Limit Object Movement Within A Window

I just started to learn Java. In my program, I created a GRect(paddle) and I would like to move it on the x axis whenever the mouse is moved.

import acm.graphics.*;
import acm.program.*;
import acm.util.*;
import java.awt.*;
import java.awt.event.*;
public class BreakoutClass extends GraphicsProgram {
/** Width and height of application window in pixels */
private static final int WIDTH = 400;
private static final int HEIGHT = 600;

[code]....

In this case, whenever the paddle reaches the right edge of the screen, it doesn't move off the windows, but it stops moving (even if you move the mouse).