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.