2.3 A Rect Subclass

Example 2-3 is a simple subclass of the Rect class of Example 2-1. This DrawableRect class inherits the fields and methods of Rect and adds its own method, draw( ), that draws a rectangle using a specified java.awt.Graphics object. (We'll see more of the Graphics object in Chapter 12.) DrawableRect also defines a constructor that simply uses the super keyword to pass its arguments up to the corresponding Rect constructor. Note the use of the extends keyword to indicate that Rect is the superclass of DrawableRect.

Example 2-3. DrawableRect.java

package je3.classes;
/**
 * This is a subclass of Rect that allows itself to be drawn on a screen.
 * It inherits all the fields and methods of Rect
 * It relies on the java.awt.Graphics object to perform the drawing.
 **/
public class DrawableRect extends Rect {
    /** The DrawableRect constructor just invokes the Rect( ) constructor */
    public DrawableRect(int x1, int y1, int x2, int y2) { super(x1,y1,x2,y2); }
    
    /** This is the new method defined by DrawableRect */
    public void draw(java.awt.Graphics g) {
        g.drawRect(x1, y1, (x2 - x1), (y2-y1));
    }
}