Write a program on Filling Rectangles ?

The drawRect() method draws an open rectangle, a box if you prefer. If you need to draw a filled rectangle, use the fillRect() method. Or else the syntax is identical.
This program draws a filled square in the center of the applet. This needs you to separate the applet width and height from the rectangle width and height. Here's the code:
import java.applet.*;
import java.awt.*;

public class FillAndCenter 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;

g.fillRect(rectLeft, rectTop, rectWidth-1, rectHeight-1);

}

}