我的 Java 程序意外停止正常工作

Question

I am programming a basic java game, but I am having an issue. Everytime that I try it out, if I wait just for 10 seconds, the program stops working correctly. I've made a class called Drawable , which has a paint function. This paint function paints a rectangle on a certain area(given on the constructor). And I have a thread that iterates over all drawables in an arraylist(added randomly, with another thread), and just substracts 1 to their x . When it stops working correctly, the character can jump and does all animations, but the drawables stop moving. At first I thought this might have given a ConcurrentModificationException error, but it didn't print it on the console. So right now I don't really know what to do. Here I add the Drawables :

Thread t2 = new Thread(new Runnable() {
    @Override
    public void run() {
        while (Game.isPlayingGame) {
            try {
                Thread.sleep((long) (Math.random()*2000));
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            obstacles.add(new Drawable(
                 Constants.WIDTH,
                 (int) (Constants.HEIGHT / 2),
                 Constants.WIDTH - 100, 
                 (int) (Constants.HEIGHT / 2) - 100, 
                 Color.BLUE));
        }
    }
});
t2.start();

Here I move the Drawables :

Thread t = new Thread(new Runnable() {  
    @Override
    public void run() {
        while (Game.isPlayingGame) {
            try {
                Thread.sleep(1);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            for (Drawable d : obstacles) {
                d.x -= 1;
                d.x2 -= 1;
                if (d.x2 < 0) {
                    obstacles.remove(d);
                }
            }
        }
    }
});
t.start();

paint method:

@Override
public void paint(Graphics g) {
    super.paint(g);
    floor.paint(g);
    Graphics2D g2d = (Graphics2D) g;
    AffineTransform ant = g2d.getTransform();
    g2d.rotate(Math.toRadians(rotation), 
               character.x - Constants.characterSize / 2, 
               character.y - Constants.characterSize / 2);
    character.paint(g);
    g2d.setTransform(ant);
    for (Drawable d : obstacles) {
        d.paint(g);
        System.out.println(rotation_down);
        if (!rotation_down) {
            if (!character.onCollision(floor)) {
                character.y += (int) gravityAccel; // gravity
                character.y2 += (int) gravityAccel; // gravity
                gravityAccel += 0.1;
            } else {
                Screen.canJump = true;
                gravityAccel = 0;
            }
        }

    }
    repaint();
}

Thanks in advance.

Answer 1

The proper synchronization depends on the exact internals of your Drawable. Also use CopyOnWriteArrayList. For the Drawable class, decrement of x and x2 should be atomic and synchronized with at least paint() method:

synchronized moveToLeft() { 
    x-=1; 
    x2-=1; 
}

However, it does not make sense to have an object with x2 < 0 but this is a separate discussion.

You would also want to have

synchronized getX2() { 
    return x2; 
}

and in your second thread do something like this:

if (d.getX2 == 0) { 
    obstacles.remove(d); 
} 
else { 
    d.moveToLeft(); 
}

The reason to do the check first is that if you do it other way around, you can and up in a situation when x2 is already -1, obstacles.remove(d) has not been called yet, and d.paint() is called. This could cause issues unless your paint() method can handle negative coordinates.