multithreading - Why doesnt this Java loop in a thread work?

Question

I'm coding a radio in java and am using a thread to of course play the stream in. I have a while loop in the run method of the thread which checks if a variable called shouldPlay is true, and if it is it runs the player. Then, I have a pause() method which sets the variable to false and naturally I want it to not run the player when the variable is false. I've created a small example of how I've set it up below:

class audiostream extends Thread {

    private boolean shouldPlay = true;

    @Override
    public void run() {
        while(true)
        { 
            if(shouldPlay)
            {
                shouldPlay = false;
                System.out.print("test");
            }
        }
    }

    public void pause() {
        shouldPlay = false;
    }

    public void play() {
        shouldPlay = true;
    }

}

What I want to happen is when the thread first runs, it should print "test" (which it does fine). Then when you run pause() it should set shouldPlay to false and not allow "test" to be printed out. But then when you call play() it should set shouldPlay to true then print out test again. The problem is it's not doing this.

About 1/100th of the time it will though. Am I right to assume the problem here is even though I call play() and pause() from another thread as not to "hang" the parent thread, that my child thread is hanging on that while loop and when calling play() or pause() from the parent the variables don't change value because of that loop? If that's the case I don't know what I should do!

Answer 1

add 'volatile' will fix it

private volatile boolean shouldPlay = true;