How do I pause a thread indefinitely and then resume it again (in Java)?

I have a program in which I want to do an animation and I have two buttons: one to start the animation and one to stop it. What I would like to do is to be able to press the Stop button to pause the animation and then press my Start button to resume it. I am using a separate thread to do the animation stuff. So far, the relevant code is as follows:

Thread t = new Thread(new Animator());

...

startButton.addActionListener(new
	    ActionListener()
	    {
		public void actionPerformed(ActionEvent e)
		{
	            t.start();    
		}
	    });

stopButton.addActionListener(new
	    ActionListener()
	    {
		public void actionPerformed(ActionEvent e)
		{
		    t.interrupt();
		}
	    });

Animator is a class that implements the Runnable interface, obviously. Initially, I can press the Start button and then press Stop to pause. Pressing Start again doesn't re-start the animation because you can't call start() more than once on a thread, so an IllegalThreadStateException is thrown. How, then, do I do what I want to do?

Thanks .

I don't know Java but I have been using threads in Python, and I assume the functions provided are similar.

The best way of communicating between threads is queues. So you'd have thread 1 which does the GUI part, and thread 2 which does the animation.

In thread 1, whenever a key is pressed, you just put the strings "stop" or "start" into that queue.

Thread 2 would go something like this, assuming the thread only starts when you hit the button:

loop
  read(queue,no_timeout) until message=='start'
  loop
    { do animation here }
    if read(queue,timeout=0)=='stop': break
  endloop
endloop

Have you read about the Java Concurrency Utilities, such as handling Interrupts?

Edit: More relevant seems to be The article explaining why Thread.stop, Thread.suspend and Thread.resume are deprecated. If you don't understand the synchronisation problems, you should read up. :-)

as was mentioned the suspend and resume are deprecated because they are not safe..

consider something like this as an alternative..

public void run(){
    while(running){
        while(paused);
        
        //real work here
    }
}

control the paused var to give the appearance of starting and stopping the thread

Originally Posted by xhi while(paused);

ouch !
You CPU isn't going to like that one ...

Yeah, this is true. I tried to do it using wait() and notify(), but I'm having problems getting that to work (I won't post code now though).

Originally Posted by jlliagre ouch ! You CPU isn't going to like that one ...

yeh, i guess ouch is right. im not sure where i was going with that one.

maybe something on the order of wrapping the code to be executed in an if

public void run(){
    while(running){
        if(!paused)
        {
          // do work
        }
    }
}

xhi, surely the crux of your code's problem is the inherent concurrency issue of reading paused==true while another thread's just changed it to false in its allocated registers, but not yet saved that back to memory. This may then mean that two threads attempt to modify variables which should have mutex access, or worse.

Wikipedia's article on Monitors isn't Java specific, but a start if you're having problems with wait() and notify(). Their Java Concurrency is a little sparse. The Sun Java Concurrency Utilites page may be more help.