Write a program on clearing Rectangles ?

It is also potential to clear a rectangle that you've drawn. The syntax is exactly what you'd expect:

public abstract void clearRect(int x, int y, int width, int height)
This program uses clearRect() to blink a rectangle on the screen.
import java.applet.*;
import java.awt.*;

public class Blink extends Applet {

public void paint(Graphics g) {

int appletHeight = this.getSize().height;
int appletWidth = this.getSize().width;
int rectHeight = appletHeight/3;
int rectWidth = appletWidth/3;
int rectTop = (appletHeight - rectHeight)/2;
int rectLeft = (appletWidth - rectWidth)/2;

for (int i=0; i < 1000; i++) {
g.fillRect(rectLeft, rectTop, rectWidth-1, rectHeight-1);
g.clearRect(rectLeft, rectTop, rectWidth-1, rectHeight-1);
}

}

}
This is not how you should do animation in practice, but this is the best we can do until we introduce threads.