ButtonApp3 (other files are same as before): ColSquare.java

import java.awt.*;
import javax.swing.*;

/**
 * Specialized subclass of Square
 */
public class ColSquare extends Square {
    private Color color; //*1 Adds new feature to Square

    /** Everything the same as superclass, except stash color */
    public ColSquare (Canvas canvas, Color color) { //*3 Pass one arg, keep the other
	super (canvas); //*3
	this.color = color; //*3
    }

    /** Piggybacks on super class' code */
    public void draw (Graphics g) {
	g.setColor (color); //*1
	super.draw (g); //*2 Exploit Square.draw() code
	g.setColor (Color.BLACK); //*1
    }
}

[download file]