Swing Tutorial - ProgressMonitors

Question

I was looking at the java examples on the Swing Tutorial examples in regard to using ProgressMonitors, and I was wondering whether the variables that are being modified (i.e. task in this case), whether they should be set as volatile, since wouldn't they be altered in different threads?
thread 1:
keeps altering the value of where the progress bar should be.
thread 2:
using the cached version of this value (since not volatile), not realising that it is updating, and so the status bar stays in the same location?
Can anyone clear this up please? Am I just very confused?
Many thanks,
Edward

Answer 1

Looking at the examples (ProgressBarDemo.java) at http://java.sun.com/docs/books/tutorial/uiswing/components/progress.html, it looks like neither ProgressBarDemo.java or LongTask.java are using volatile. They're not even declaring their methods synchronized. I don't think it matters, as long as only one thread will ever be writing the variable value. If the progress bar thread reads it at the wrong time, it will just get the old value of the variable, right before it is updated by the task thread.
I don't think you need to worry about volatility unless both threads are working on the same variable in the same scope. For instance, if you progress bar thread is calling a getProgressValue() type method on a task object (as in the tutorial code), the value will be returned fresh from the method, and will not be cached. However, if your task thread is writing a progress value directly into the scope of the progress bar thread, your progress bar thread might not get the new value unless the value is declared volatile.
That's just my take after Googling "java volatile" and reading a bit on it. I've never had to use the volatile keyword before. I think generally it is better to avoid situations where you would have to use it. For instance, make the progress bar thread retrieve the value through a method call from the task object, as in the example.
Nick