growingdot.java

Java程序设计(美) David D. Riley著 机械工业出版社 书籍配套 代码

// see FIgure 15.12

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

/**	Class Invariant:	
 *		redDot.getWidth() == redDot.getHeight()
 *		and  redDot.getX() == 100 - redDot.getWidth()/2
 *		and  redDot.getY() == 100 - redDot.getHeight()/2 
 */
public class GrowingDot  extends JApplet 
		implements ActionListener {
	private Oval redDot;
	private Timer clock;
	private ButtonFrame startBtn;
	private int darkness;

	/**	post:	redDot is added on this 
	 *			and startBtn != null
	 *			and this is colored black
	 *			and clock is scheduled for 0.1 sec. repeated events  
	 */
	public GrowingDot()  {
		setLayout(null);
		redDot = new Oval(98, 98, 4, 4);
		redDot.setBackground( Color.red );
		add(redDot);
		clock = new Timer(100, this);
		clock.setRepeats(true);
		startBtn = new ButtonFrame(clock);
		darkness = 0;
		setBackground(new Color(darkness, darkness, darkness));
	}

	/**	pre:	redDot != null  <br>
	 *	post:	([e-mail]() == 200) implies
	 *					redDot is resized to 4 by 4 and this recolored to black
	 *			and  ([e-mail]() != 200) implies
	 *					( redDot resize to 2 by 2 larger than redDot@pre
	 *					and this is recolored whiter ) 
	 */
	public void actionPerformed(ActionEvent e) {
		if (redDot.getWidth() == 200)  {
			redDot.setSize(4, 4);
			redDot.setLocation(98, 98);
			darkness = 0;
			setBackground(new Color(darkness, darkness, darkness));
			clock.stop();
		} else {
			redDot.setSize(redDot.getWidth()+2, redDot.getHeight()+2);
			redDot.setLocation(100-redDot.getWidth()/2, 100-redDot.getHeight()/2);
			darkness = darkness + 5;
			if (darkness > 255)
				darkness = 255;
			setBackground( new Color(darkness, darkness, darkness) );
		}
		redDot.repaint();
		repaint();
	}
}