When to use synchronized in Java

Question

I hope this is going to be enough information, so here it goes. If you need more info, lemme know in the comments.

I have a class that has two inner classes. The inn

Answer 1

Wrap everything in outerMethod that you want to run only once in a synchronized block:

private void outerMethod() {
    synchronized (this) {
        if(!outerMethodHasBeenCalled) {
            // do stuff
        }

        outerMethodHasBeenCalled = true;
    }
}

That way, the first time the method is called, only one thread will be allowed into the synchronized block at a time. The first one will execute the code in the if statement, then set outerMethodHasBeenCalled to true. The other threads will see that it is true, and skip the if code.

Answer 2

Yup, given what you've laid out above, I'd go with:

private synchronized void outerMethod() {
...
}

Note, this will have the side-effect of blocking one of the callers until the outerMethod() completes. If that is acceptable, cool. If the intent is merely that the code in outerMethod() is run once, and it is OK for the second caller not to be delayed if the first caller is running outerMethod(), you might consider:

public OuterClass {
    private AtomicBoolean outerMethodHasBeenCalled = new AtomicBoolean();

    private void outerMethod() {
        if (outerMethodHasBeenCalled.compareAndSet(false, true)) {
            // do stuff
        }
    }
...

See the JavaDoc for AtomicBoolean to grok what is going on there (assuming it is available in Android's Java).