Java Installation

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					Object Oriented Programming

Lecture 3

   In discussing Java, some items
    need to be clarified
    The Java programming language
    The Java virtual machine (JVM)
    The Java platform
Java Programming Language
   An       object-oriented     programming
    language, which is developed by Sun
    A familiar C/C++ style of notation.
    Use the object-oriented programming
    Allow the same program to be executed
    on multiple operating systems.
   Contain build-in support for using
    computer networks.
    The Java Platform

   The Java platform, is the environment
    in which a program runs.
   Two components:
   The Java virtual machine
   The Java application programming
    interface (API)
      The Java Virtual Machine
   The heart of the Java Platform
    is the concept of a "virtual
    machine" that executes Java
    bytecode programs.
   Sits    between      the   Java
    program and the machine it is
    running on.
   Offers    the     program     an
    “abstract    computer”      that
    executes the Java code
   virtual machine isn't running
    on a CPU - it is being
    emulated on the CPU of the
    host machine.
   Through the Java VM,
    the same application is
    capable of running on
    multiple platforms.
Java Programming Process
Key benefits of Java

   Simple: automatic memory allocation
    and garbage collection
   Platform independent.
    Network-Centric programming, Java
    is designed to make distributed
    computing easy with the networking
    capability    that    is inherently
    integrated into it.
    Java Editions
   J2ME (Micro Edition) – Used to create programs that
    run on small handheld devices, such as phones,
    PDAs (personal digital assistants), and appliances.

   J2SE (Standard Edition) – Used primarily to create
    programs for desktop computers or for many
    computers too large for J2ME and too small for J2EE.

   J2EE (Enterprise Edition) – Used to create very large
    programs that run on servers managing heavy traffic
    and complicated transactions. These programs are
    the backbone of many online services, such as
    banking, e-commerce systems etc.
Installing Java
   The Java Development Kit (JDK) is a
    collection of software available at no charge
    from Sun Microsystems, Inc.
   The v1.6 download is available at
 Sample Java Program

class Hello
public static void main ( String[] args )
    System.out.println("Hello World!");
Getting Started:
Create the source file:
     open a text editor, type in the code which
      defines a class Hello and then save it in a file
     file and class name are case sensitive and must
      be matched exactly (except the .java part)

Be Careful When You Type
 Type all code, commands, and file names
  exactly as shown. Both the compiler (javac)
  and launcher tool (java) are case-sensitive,
  HelloWorldApp helloworldapp
 The compiler requires the source code files to
  be named according to specific rules:
 Correct:
 Incorrect: (wrong case) (wrong extension) (Microsoft Windows short-
  names not allowed)
(2) Compile the program:
      compile by using the following
      it generates a file named Hello.class
At the prompt, type the following command and press Enter.
The compiler has generated a bytecode file, HelloWorldApp.class.
At the prompt, type dir to see the new file that was generated, as
shown in the following figure.
3) Run the program:
     run the code through:

  java Hello
     Note that the command is java,
      not javac, and you refer to
      Hello, not or
Compiler Problems :Common Error Messages

 'javac' is not recognized as an internal or
  external command, operable program or
  batch file
 If you receive this error, Window
  cannot find the compiler (javac).
   Suppose you installed the JDK in C:\jdk6. At the prompt you would type
    the following command and press Enter: C:\jdk6\bin\javac If you choose this option, you'll have to precede your
    javac and java commands with C:\jdk6\bin\ each time you compile or run

    a program.
Compiler Problems :Common Error Messages

   Class names, 'HelloWorldApp', are
    only    accepted      if    annotation
    processing is explicitly requested
   If you receive this error, you forgot to include the .java
    suffix when compiling the program. Remember, the
    command is       javac not javac
Runtime Problems :Common Error Messages

   Exception in thread "main"
    java.lang.NoClassDefFoundError: HelloWorldApp
   If you receive this error, java cannot find your bytecode
    file, HelloWorldApp.class.
   One of the places java tries to find your .class file is
    your current directory. So if your .class file is in C:\java,
    you should change your current directory to that. To
    change your directory, type the following command at
    the prompt and press Enter:
   cd c:\java The prompt should change to C:\java>. If
    you enter dir at the prompt, you should see your .java
    and .class files. Now enter java HelloWorldApp again.
Runtime Problems :Common Error Messages

   Exception in thread "main"
    java.lang.NoClassDefFoundError: HelloWorldApp/class
   A common mistake made by beginner programmers is to try
    and run the java launcher on the .class file that was created
    by the compiler. For example, you'll get this error if you try to
    run your program with java HelloWorldApp.class instead of
    java HelloWorldApp. Remember, the argument is the name of
    the class that you want to use, not the filename.

   Exception in thread "main"
    java.lang.NoSuchMethodError: main
   The Java VM requires that the class you execute with it have a
    main method at which to begin execution of your application.
Java Class
   A class is an object oriented construct. It
    is designed to perform a specific task.
    A Java class is defined by its class name,
    an open curly brace, a list of methods and
    fields, and a close curly brace.
    The name of the class is made of
    alphabetical characters and digits without
    spaces, the first character must be
   The line public static void main ( String[]
    args ) shows where the program will start
    running. The word main means that this
    is the main method –
   The JVM starts running any program by
    executing this method first.
    The main method in consists
    of a single statement
    System.out.println("Hello World!");
    The statement outputs the characters
    between quotes to the console.
Syntax Errors
   public Class Hello {
   public static void main ( String[]
   args) {
   System.out.println("Hello World!");
   }
   }
   The required word "class" has been changed to "Class"
    with a capital "C". This is called a syntax error.
   •A syntax error is a “spelling or grammatical error" in
    the program.
   The error message is not very clear. But at least it
    shows where the error is.
   The compiler will not create a new bytecode file
    because it stops compiling when it gets to an error.
   // This program outputs “Hello World I am
   // learning Java” to screen
   public class MyFirstProgram {
   public static void main ( String[] args ) {
   // the output statement starts here
   System.out.println("Hello World!"); // this statement outputs “Hello
   World”
   System.out.println(“I am learning Java”);
   }
   }
   A comment is a note written to a human reader of a
    program. The program compiles and runs exactly the
    same with or without comments. Comments start with
    the two characters "//" (slash slash). Those characters
    and everything that follows them on the same line are
    ignored by the java compiler.
   /*
   This is my first Java program
   and I am pretty excited about it.
   */
   public class MyFirstProgram {
   public static void main ( String[] args ) {
   System.out.println("Hello World!"); // this statement outputs
    “Hello World”
   System.out.println(“I am learning Java”);
   }
   }
   With this style of comment, everything between the
    two characters "/*" and the two characters "*/" are
    ignored by the compiler. There can be many lines of
    comments between the "/*" and the "*/".

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