java

							  Java

Steve Haroz
       How does Java work?
• Code (similar to c++) compiled to byte
  code
• Byte code runs on a virtual machine
• A virtual machine is created for each OS
• The same byte code (i.e. executable or
  library) can run on any Java VM
               Java vs. C++
•   Efficiency              C++
•   Portability             Java
•   Object Oriented         Both
•   Strongly Typed          Java
•   Pointers                C++
•   Garbage Collection      Java
•   Consistent Primitives   Java
         Java does not have…
•   Header files
•   Macros
•   Pointers and references
•   Unions
•   Operator overloading
•   Templates (maybe?)
                   Hello World
public class HelloWorld {
  public static void main(String[] args) {
      String s = "Hello World";
      char c = '!';
      s = s + c;

        for (int i = 0; i < 5; i++)
               System.out.println(i);

        System.out.println(s);
    }
}
            Compile and Run
• Compile
    javac HelloWorld.java
  Output File: HelloWorld.class


• Run
    java HelloWorld
  Output: Hello, World
         Arrays and Objects
// notice location of brackets
 String[] days = new String[] {"Su", "Mo",
   "Tu", "We", "Th", "Fr", "Sa"};

Object x;
x = new String[] {"Su", "Mo", "Tu", "We",
 "Th", "Fr", "Sa"};
             Exceptions
try {
          f();
    } catch (MyException e) {
          e.printStackTrace();
    } catch (YourException e) {
          e.printStackTrace();
    } finally {
          ... // always executed
}
Next few slides blatantly stolen from
          Stoney Jackson
 How are instance methods defined?
Instance methods take an implicit parameter:
  instance on which method is invoked
   public class Movie {
       public int script, acting, directing;
       public int rating() {
           return script + acting + directing;
       }
   }
   public class Demo {
       public static void main (String argv[]) {
           Movie m = new Movie();
           m.script = 6; m.acting = 9; m.directing = 8;
           System.out.print(“The rating of this movie is”);
           System.out.println(m.rating());
       }
   }
                How to extend classes?
   Inheritance: mechanism for extending behavior
    of classes; leads to construction of hierarchy of
    classes [Note: no multiple inheritance]
   What happens when class C extends class D:
       Inherits instance variables
       Inherits static variables
       Inherits instance methods
       Inherits static methods
       C can:
            Add new instance variables
            Add new methods (static and dynamic)
            Modify methods (only implementation)
            Cannot delete anything
              How to extend classes?
public class Attraction {
    public int minutes;
    public Attraction() {minutes = 75;}
    public int getMinutes() {return minutes;}
    public void setMinutes(int d) {minutes = d;}
}
public class Movie extends Attraction {
    public int script, acting, directing;
    public Movie() {script = 5; acting = 5; directing = 5;}
    public Movie(int s, int a, int d) {
         script = s; acting = a; directing = d;
    }
    public int rating() {return script + acting + directing;}
}
public class Symphony extends Attraction {
    public int playing, music, conducting;
    public Symphony() {playing = music = conducting = 5;}
    public Symphony(int p, int m, int c) {
         playing = p; music = m; conducting = c;
    }
    public int rating() {return playing + music + conducting;}
}
           What are abstract classes?
   Abstract class: Merely a place holder for class
    definitions; cannot be used to create instances.;
    public abstract class Attraction {
        public int minutes;
        public Attraction() {minutes = 75;}
        public int getMinutes() {return minutes;}
        public void setMinutes(int d) {minutes = d;}
        public abstract void m();
    }
    Following is an error:
    Attraction x;
    x = new Attraction();


    Following is not an error:
    public class Movie extends Attraction { … }
    public class Symphony extends Attraction { … }
    Attraction x;
    x = new Movie ();
    x = new Symphony();
     Java Does Not Have Const
public class MyClass
{
  public static final int x = 5; // const

    // Members and methods…
}
       Your Own Exception
public class MyException extends Exception
{
  public MyException() {}
  public MyException(String message)
  {
     super(String message);
  }
}
          Java Resources
• Eclipse - www.eclipse.org – good IDE
• Java API -
  java.sun.com/j2se/1.5.0/docs/api/
• Google -
  directory.google.com/Top/Computers/Prog
  ramming/Languages/Java/