Question

Why does making the lock object private encapsulate the lock so that client code cannot acquire it ?

Object’s Intrinsic Lock

   public class A{
        private final Set<Integer> set = new HashSet<Integer>();

        public synchronized void addInt(int i){
            set.add(i);
        }
   }

Private Lock

   public class B{
        private final Set<Integer> set = new HashSet<Integer>();

        public void addInt(int i){
             synchronized(set){
                  set.add(i);
             }
        }
   }

Answer 1

Well, another class can’t access set at all, because it’s private. Of the many things everyone else can’t do because they don’t have access to that reference, synchronizing on it is one.

If a getter returns that reference directly (without wrapping or copying the object), someone else can synchronize on it, defeating the benefits of a lock on a private object.