fallingball.java

书籍"Java_面向事件编程"的附带光盘代码

import objectdraw.*;

public class FallingBall extends ActiveObject
{
    // size and speed of balls
    private static final int BALLSIZE = 30;
    private static final double Y_SPEED = 0.1;
    private static final int DELAY_TIME = 33;
    private static final int TOP = 50;
    
    // relevant dimensions of playing area
    private static final int BOTTOM = 600;
    private static final int SCREENWIDTH = 400;
    
    private FilledOval ball;
    
    public FallingBall(DrawingCanvas aCanvas)
    {
        
        ball = new FilledOval(SCREENWIDTH/2 ,TOP,
                              BALLSIZE, BALLSIZE, aCanvas);
        start();
    }
    
    public void run()
    {
        double lastTime, currentTime, elapsedTime;
        lastTime = System.currentTimeMillis();

        // move the ball repeatedly until it falls 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, Y_SPEED * elapsedTime);
            pause(DELAY_TIME);
        }
        
        ball.removeFromCanvas();
    }
    
}