bouncingball.java~

书籍"Java_面向事件编程"的附带光盘代码

import objectdraw.*;

// animate the motion of a simple, falling ball
public class BouncingBall extends ActiveObject
{
    // size and speed of balls
    private static final double BALLSIZE = 30;
    private static final double DELAY_TIME = 33;
    private double ySpeed = 0.1;
    
    // y coordinate where ball disappears
    private double bottom;
    
    // the ball
    private FilledOval ball;
    
    // the paddle
    private FilledRect paddle;
    
    public BouncingBall(double x, double y, FilledRect thePaddle, DrawingCanvas aCanvas)
    {
        
        ball = new FilledOval(x - BALLSIZE/2 , y,
                              BALLSIZE, BALLSIZE, aCanvas);
        bottom = aCanvas.getHeight();
        paddle = thePaddle;
    }
    
    public void run()
    {
        double top = ball.getY();
        
        // move the ball repeatedly until it fall off screen
        while (ball.getY() < bottom ) {
     
            // determine how much time has passed
            currentTime = System.currentTimeMillis();
            elapsedTime = currentTime - lastTime;
            // restart timing
            lastTime = currentTime;
            ball.move(0, ySpeed * elapsedTime);
            
            if (ball.overlaps(paddle) || ball.getY() < top){
                ySpeed = -ySpeed;
            }
            pause(DELAY_TIME);
        }
        
        ball.removeFromCanvas();
    }
    
}