abstract class A {
    public abstract void show();
}
------------------------------------------------------------------
public
    class B extends A{

    public void show(){
        System.out.println("Hej tu klasa - B\t"+getClass());
    };
}
------------------------------------------------------------------
import java.awt.Graphics;

import figury.Kwadrat;

public class Main {

    public static void main(String[] args){
        A a = null;
        a = new B();
        a.show();

        a = new A(){
            public void show(){
                System.out.println("Hej tu klasa anonimowa\t"+getClass());
            }
        };
        a.show();
        new MyFrame();
    }
}
------------------------------------------------------------------
import java.awt.*;
import java.awt.event.*;
import figury.*;

public class MyFrame
    extends Frame{

    Figura fig;

    public MyFrame(){

        addWindowListener(
            new WindowAdapter(){
                public void windowClosing(WindowEvent e){
                    System.exit(0);
                }
            }
        );
        addMouseListener(
            new MouseAdapter(){
                public void mouseClicked(MouseEvent e){
                    System.out.println(e.getX()+" "+e.getY());
                    switch((int)(Math.random()*3)){
                    case 0:
                        fig = new Kwadrat(e.getX(),e.getY());
                    break;
                    case 1:
                        fig = new Kolo(e.getX(),e.getY());
                    break;
                    case 2:
                        fig = new Trojkat(e.getX(),e.getY());
                    break;
                    }
                    repaint();
                }
            }

        );
        setSize(800,400);
        setVisible(true);
    }

    public void paint(Graphics g){
        if(fig !=null)
        fig.rysuj(g);
    }

}
------------------------------------------------------------------
package figury;
import java.awt.Graphics;

public class Kwadrat
    extends Figura{

    public Kwadrat(int x, int y) {
        super(x, y);
    }

    public void rysuj(Graphics g) {
        g.drawRect(x, y, 50, 50);
    }

}
------------------------------------------------------------------
package figury;
import java.awt.Graphics;

public class Trojkat
    extends Figura{

    int punktyX[];
    int punktyY[];
    int promien;

    public Trojkat(int x, int y) {
        super(x, y);

        promien = 25;

        double step =(Math.PI*2)/3;

        punktyX=new int[]{
            (int)(Math.sin(step*0)*promien) +x,
            (int)(Math.sin(step*1)*promien) +x,
            (int)(Math.sin(step*2)*promien) +x
        };
        punktyY=new int[]{
                (int)(Math.cos(step*0)*promien) +y,
                (int)(Math.cos(step*1)*promien) +y,
                (int)(Math.cos(step*2)*promien) +y
            };

    }

    public void rysuj(Graphics g) {
        g.drawPolygon(punktyX,punktyY,3);
    }

}
------------------------------------------------------------------
package figury;
import java.awt.*;

public abstract
    class Figura {
    protected int x,y;
    public Figura(int x,int y){
        this.x=x;
        this.y=y;
    }
    public abstract void rysuj(Graphics g);
}
------------------------------------------------------------------
package figury;
import java.awt.Graphics;

public class Kolo
    extends Figura{

    public Kolo(int x, int y) {
        super(x, y);
    }

    public void rysuj(Graphics g) {
        g.drawOval(x, y, 50, 50);
    }

}