synchronizing by this vs dummy object

Question

I have seen usage of synchronized block by this so far but recently I learned that using dummy object is preferable. I found the following topic related to this.

Java sy

Answer 1

You are correct. In this case you need two different objects to synchronize on them separately.

For locking purpose the easiest way is to create Object objects.

Object lock1 = new Object();
Object lock2 = new Object();

Answer 2

That is exactly the point of lock objects - you can use different locks for different operations. Assuming it makes sense to run addA and addB concurrently (and from the looks of it - it definitely does), you should indeed have two separate locks, one for each method.