope:
           &l;% SomeObj x = new SomeObj(...);
           bar.FooBean foobar = new FooBean(x);
           %> You can now access this bean within any other page that is part of the same session as:
           bar.FooBean foobar = session.getValue("foobar");
                             Java Interview Questions                             46
           To give the bean "application scope", you will have to place it within the ServletContext as:
           To give the bean "request scope", you will have to place it within the request object as:
           If you do not place the bean within the request, session or application scope, the bean can be
           accessed only within the current JSP page (page scope).
           Once the bean is instantiated, it can be accessed in the usual way:
           jsp:getProperty name="foobar" property="someProperty"/ jsp:setProperty name="foobar"
           property="someProperty" value="someValue"/

Question     Can I invoke a JSP error page from a servlet? (JSP)
             Yes, you can invoke the JSP error page and pass the exception object to it from within a
           servlet. The trick is to create a request dispatcher for the JSP error page, and pass the
           exception object as a javax.servlet.jsp.jspException request attribute. However, note that you
           can do this from only within controller servlets. If your servlet opens an OutputStream or
           PrintWriter, the JSP engine will throw the following translation error:
           java.lang.IllegalStateException: Cannot forward as OutputStream or Writer has already been
           The following code snippet demonstrates the invocation of a JSP error page from within a
           controller servlet:
           protected void sendErrorRedirect(HttpServletRequest request, HttpServletResponse response,
           String errorPageURL, Throwable e) throws ServletException, IOException {
           request.setAttribute ("javax.servlet.jsp.jspException", e);
           getServletConfig().getServletContext(). getRequestDispatcher(errorPageURL).forward(request,
           public void doPost(HttpServletRequest request, HttpServletResponse response) {
           try {
           // do something
           } catch (Exception ex) {
           try {
           } catch (Exception e) {

Question    How can you store international / Unicode characters into a cookie?
            One way is, before storing the cookie URLEncode it.
Answer     URLEnocder.encoder(str);
           And use URLDecoder.decode(str) when you get the stored cookie.

Question    What are the implicit objects? (JSP)
            Implicit objects are objects that are created by the web container and contain information
           related to a particular request, page, or application. They are:
                             Java Interview Questions                                47

Question    Is JSP technology extensible? (JSP)
            YES. JSP technology is extensible through the development of custom actions, or tags, which
           are encapsulated in tag libraries

             How can I implement a thread-safe JSP page? What are the advantages and Disadvantages
           of using it? (JSP)
             You can make your JSPs thread-safe by having them implement the SingleThreadModel
           interface. This is done by adding the directive
           <%@ page isThreadSafe="false" %> within your JSP page.
           With this, instead of a single instance of the servlet generated for your JSP page loaded in
           memory, you will have N instances of the servlet loaded and initialized, with the service
           method of each instance effectively synchronized. You can typically control the number of
           instances (N) that are instantiated for all servlets implementing SingleThreadModel through
           the admin screen for your JSP engine.
           More importantly, avoid using the tag for variables. If you do use this tag, then you should set
           isThreadSafe to true, as mentioned above. Otherwise, all requests to that page will access
           those variables, causing a nasty race condition.
           SingleThreadModel is not recommended for normal use. There are many pitfalls, including the
           example above of not being able to use . You should try really hard to make them thread-safe
           the old fashioned way: by making them thread-safe

Question    How does JSP handle run-time exceptions? (JSP)
            You can use the errorPage attribute of the page directive to have uncaught run-time
           exceptions automatically forwarded to an error processing page. For example:
           <%@ page errorPage="error.jsp" %>
           redirects the browser to the JSP page error.jsp if an uncaught exception is encountered during
           request processing. Within error.jsp, if you indicate that it is an error-processing page, via the
           <%@ page isErrorPage="true" %>
           the Throwable object describing the exception may be accessed within the error page via the
           exception implicit object.
           Note: You must always use a relative URL as the value for the errorPage attribute.

            How do I prevent the output of my JSP or Servlet pages from being cached by the browser?
Question    (JSP)

            You will need to set the appropriate HTTP header attributes to prevent the dynamic content
           output by the JSP page from being cached by the browser. Just execute the following scriptlet
           at the beginning of your JSP pages to prevent them from being cached at the browser. You
           need both the statements to take care of some of the older browser versions.
Answer     <%
           response.setHeader("Cache-Control","no-store"); //HTTP 1.1
           response.setHeader("Pragma","no-cache"); //HTTP 1.0
           response.setDateHeader ("Expires", 0); //prevents caching at the proxy server

Question     How do I use comments within a JSP page? (JSP)
             You can use "JSP-style" comments to selectively block out code while debugging or simply to
           comment your scriptlets. JSP comments are not visible at the client. For example:
           <%-- the scriptlet is now commented out
           out.println("Hello World");
           You can also use HTML-style comments anywhere within your JSP page. These comments are
           visible at the client. For example:
           <!-- (c) 2004 -->
                             Java Interview Questions                               48
           Of course, you can also use comments supported by your JSP scripting language within your
           scriptlets. For example, assuming Java is the scripting language, you can have:
           //some comment
           yet another comment

Question     Response has already been commited error. What does it mean? (JSP)
             This error show only when you try to redirect a page after you already have written
           something in your page. This happens because HTTP specification force the header to be set
           up before the lay out of the page can be shown (to make sure of how it should be
           displayed...content-type="text/html" or "text/xml" or "plain-text" or "image/jpg", etc...) When
           you try to send a redirect status (Number is line_status_402), your HTTP server cannot send it
           right now if it hasn't finished to set up the header. If not starter to set up the header, there
           are no problems, but if it 's already begin to set up the header, then your HTTP server expects
           these headers to be finished setting up and it cannot be the case if the stream of the page is
           not over... In this last case it's like you have a file started with <HTML Tag> <Some Headers>
           <Body> some output (like testing your variables...) ... and before you indicate that the file is
           over (and before the size of the page can be setted up in the header), you try to send a
           redirect status... It s simply impossible due to the specification of HTTP 1.0 and 1.1

Question     How do I use a scriptlet to initialize a newly instantiated bean? (JSP)
             A jsp:useBean action may optionally have a body. If the body is specified, its contents will be
           automatically invoked when the specified bean is instantiated. Typically, the body will contain
           scriptlets or jsp:setProperty tags to initialize the newly instantiated bean, although you are not
           restricted to using those alone.
           The following example shows the "today" property of the Foo bean initialized to the current
           date when it is instantiated. Note that here, we make use of a JSP expression within the
           jsp:setProperty action.
           <jsp:useBean id="foo" class="com.Bar.Foo" >
           <jsp:setProperty name="foo" property="today"
           value="<%=java.text.DateFormat.getDateInstance().format(new java.util.Date()) %>"/ >
           <%-- scriptlets calling bean setter methods go here --%>
           </jsp:useBean >

Question    How can I enable session tracking for JSP pages if the browser has disabled cookies? (JSP)
            We know that session tracking uses cookies by default to associate a session identifier with a
           unique user. If the browser does not support cookies, or if cookies are disabled, you can still
           enable session tracking using URL rewriting.
           URL rewriting essentially includes the session ID within the link itself as a name/value pair.
           However, for this to be effective, you need to append the session ID for each and every link
           that is part of your servlet response.
           Adding the session ID to a link is greatly simplified by means of of a couple of methods:
           response.encodeURL() associates a session ID with a given URL, and if you are using
           redirection, response.encodeRedirectURL() can be used by giving the redirected URL as input.
           Both encodeURL() and encodeRedirectedURL() first determine whether cookies are supported
Answer     by the browser; if so, the input URL is returned unchanged since the session ID will be
           persisted as a cookie.
           Consider the following example, in which two JSP files, say hello1.jsp and hello2.jsp, interact
           with each other. Basically, we create a new session within hello1.jsp and place an object within
           this session. The user can then traverse to hello2.jsp by clicking on the link present within the
           page.Within hello2.jsp, we simply extract the object that was earlier placed in the session and
           display its contents. Notice that we invoke the encodeURL() within hello1.jsp on the link used
           to invoke hello2.jsp; if cookies are disabled, the session ID is automatically appended to the
           URL, allowing hello2.jsp to still retrieve the session object.
           Try this example first with cookies enabled. Then disable cookie support, restart the brower,
           and try again. Each time you should see the maintenance of the session across pages.
                             Java Interview Questions                               49
           Do note that to get this example to work with cookies disabled at the browser, your JSP
           engine has to support URL rewriting.
           <%@ page session="true" %>
           Integer num = new Integer(100);
           String url =response.encodeURL("hello2.jsp");
           <a href='<%=url%>'>hello2.jsp</a>
           <%@ page session="true" %>
           Integer i= (Integer )session.getValue("num");
           out.println("Num value in session is "+i.intValue());

Question     How can I declare methods within my JSP page? (JSP)
             You can declare methods for use within your JSP page as declarations. The methods can then
           be invoked within any other methods you declare, or within JSP scriptlets and expressions.
           Do note that you do not have direct access to any of the JSP implicit objects like request,
           response, session and so forth from within JSP methods. However, you should be able to pass
           any of the implicit JSP variables as parameters to the methods you declare. For example:
           public String whereFrom(HttpServletRequest req) {
           HttpSession ses = req.getSession();
           return req.getRemoteHost();
           out.print("Hi there, I see that you are coming in from ");
           <%= whereFrom(request) %>
           Another Example
           <%@page contentType="text/html"%>
           public void test(JspWriter writer) throws IOException{

           <%@include file="file1.jsp"%>
           <%test(out);% >

Question    Is there a way I can set the inactivity lease period on a per-session basis? (JSP)
            Typically, a default inactivity lease period for all sessions is set within your JSP engine admin
           screen or associated properties file. However, if your JSP engine supports the Servlet 2.1 API,
           you can manage the inactivity lease period on a per-session basis. This is done by invoking the
           HttpSession.setMaxInactiveInterval() method, right after the session has been created. For
                              Java Interview Questions                                50
           would reset the inactivity period for this session to 5 minutes. The inactivity interval is set in

Question     How can I set a cookie and delete a cookie from within a JSP page?
             A cookie, mycookie, can be deleted using the following scriptlet:
           //creating a cookie
           Cookie mycookie = new Cookie("aName","aValue");
Answer     //delete a cookie
           Cookie killMyCookie = new Cookie("mycookie", null);

Question     How does a servlet communicate with a JSP page? (JSP)
             The following code snippet shows how a servlet instantiates a bean and initializes it with
           FORM data posted by a browser. The bean is then placed into the request, and the call is then
           forwarded to the JSP page, Bean1.jsp, by means of a request dispatcher for downstream
           public void doPost (HttpServletRequest request, HttpServletResponse response) {
           try {
           govi.FormBean f = new govi.FormBean();
           String id = request.getParameter("id");
           //use the id to compute
           //additional bean properties like info
           //maybe perform a db query, etc.
Answer     // . . .
           ("/jsp/Bean1.jsp").forward(request, response);
           } catch (Exception ex) {
           The JSP page Bean1.jsp can then process fBean, after first extracting it from the default
           request scope via the useBean action.
           jsp:useBean id="fBean" class="govi.FormBean" scope="request"/ jsp:getProperty
           name="fBean" property="name" / jsp:getProperty name="fBean" property="addr" /
           jsp:getProperty name="fBean" property="age" / jsp:getProperty name="fBean"
           property="personalizationInfo" /

            How do I have the JSP-generated servlet subclass my own custom servlet class, instead of
           the default? (JSP)
            One should be very careful when having JSP pages extend custom servlet classes as opposed
           to the default one generated by the JSP engine. In doing so, you may lose out on any
           advanced optimization that may be provided by the JSP engine. In any case, your new
           superclass has to fulfill the contract with the JSP engine by:
           Implementing the HttpJspPage interface, if the protocol used is HTTP, or implementing
           JspPage otherwise Ensuring that all the methods in the Servlet interface are declared final
           Additionally, your servlet superclass also needs to do the following:
           The service() method has to invoke the _jspService() method
           The init() method has to invoke the jspInit() method
           The destroy() method has to invoke jspDestroy()
                             Java Interview Questions                               51
           If any of the above conditions are not satisfied, the JSP engine may throw a translation error.
           Once the superclass has been developed, you can have your JSP extend it as follows:
           <%@ page extends="packageName.ServletName" %<

             How can I prevent the word "null" from appearing in my HTML input text fields when I
           populate them with a resultset that has null values? (JSP)
             You could make a simple wrapper function, like
           String blanknull(String s) {
           return (s == null) ? "" : s;
           then use it inside your JSP form, like
           <input type="text" name="shoesize" value="<%=blanknull(shoesize)% >" >

Question    How can I get to print the stacktrace for an exception occuring within my JSP page? (JSP)
            By printing out the exception's stack trace, you can usually diagonse a problem better when
           debugging JSP pages. By looking at a stack trace, a programmer should be able to discern
           which method threw the exception and which method called that method. However, you
           cannot print the stacktrace using the JSP out implicit variable, which is of type JspWriter. You
           will have to use a PrintWriter object instead. The following snippet demonstrates how you can
           print a stacktrace from within a JSP error page:
           <%@ page isErrorPage="true" %>
Answer     <%
            PrintWriter pw = response.getWriter();


Question     How do you pass an InitParameter to a JSP? (JSP)
             The JspPage interface defines the jspInit() and jspDestroy() method which the page writer
           can use in their pages and are invoked in much the same manner as the init() and destory()
           methods of a servlet. The example page below enumerates through all the parameters and
           prints them to the console.
           <%@ page import="java.util.*" %>
           ServletConfig cfg =null;
           public void jspInit(){
           ServletConfig cfg=getServletConfig();
           for (Enumeration e=cfg.getInitParameterNames(); e.hasMoreElements();) {
           String name=(String)e.nextElement();
           String value = cfg.getInitParameter(name);

Question     How can my JSP page communicate with an EJB Session Bean? (JSP)
             The following is a code snippet that demonstrates how a JSP page can interact with an EJB
           session bean:
           <%@ page import="javax.naming.*, javax.rmi.PortableRemoteObject, foo.AccountHome,
Answer     foo.Account" %>
           //declare a "global" reference to an instance of the home interface of the session bean
           AccountHome accHome=null;
                             Java Interview Questions                               52
           public void jspInit() {
           //obtain an instance of the home interface
           InitialContext cntxt = new InitialContext( );
           Object ref= cntxt.lookup("java:comp/env/ejb/AccountEJB");
           accHome = (AccountHome)PortableRemoteObject.narrow(ref,AccountHome.class);
           //instantiate the session bean
           Account acct = accHome.create();
           //invoke the remote methods
           // etc etc...

Question    How do you prevent the Creation of a Session in a JSP Page and why? (JSP)
            By default, a JSP page will automatically create a session for the request if one does not
           exist. However, sessions consume resources and if it is not necessary to maintain a session,
           one should not be created. For example, a marketing campaign may suggest the reader visit a
           web page for more information. If it is anticipated that a lot of traffic will hit that page, you
           may want to optimize the load on the machine by not creating useless sessions.

           The page directive is used to prevent a JSP page from automatically creating a session:
           <%@ page session="false">

Question     What is the difference between page and request scopes? (JSP)
             If you specify scope as Page then its life span is until page is displayed or control is
           forwarded to a new page. Accessibility is current page only.
Answer     But if u specify as Request, its life span is until the request has been completely processed and
           the response has been sent back to the user. Accessibility is current page and any included or
           for-warded pages

Question    How do you precompile a jsp page? (JSP)
            To precompile a JSP page, access the page with a query string of ?jsp_precompile
           the jsp page will not get executed by this action.

Question    How do you print the contents of a.jsp in b.jsp. How do you include the file?
           public int i=10;

Answer     <%@ include file="a.jsp" %>

           do not use as this will include the file at runtime where as the above includes (directive
           include) the file at compile time

Question    Why would a client application use JTA transactions? (JTA)
            One possible example would be a scenario in which a client needs to employ two (or more)
           session beans, where each session bean is deployed on a different EJB server and each bean
Answer     performs operations against external resources (for example, a database) and/or is managing
           one or more entity beans. In this scenario, the client's logic could required an all-or-nothing
           guarantee for the operations performed by the session beans; hence, the session bean usage
                             Java Interview Questions                              53
           could be bundled together with a JTA UserTransaction object.
           In the previous scenario, however, the client application developer should address the
           question of whether or not it would be better to encapsulate these operations in yet another
           session bean, and allow the session bean to handle the transactions via the EJB container. In
           general, lightweight clients are easier to maintain than heavyweight clients. Also, EJB
           environments are ideally suited for transaction management.
            ... Context c = new InitialContext(); UserTransaction ut = (UserTransaction)
           c.lookup("java:comp/UserTransaction"); ut.begin(); // perform multiple
           operations... ut.commit() ...

            How do I convert a numeric IP address like into a hostname like
Question    (Networking)

Answer      String hostname = InetAddress.getByName("").getHostName();

Question    What information is needed to create a TCP Socket? (Networking)
            The Local System?s IP Address and Port Number. And the Remote System's IPAddress and
           Port Number.

Question    What is the difference between URL instance and URLConnection instance? (Networking)
            A URL instance represents the location of a resource, and a URLConnection instance
           represents a link for accessing or communicating with the resource at the location.

Question    What are the two important TCP Socket classes? (Networking)
            Socket and ServerSocket. ServerSocket is used for normal two-way socket communication.
Answer     Socket class allows us to read and write through the sockets. getInputStream() and
           getOutputStream() are the two methods available in Socket class.

Question    how to make default checkbox selecte(in struts)
Answer      useing the set and get methods it can be done.

Question    How many types of protocol implementations does RMI have? (RMI)
            RMI has at least three protocol implementations: Java Remote Method Protocol(JRMP),
           Internet Inter ORB Protocol(IIOP), and Jini Extensible Remote Invocation(JERI). These are
           alternatives, not part of the same thing, All three are indeed layer 6 protocols for those who
           are still speaking OSI reference model.

Question     Does RMI-IIOP support dynamic downloading of classes? (RMI)
             No, RMI-IIOP doesn't support dynamic downloading of the classes as it is done with CORBA
           in DII (Dynamic Interface Invocation).Actually RMI-IIOP combines the usability of Java
           Remote Method Invocation (RMI) with the interoperability of the Internet Inter-ORB Protocol
           (IIOP).So in order to attain this interoperability between RMI and CORBA,some of the features
           that are supported by RMI but not CORBA and vice versa are eliminated from the RMI-IIOP

            Does RMI-IIOP support code downloading for Java objects sent by value across an IIOP
           connection in the same way as RMI does across a JRMP connection? (RMI)
Answer      Yes. The JDK 1.2 support the dynamic class loading.

Question    Can RMI and Corba based applications interact ? (RMI)
Answer      Yes they can. RMI is available with IIOP as the transport protocol instead of JRMP.

Question    Request parameter How to find whether a parameter exists in the request object?
            1.boolean hasFoo = !(request.getParameter("foo") == null ||
           2. boolean hasParameter = request.getParameterMap().contains(theParameter);
           (which works in Servlet 2.3+)
                             Java Interview Questions                                54
Question    How can I send user authentication information while makingURLConnection? (Servlets)
            You'll want to use HttpURLConnection.setRequestProperty and set all the appropriate headers
           to HTTP authorization.

Question    Can we use the constructor, instead of init(), to initialize servlet? (Servlets)
            Yes , of course you can use the constructor instead of init(). There's nothing to stop you. But
           you shouldn't. The original reason for init() was that ancient versions of Java couldn't
Answer     dynamically invoke constructors with arguments, so there was no way to give the constructur
           a ServletConfig. That no longer applies, but servlet containers still will only call your no-arg
           constructor. So you won't have access to a ServletConfig or ServletContext.

Question    How can a servlet refresh automatically if some new data has entered the database?
Answer      You can use a client-side Refresh or Server Push

Question    The code in a finally clause will never fail to execute, right? (Servlets)
Answer      Using System.exit(1); in try block will not allow finally code to execute.

Question    What is HttpTunneling? (Servlets)
            HTTP tunneling is used to encapsulate other protocols within the HTTP or HTTPS protocols.
           Normally the intra-network of an organization is blocked by a firewall and the network is
Answer     exposed to the outer world only through a specific web server port , that listens for only HTTP
           requests. To use any other protocol, that by passes the firewall, the protocol is embedded in
           HTTP and send as HttpRequest.

Question     What is Server Side Push and how is it implemented and when is it useful? (Servlets)
             Server Side push is useful when data needs to change regularly on the clients application or
           browser, without intervention from client. Standard examples might include apps like Stock's
           Tracker, Current News etc. As such server cannot connect to client's application automatically.
           The mechanism used is, when client first connects to Server, (Either through login etc..), then
           Server keeps the TCP/IP connection open.
Answer     It's not always possible or feasible to keep the connection to Server open. So another method
           used is, to use the standard HTTP protocols ways of refreshing the page, which is normally
           supported by all browsers.
           <META HTTP-EQUIV="Refresh" CONTENT="5;URL=/servlet/stockquotes/">
           This will refresh the page in the browser automatically and loads the new data every 5

Question    What are the phases in JSP? (Servlets)
            a) Translation phase ? conversion of JSP to a Servlet source, and then Compilation of servlet
           source into a class file. The translation phase is typically carried out by the JSP engine itself,
           when it receives an incoming request for the JSP page for the first time
           b) init(), service() and destroy() method as usual as Servlets.

            How many cookies can one set in the response object of the servlet? Also, are there any
           restrictions on the size of cookies? (Servlets)
            If the client is using Netscape, the browser can receive and store 300 total cookies
Answer     4 kilobytes per cookie (including name)
           20 cookies per server or domain

Question    What?s the difference between sendRedirect( ) and forward( ) methods? (Servlets)
            A sendRedirect method creates a new request (it?s also reflected in browser?s URL ) where
           as forward method forwards the same request to the new target(hence the chnge is NOT
           reflected in browser?s URL).
           The previous request scope objects are no longer available after a redirect because it results in
           a new request, but it?s available in forward.
           SendRedirectis slower compared to forward.
                             Java Interview Questions                               55
             Is there some sort of event that happens when a session object gets bound or unbound to
           the session? (Servlets)
             HttpSessionBindingListener will hear the events When an object is added and/or remove from
           the session object, or when the session is invalidated, in which case the objects are first
           removed from the session, whether the session is invalidated manually or automatically

Question     What do the differing levels of bean storage (page, session, app) mean? (Servlets)
             page life time - NO storage. This is the same as declaring the variable in a scriptlet and using
           it from there.
           session life time - request.getSession(true).putValue "myKey", myObj);
           application level ? getServletConfig().getServletContext().setAttribute("myKey ",myObj )
           request level - The storage exists for the lifetime of the request, which may be forwarded
           between jsp's and servlets

           Is it true that servlet containers service each request by creating a new thread? If that is
Question true, how does a container handle a sudden dramatic surge in incoming requests without
         significant performance degradation? (Servlets)
           The implementation depends on the Servlet engine. For each request generally, a new
         Thread is created. But to give performance boost, most containers, create and maintain a
         thread pool at the server startup time. To service a request, they simply borrow a thread from
Answer   the pool and when they are done, return it to the pool.
         For this thread pool, upper bound and lower bound is maintained. Upper bound prevents the
         resource exhaustion problem associated with unlimited thread allocation. The lower bound can
         instruct the pool not to keep too many idle threads, freeing them if needed.

Question    Can I just abort processing a JSP? (Servlets)
            Yes. Because your JSP is just a servlet method, you can just put (whereever necessary) a <
           % return; % >

Question What is URL Encoding and URL Decoding ? (Servlets)
         URL encoding is the method of replacing all the spaces and other extra characters into their correspon
        Characters and Decoding is the reverse process converting all Hex Characters back their normal form.
        For Example consider this URL, /ServletsDirectory/Hello'servlet/
Answer When Encoded using URLEncoder.encode("/ServletsDirectory/Hello'servlet/") the output is can be
        back using

            Do objects stored in a HTTP Session need to be serializable? Or can it store any object?
Question    (Servlets)

            Yes, the objects need to be serializable, but only if your servlet container supports persistent
           sessions. Most lightweight servlet engines (like Tomcat) do not support this. However, many
           EJB-enabled servlet engines do. Even if your engine does support persistent sessions, it is
           usually possible to disable this feature.

Question     What is the difference between session and cookie? (Servlets)
             The difference between session and a cookie is two-fold.
           1) session should work regardless of the settings on the client browser. even if users decide to
           forbid the cookie (through browser settings) session still works. there is no way to disable
           sessions from the client browser.
Answer     2) session and cookies differ in type and amount of information they are capable of storing.
           Javax.servlet.http.Cookie class has a setValue() method that accepts Strings.
           javax.servlet.http.HttpSession has a setAttribute() method which takes a String to denote the
           name and java.lang.Object which means that HttpSession is capable of storing any java
           object. Cookie can only store String objects.
                              Java Interview Questions                               56
Question     What is the difference between ServletContext and ServletConfig?
             Both are interfaces.
           The servlet engine implements the ServletConfig interface in order to pass configuration
           information to a servlet. The server passes an object that implements the ServletConfig
           interface to the servlet's init() method.
           The ServletContext interface provides information to servlets regarding the environment in
           which they are running. It also provides standard way for servlets to write events to a log file.

Question    What are the differences between GET and POST service methods? (Servlets)
            A GET request is a request to get a resource from the server. Choosing GET as the "method"
           will append all of the data to the URL and it will show up in the URL bar of your browser. The
           amount of information you can send back using a GET is restricted as URLs can only be 1024
           characters. A POST request is a request to post (to send) form data to a resource on the
           server. A POST on the other hand will (typically) send the information through a socket back
           to the webserver and it won't show up in the URL bar. You can send much more information to
           the server this way - and it's not restricted to textual data either. It is possible to send files
           and even binary data such as serialized Java objects!

Question    What is the difference between GenericServlet and HttpServlet? (Servlets)
            GenericServlet is for servlets that might not use HTTP, like for instance FTP service.As of only
           Http is implemented completely in HttpServlet.
Answer     The GenericServlet has a service() method that gets called when a client request is made. This
           means that it gets called by both incoming requests and the HTTP requests are given to the
           servlet as they are

Question     What is the Max amount of information that can be saved in a Session Object ? (Servlets)
             As such there is no limit on the amount of information that can be saved in a Session Object.
           Only the RAM available on the server machine is the limitation. The only limit is the Session ID
           length(Identifier) , which should not exceed more than 4K. If the data to be store is very
           huge, then it's preferred to save it to a temporary file onto hard disk, rather than saving it in
           session. Internally if the amount of data being saved in Session exceeds the predefined limit,
           most of the servers write it to a temporary cache on Hard disk.

Question    Java Servlets

          I am using servlets. I need to store an object NOT a string in a cookie. Is that possible? The
Question helpfile says BASE64 encoding is suggested for use with binary values. How can I do that?

             You could serialize the object into a ByteArrayOutputStream and then Base64 encode the
           resulting byte[]. We must keep in mind the size limitations of a cookie and the overhead of
           transporting it back and forth between the browser and the server.
           Limitations are:
Answer     * at most 300 cookies
           * at most 4096 bytes per cookie (as measured by the characters that comprise the cookie
           non-terminal in the syntax description of the Set-Cookie2 header, and as received in the Set-
           Cookie2 header)
           * at most 20 cookies per unique host or domain name

            Why does it take so much time to access an Applet having Swing Components the first time?
Question    (Swing)

             Because behind every swing component are many Java objects and resources. This takes
Answer     time to create them in memory. JDK 1.3 from Sun has some improvements which may lead to
           faster execution of Swing applications.

Question    Why does JComponent have add() and remove() methods but Component does not? (Swing)
Answer      because JComponent is a subclass of Container, and can contain other components and
                             Java Interview Questions                               57

Question    How would you create a button with rounded edges? (Swing)
            There are 2 ways. The first thing is to know that a JButton?s edges are drawn by a Border.
           so you can override the Button?s paintComponent(Graphics) method and draw a circle or
           rounded rectangle (whatever), and turn off the border. Or you can create a custom border that
           draws a circle or rounded rectangle around any component and set the button?s border to it.

Question    How would you detect a keypress in a JComboBox? (Swing)
            Add a KeyListener to the JComboBox?s editor component instead of adding a KeyListener to
           the JComboBox itself

            Why should the implementation of any Swing callback (like a listener) execute quickly?
Question    (Swing)

            Because callbacks are invoked by the event dispatch thread which will be blocked processing
           other events for as long as your method takes to execute.

Question    Why would you use SwingUtilities.invokeAndWait or SwingUtilities.invokeLater? (Swing)
            I want to update a Swing component but I?m not in a callback. If I want the update to
Answer     happen immediately (perhaps for a progress bar component) then I?d use invokeAndWait. If I
           don?t care when the update occurs, I?d use invokeLater.

Question    If your UI seems to freeze periodically, what might be a likely reason? (Swing)
            A callback implementation like ActionListener.actionPerformed or
Answer     MouseListener.mouseClicked is taking a long time to execute thereby blocking the event
           dispatch thread from processing other UI events.

Question    Which Swing methods are thread-safe? (Swing)
Answer      The only thread-safe methods are repaint(), revalidate(), and invalidate()

Question    Why won?t the JVM terminate when I close all the application windows? (Swing)
            The AWT event dispatcher thread is not a daemon thread. You must explicitly call
           System.exit to terminate the JVM.

            JFrame is a heavy weight component. Since it extends an awt Frame, is it Thread Safe?
Question    (Swing)

             JFrame itself is, since it is just a java.awt.Frame in essence, but the root pane/content pane
Answer     is not, so it effectively follows the same rules for Swing containers and is not considered
           thread safe.

Question     What is the difference between invokeAndWait() and invokeLater()? (Swing)
             invokeAndWait() blocks until the Runnable task is complete; it's synchronous.
Answer     invokeLater() posts an action event to the event queue and returns immediately; it's

Question    What is the difference between a Scrollbar and a ScrollPane? (Swing)
            A Scrollbar is a Component, but not a Container. A ScrollPane is a Container. A ScrollPane
           handles its own events and performs its own scrolling

Question    Difference between paint() and paintComponent() (Swing)
            The key point is that the paint() method invokes three methods in the following order:
           a) paintComponent()
           b) paintBorder()
           c) paintChildren()

           As a general rule, in Swing, you should be overriding the paintComponent method unless you
                             Java Interview Questions                                  58
           know what you are doing.
           paintComponent() paints only component (panel) but paint() paints component and all its

             What is the difference between paint(), repaint() and update() methods within an applet
           which contains images? (Swing)
             paint : is only called when the applet is displayed for the first time, or when part of the
           applet window has to be redisplayed after it was hidden.
           repaint : is used to display the next image in a continuous loop by calling the update method.
           update : you should be aware that, if you do not implement it yourself, there is a standard
           update method that does the following :
           · it will reset the applet window to the current background color (i.e. it will erase the current
           it will call paint to construct the new image

Question     What is the difference between yielding and sleeping? (Threads)
             When a task invokes its yield() method, it returns to the ready state. When a task invokes its
           sleep() method, it returns to the waiting state.

Question    When a thread blocks on I/O, what state does it enter?
Answer      A thread enters the waiting state when it blocks on I/O.

Question    When a thread is created and started, what is its initial state? (Threads)
Answer      A thread is in the ready state after it has been created and started.

Question    What invokes a thread's run() method? (Threads)
            After a thread is started, via its start() method or that of the Thread class, the JVM invokes
           the thread's run() method when the thread is initially executed.

Question    What method is invoked to cause an object to begin executing as a separate thread? (Threads)
            The start() method of the Thread class is invoked to cause an object to begin executing as a
           separate thread.

Question    What is the purpose of the wait(), notify(), and notifyAll() methods? (Threads)
            The wait(),notify(), and notifyAll() methods are used to provide an efficient way for threads
           to wait for a shared resource. When a thread executes an object's wait() method, it enters the
           waiting state. It only enters the ready state after another thread invokes the object's notify()
           or notifyAll() methods.

Question    What are the high-level thread states? (Threads)
Answer      The high-level thread states are ready, running, waiting, and dead

Question    What happens when a thread cannot acquire a lock on an object? (Threads)
            If a thread attempts to execute a synchronized method or synchronized statement and is
           unable to acquire an object's lock, it enters the waiting state until the lock becomes available.

Question    How does multithreading take place on a computer with a single CPU? (Threads)
            The operating system's task scheduler allocates execution time to multiple tasks. By quickly
           switching between executing tasks, it creates the impression that tasks execute sequentially.

            What happens when you invoke a thread's interrupt method while it is sleeping or waiting?
Question    (Threads)

            When a task's interrupt() method is executed, the task enters the ready state. The next time
           the task enters the running state, an InterruptedException is thrown.

Question    What state is a thread in when it is executing?
                             Java Interview Questions                             59
Answer      An executing thread is in the running state

Question    What are three ways in which a thread can enter the waiting state? (Threads)
            A thread can enter the waiting state by invoking its sleep() method, by blocking on I/O, by
Answer     unsuccessfully attempting to acquire an object's lock, or by invoking an object's wait()
           method. It can also enter the waiting state by invoking its (deprecated) suspend() method.

Question    What method must be implemented by all threads? (Threads)
            All tasks must implement the run() method, whether they are a subclass of Thread or
           implement the Runnable interface.

            What are the two basic ways in which classes that can be run as threads may be defined?
Question    (Threads)

             A thread class may be declared as a subclass of Thread, or it may implement the Runnable

Question    Explain about Deadlock state of a thread (Threads)
            Synchronizing run() is a good example of a simple deadlock scenario, where a thread is
           blocked forever, waiting for something to happen that can't. Let's look at a few examples that
           are more realistic than this.

           The most common deadlock scenario occurs when two threads are both waiting for each other
           to do something. The following (admittedly contrived) code snippet makes what's going on
           painfully obvious:

           see code at:

           Now, imagine a scenario whereby one thread (call it Wilma) calls fred(), passes through the
           synchronization of lock_1, and is then preempted, allowing another thread (call it Betty) to
           execute. Betty calls barney(), acquires lock_2, and tries to acquire lock_1, but can't because
           Wilma has it. Betty is now blocked, waiting for lock_1 to become available, so Wilma wakes up
           and tries to acquire lock_2 but can't because Betty has it. Wilma and Betty are now
           deadlocked. Neither one can ever execute.

           (Note that lock_1 and lock_2 have to be one-element arrays rather than simple ints, because
           only objects have monitors in Java; the argument to synchronized must be an object. An array
           is a first-class object in Java; a primitive-type such as int is not. Consequently, you can
           synchronize on it. Moreover, a one-element array is efficient to bring into existence compared
           to a more elaborate object (like an Integer) since it's both small and does not require a
           constructor call. Also, note that I can keep the reference to the lock as a simple Object
           reference, since I'll never access the array elements.

Question    Can you explain the difference between green threads and native threads? (Threads)
            Green threads is thread mechanism implemented in JVM itself. It is blind and can run on any
           OS, so actually all threads are run in one native thread and scheduling is up to JVM. This is
           disadvantageously for SMP systems, since only one processor can serve Java application.
           Native threads is a mechanism based on OS threading mechanism. This allows to use features
           of hardware and OS. For example,there is IBM's JDK for AIX that supports native threads. The
           perfomance of applications can be highly imploved by this.

             I have created a program with a main method that instantiates and starts three threads, the
           first two of which are daemons. Why daemons does die when normal thread die? (Threads)
             Because of nature of daemon threads. They are alive if exists at least one "normal user's"
           thread. Otherwise they die immediately

Question    When will a Thread Object be garbage collected?
                             Java Interview Questions                              60
             Since Thread is also an Object, it will only garbage collected when the reference count is
           zero. You may think it is quite non-sense. the thread is useless when it enter "dead" state.
Answer     why not garbage collect it? That's because the thread object itself may contain some other
           useful information even the thread dead , e.g. the result of the execution of the thread. Thus,
           it is not sensible to do garbage collect when the reference count is not zero.

Question    What happens when you call yield() on a thread? (Threads)
            Causes the currently executing thread object to temporarily pause and allow other threads to

Question     Why is XML such an important development? (XML)
             It removes two constraints which were holding back Web developments:<br> 1. §
           dependence on a single, inflexible document type (HTML) which was being much abused for
           tasks it was never designed for;<br> 2. the complexity of full SGML, whose syntax allows
Answer     many powerful but hard-to-program options.<br> § XML allows the flexible development of
           user-defined document types. It provides a robust, non-proprietary, persistent, and verifiable
           file format for the storage and transmission of text and data both on and off the Web; and it
           removes the more complex options of SGML, making it easier to program for.

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