Lec 6 14_aug [compatibility mode]

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Lec 6 14_aug [compatibility mode]

  1. 1. Introduction to Java Lecture 6 Naveen Kumar
  2. 2. Class definition class A { int i; char ch; void set() {…} int get(int b) {…} }
  3. 3. Method Declarations  General format of method declaration: Modifier return-type method-name( parameter1, …, parameterN ) { body (declarations and statements); }  Modifiers—such as public, private, and others you will learn later.  return type—the data type of the value returned by the method, or void if the method does not return a value.  Method body can also return values: return expression;
  4. 4. Access members of a class Class A { int i; char ch; void set() { i=20; } int get() {return i; } } stack Heap i ch A How to access member of class A ? A a= new A(); a.i; a.ch; a.set();
  5. 5. Types of Methods (4 basic types ) – Modifier (sometimes called a mutator)  Changes the value associated with an attribute of the object  E.g. A method like set() – Accessor  Returns the value associated with an attribute of the object  E.g. A method like Get() – Constructor  Called once when the object is created (before any other method will be invoked)  E.g. A(int i) – Destructor  Called when the object is destroyed  E.g.~A( )
  6. 6. Constructor  Same name as class name  No return type (as methods) Why we need constructors?  Initialize an object Default cons (if we not defined) – No parameter – Ex: A() { }
  7. 7. Parameterized constructor A(int in) A(int in, char c) { { i=in; i=in; } ch=c; }  Created when object init  Can define any number of constructors
  8. 8. Example 2: two classes class aa2 { int i; char ch; void set() { i=20;} int get() {return i;} } 8 public class aa4 { public static void main(String args[]) { aa2 obj= new aa2(); int b; obj.set(); b= obj.get(); System.out.println("i="+ obj.i); System.out.println("i="+ b); } }
  9. 9. Example 3: two classes uses cons. class aa2 { int i; char ch; void set() { i=20;} int get() {return i;} aa2 (int in, char c) { i=in; ch=c; } }9 public class aa4 { public static void main(String args[]) { aa2 obj= new aa2(20,‘g’); System.out.println("i="+ obj.i); System.out.println("i="+ obj.ch); } }
  10. 10. Example 4: single class public class aa1 { int i; char ch; void set() { i=20;} int get() {return i;} 10 public static void main(String args[]) { aa1 a= new aa1(); int b; a.set(); b=a.get(); System.out.println("i="+ a.i); System.out.println("i="+ b); } }
  11. 11. Introduction to Applets  Java applet is a small appln. written in Java  delivered to users in the form of bytecode  user can launches Java applet from a web page  it can appear in a frame of the web page, in a new application window, or in Sun's AppletViewer, a stand-alone tool for testing applets 11
  12. 12. Applet Example 1 /* <APPLET CODE="app1.class" WIDTH=150 HEIGHT=100> </APPLET> */ import java.applet.Applet; import java.awt.Graphics; public class app1 extends Applet { public void paint (Graphics g) { g.drawString("Hello!",50,20); } } 12
  13. 13. Applet program execution Compile javac app1.java Execution appletviewer app1.java 13
  14. 14. Execution through HTML file <HTML> <HEAD> <TITLE> A simple Program</TITLE> </HEAD> <BODY> Here is the output: <APPLET CODE="app1.class" WIDTH=150 HEIGHT=100> </APPLET> <BODY> </HTML> Store with name app1.htm Execute from browser: C:javaapp1.htm14

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