/* <applet code="Tomato.class" width=400 height=400></applet> */

import java.applet.Applet; 
import java.awt.Graphics; 
import java.awt.Color; 
import java.awt.event.MouseEvent; 
import java.awt.event.MouseMotionListener; 

public class Tomato extends Applet {
  public void init() {
    this.addMouseMotionListener(new Umpire());
  } 
  int n;
  public void paint(Graphics g) {
    g.setColor(Color.red); 
    g.fillOval(50, 50, 300, 300); 
    g.setColor(Color.green);
    g.drawOval(50, 50, 300, 300); 
    this.n += 1; 
    System.out.println("I am paint and I am working " + this.n); 
  }   
} 

class Umpire implements MouseMotionListener {
  public void mouseMoved(MouseEvent e) { 
    int x = e.getX(), y = e.getY(); 
    System.out.println("I am moving the mouse...(" + x + ", " + y + ")"); 
  }
  public void mouseDragged(MouseEvent e) { 
    int x = e.getX(), y = e.getY(); 
    System.out.println("Mouse being dragged (" + x + ", " + y + ")"); 
  }
} 

-----------------

Classes and objects are containers.

Classes contain: 

a) blueprint for objects of that type
b) static members (methods and variables)

Objects are instantiations of blueprints and they contain: 

a) instance members (methods and variables)


class One {
  int a;
  static int b;

  public static void main(String[] args) {
    One a = new One(); 
    One b = new One(); 
    a.a = 1; 
    a.b += 2;
    b.a += 9; 
    b.b += 4; 
    System.out.println(a.a + ", " + b.b); 
    System.out.println(b.a + ", " + b.b); 
  }
} 

The class extension mechanism: inheritance, polymorphism, dynamic method lookup. 

The meaning of "this":

a) if we had only variables "this" wouldn't be, couldn't exist

b) "this" can be used and makes sense only from inside a method

c) "this" can't appear in a static method, it is not allowed, not accepted

d) "this" therefore can only appear in an instance method, where it refers to
the instance where that method is.