Strange behavior of java game loop

Question

I have a simple game loop in java:

public void run(){
    while(running){
        start = System.nanoTime();
        gamePanel.update();
        gamePanel.repaint();
        elapsed = System.nanoTime() - start;
        wait = (TARGET_TIME - elapsed) / 1000000;
        if(wait < 0){wait = TARGET_TIME;}
        try {Thread.sleep(wait);}catch(Exception e) {e.printStackTrace();}
    }
}

Now the problem: When i add a sysout in update() outing "updating" and one in paintComponent() outing "repainting", i get following result:

updating
updating
updating
updating
updating
(x1000 and more)
repainting

So when i repaint one time, the game is updating 1000 and more times. Is it normal? I think its abit strange, isnt it? For example can do 1000 steps with the player until the game is painting it... That means, the update() method doesnt wait for repaint finishing? Why?

Thank you!

Edit: Here the update code:

public void update(){
    if(moveUp){
        changeMapY(map, -map.getSpeed());
    }
    if(moveDown){
        changeMapY(map, map.getSpeed());
    }
    if(moveRight){
        changeMapX(map, map.getSpeed());
    }
    if(moveLeft){
        changeMapX(map, -map.getSpeed());
    }
}

Edit 2: And this is what changeMap does: (maybe this is the problem?)

public void changeMapX(Map map, int amount){
    for(int i = 0; i < blocks.length; i++){
        for(int j = 0; j < blocks[0].length; j++){
            blocks[i][j].addX(amount);
        }
    }
    map.addOffsetx(amount);
}

public void changeMapY(Map map, int amount){
    for(int i = 0; i < blocks.length; i++){
        for(int j = 0; j < blocks[0].length; j++){
            blocks[i][j].addY(amount);
        }
    }
    map.addOffsety(amount);
}

Maybe you have loop in `gamePanel.update();`, which shows `updating` 1000 times? — Pawel, Sep 03 '13 at 17:41
Could we see some of the code in update? (I assume update is also not recursive correct?) — Jordan, Sep 03 '13 at 17:45
@T_01 ok, does `getSpeed()` happen to call `update()` in some way? — Jordan, Sep 03 '13 at 17:55
no no getSpeed just returns an int. Look on my edit, i postet some more code. — T_01, Sep 03 '13 at 18:04
Have you tried without the loop? Just to see, what the output is. — Fildor, Sep 03 '13 at 18:09

score 0 · Answer 1 · answered Sep 03 '13 at 17:44

0

You calculate wait in seconds. (nano-seconds / 1 million)

The parameter to sleep should be in miliseconds. So change this line:

wait = (TARGET_TIME - elapsed) / 1000;

answered Sep 03 '13 at 17:44

Lee Meador

12,829
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That's true, but it does not answer the question, does it? – Fildor Sep 03 '13 at 17:49
@Fildor If he waits 1/1000th as long as he should before looping back, it will cause the loop to be executed 1000 times as often. I think that constitutes an answer. – Lee Meador Sep 03 '13 at 17:51
Hmmm its okay, because hes right and it does help... Now it updates 1 to 3 times by redrawing 1 time. But anyway, the update method isnt wating... – T_01 Sep 03 '13 at 17:52
@LeeMeador true, but if it was simply a problem of his `while` loop running too often, wouldn't he be getting `Updating, Repainting, Updating, Repainting` and so on? – Jordan Sep 03 '13 at 17:57
@LeeMeador I disagree. I think he'd expect always an update followed by a repaint immediately. Waiting for too long should cause them both to be executed too often, or am I missing something perhaps? It's part of the answer though, you're right. – Fildor Sep 03 '13 at 17:58
@Fildor We haven't seen the repaint code, perhaps it only paints if something changes or a certain time has elapsed or whatever. – Lee Meador Sep 03 '13 at 18:05
@LeeMeador See his comment to my question what the output without the loop is. I guess you are right that something probably keeps repaint from being executed, when using that loop. Could be that it has to do with it not being executed on the EDT or something like that? – Fildor Sep 03 '13 at 19:13

score 0 · Answer 2 · answered Sep 03 '13 at 17:50

0

I tested your source code and "updating", "repainting" are one after each other. The problem is connected with your update() method. Maybe recursion, maybe loop.

answered Sep 03 '13 at 17:50

Pawel

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score 0 · Answer 3 · answered Sep 04 '13 at 22:03

0

Assuming that gamePanel is a Swing component, what you're probably observing is that repaint() doesn't actually call JComponent.paintComponent(). All it does is essentially invalidates the component's graphics and tells Swing to repaint the component at a future point in time, on the Swing thread.

Multiple calls to repaint() don't necessarily result in multiple calls to paintComponent() - redundant repaint requests are collapsed into one.

answered Sep 04 '13 at 22:03

TrogDor

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repaint calls paintComponent of course, and paintComponent is calling super.paintComponents(); – T_01 Sep 05 '13 at 07:51
The default implementation of repaint() does not. If you override it, it would be a useful thing to mention. – TrogDor Sep 06 '13 at 20:41

Strange behavior of java game loop

3 Answers3