Question

This seems obvious but I want to make sure that I’m doing this right. If I try to attain a lock on a null reference this will not work correct? Because it uses a property of an object not a reference?

ex

Object lock = null;

synchronized(lock)
{
}

Answer 1

If I try to attain a lock on a null reference this will not work correct?

That is correct. You’ll get a NullPointerException if you try to synchronize using null.

Because it uses a property of an object not a reference?

Not exactly. The lock state is not a property in the normal sense because there is no way you can interrogate it. But yes, the state is part of the object.

(But think about it. How could could the lock state possibly be part of the reference? If it was, how would you be able to synchronize on this or an object reference passed as a parameter? References are passed as copies … and a change to one copy can’t propagate to the others. And if the lock state can’t propagate, then two threads with their own copies of the reference can’t synchronize in any meaningful way.)