movingball.java

书籍"Java_面向事件编程"的附带光盘代码

import objectdraw.*;

public class MovingBall extends ActiveObject
{
    private static final int BALLSIZE = 30;
    private static final int SCREENGAP = 130;
    
    // Constants to determine ball speeds
    private static final double MINSPEED = 0.08;
    private static final double SPEEDRANGE = 0.15;
    private static final int PAUSETIME = 33;
    
    private FilledOval ball;
    
    // components of speed vector
    private double xspeed, yspeed, initYspeed;
    
    // boundaries of playing area
    private double bleft,bright,btop,offScreen;
    
    // the paddle
    private Rect paddle;
    
    private RandomDoubleGenerator speedGen;
    private DrawingCanvas canvas;
    
    public MovingBall(DrawingCanvas aCanvas, Rect thePaddle,
                      Rect boundary)
    {
        canvas = aCanvas;
        paddle = thePaddle;
        
        ball = new FilledOval(boundary.getLocation() ,
                              BALLSIZE,BALLSIZE,canvas);
        ball.move(1,1);
        
        // pick random pixel/pause speeds
        speedGen = new RandomDoubleGenerator(MINSPEED,SPEEDRANGE);
        xspeed = speedGen.nextValue()-SPEEDRANGE/2;
        if (xspeed > 0){
            xspeed = xspeed + MINSPEED;
        }
        else {
            xspeed = xspeed - MINSPEED;
        }

        yspeed = speedGen.nextValue() + MINSPEED;
        
        initYspeed = yspeed;
        
        // give names to boundary values
        bleft = boundary.getX();
        btop = boundary.getY();
        bright = bleft + boundary.getWidth() - BALLSIZE;
        
        offScreen = btop + boundary.getHeight() + SCREENGAP;
        
        start();
    }
    
    public void run()
    {
        // used to record times and compute time between moves
        double lastTime, currentTime, elapsedTime;
        
        lastTime = System.currentTimeMillis();
        
        while (ball.getY() < offScreen) {

            // determine how much time has passed
            currentTime = System.currentTimeMillis();
            elapsedTime = currentTime - lastTime;
            // restart timing
            lastTime = currentTime;

            ball.move(xspeed*elapsedTime,yspeed*elapsedTime);
            
            if ( ball.getX() < bleft  || ball.getX() > bright ) {
                xspeed = -xspeed;
            }
            
            if ( ball.getY() < btop ) {
                yspeed = initYspeed;
            } else if ( ball.overlaps(paddle) ) {
                yspeed = -initYspeed;
            }
            
            pause(PAUSETIME);
        }
        ball.removeFromCanvas();
    }
    
    
}