public class MyClass {

    private HashSet mySet = new HashSet();

    public synchronized void add(Object in) { // get 'this' monitor'
        synchronized (mySet) {                // get 'mySet' monitor

            mySet.add(in);
        }
    }

    private void doRemove(Object out) {
    }

    public void remove(Object out) {
        synchronized (mySet) {                // get 'mySet' monitor
            doRemove(out);                    // get 'this' monitor

        }
    }
 }