What's the best way to return variables from a syncExec?

Question

In my SWT Java app I often want to return information from inside a Display.syncExec() call. The best way I've found so far to do this is:

final ArrayList<Integer> result = new ArrayList<Integer>();
GUI.display().syncExec(new Runnable(){ public void run() {
   MessageBox mb = /* ... */;
    /* set up messagebox */
   result.add(mb.open());
}});
if (SWT.OK == result.get(0)) { /* ... */ }

I think this is allowed because ArrayList is thread-safe, but is there a better container I should be using, or an easier way altogether?

Answer 1

ArrayList is not thread-safe. From the relevant Javadoc:

Note that this implementation is not synchronized. If multiple threads access an ArrayList instance concurrently, and at least one of the threads modifies the list structurally, it must be synchronized externally.

If you need a thread-safe implementation of List, there are (at least) two provided in the JDK: CopyOnWriteArrayList and Vector.

Answer 2

final AtomicInteger resultAtomicInteger = new AtomicInteger();
Display.getCurrent().syncExec(new Runnable() { 
    public void run() {
        MessageBox mb = /* ... */;
            /* set up messagebox */
        resultAtomicInteger.set(mb.open());
}});
if (SWT.OK == resultAtomicInteger.get()) { /* ... */ }

Answer 3

You could use an Integer[1] array to make it more concise but I don't think it can directly update a non-final variable from inside an anonymous inner class.

final Integer[] result = new Integer[1];

I thought that you had to declare results as final (but that change wouldn't affect your code). Since the current Thread blocks until the inner Thread is finished I don't think you need to worry about synchronization (but you might need to make the variable violate so that you see the result).

Answer 4

If this happens often, you better use subscribe/notify model between your process and view. Your view subscribes to the event which should trigger that message box and gets notified when conditions met.