Multithreading multiball game

Question

I am working on a school project about multiple balls bouncing. So far, I managed to create the app and everything works ok. But, I also need to implement multi-threading in the app, and this is where I am stuck. I was thinking of one ball one thread, but I am not sure of how to implement it. Here is my code so far (part):

public void paintComponent(Graphics g)
        {
            super.paintComponent(g);

            //The balls are painted only after the timer is started
            if(bTimer)
            {
                for(Ball ball:ballList.ballsArrayList) 
                {
                    Thread ballThread = new Thread(ball);
                    ballThread.start();
                    ball.draw(g);

                    /*other code for moving the ball*/
                }
            }
        }

In the class Ball:

     public void draw(Graphics g) {
          Color color = new Color(this.getColorR(),this.getColorG(),this.getColorB());
          g.setColor(color);
          int radius = this.getsize();
          g.fillOval((int)(this.getX() - radius), (int)(this.getY() - radius), (int)(2 *     
          radius), (int)(2 * radius));

       }

     public void run() {  

            String name = Thread.currentThread().getName();  

            for (int i = 0; i < 200; i++) {  

                //ball.draw(g); ??

                try {  
                    Thread.sleep(50); 
                    System.out.println("Sleeping");
                } catch (Exception ex) {}  
            }  
        }

I was thinking that I could put the ball.draw() function in the run() function for the thread. But I don't know how I can do that or if it's a good idea. Multi-threading is still difficult for me to understand and implement =((

Having one ball per thread is certainly an overkill. You can have thousands of balls! Just process all the balls sequentially in one thread. — Vlad, Nov 02 '12 at 10:35
In any case, please please don't put your business logic into paint function! You need to separate business logic managing the balls, model containing all the physics, and UI which only displays the results. — Vlad, Nov 02 '12 at 10:38
@Burkhard Swing is single-threaded, but it has its own dedicated thread (the Event Dispatch Thread) — Robin, Nov 02 '12 at 10:39
@Vlad: Thanks for the advice! I will change my code for that part. — user1719821, Nov 02 '12 at 10:41
I would suggest you have a look at [this](http://stackoverflow.com/questions/13022754/java-bouncing-ball/13022788#13022788), in particular, the "A Different Approach" section ;) — MadProgrammer, Nov 02 '12 at 22:33
Ok I moved the business logic out of paint function and creat a move() function in the Ball class. I read a bit more on EDT and changed my main function to include an invokeLater method, so the GUI is created on the EDT thread. In the project, an XML file needs to be parsed to get information like speed, size etc about the ball. So I was thinking of moving that parsing part into a thread. But I am not sure how to do it. Should I create another post for this question? — user1719821, Nov 03 '12 at 10:59

score 2 · Answer 1 · answered Nov 02 '12 at 10:38

Not a real answer, but too long to put in a comment.

You should note that Swing is not thread-safe. All Swing components should be accessed on the Event Dispatch Thread, and on that thread only. See the Concurrency in Swing guide for more information.

This means that you can have one thread per ball which updates the position of the ball. However, if you access the position of the ball during the painting, this access happens on the EDT. Meaning that you cannot update the position of the ball in your background thread at any moment. You will have to implement some locking or simply update the position on the EDT.

I am not sure what you try to achieve, but if you simply want to update the position of a ball at certain time intervals I would opt for a javax.swing.Timer. This timer is triggered on the EDT, allowing you to update the position in a thread-safe manner. The Swing wiki tag contains some more links for implementing animation in Swing.

I am already using Timer to update the position of the ball. And I also thought that one thread per ball might be a little too much because like Vlad said above, a thousand balls = a thousand threads! But, the question was asking for multi-threading and that got me a bit confused as to what to do next. I will check the link you gave me. Thanks!! — user1719821, Nov 02 '12 at 10:43

score 1 · Answer 2 · answered Nov 02 '12 at 11:04

All swing code has to run on the event dispatching thread. Therefore what you're doing in the code snippets is bad.

However, if calculating ball positions is cpu intensive and requires time, you do want to move the logic in a separate thread, otherwise your UI will become unresponsive.

This would become a typical producer/consumer problem: one thread produces cooridnates and the event dispatch thread consumes them by drawing the balls.

Multithreading multiball game

2 Answers2