IllegalMonitorStateException on notify() when synchronized on an Integer

Question

I\'m new to using wait() and notify() in Java and I\'m getting an IllegalMonitorStateException.

Main Code

public class ThreadTest          


        

Answer 1

This

private static Integer state = 0;

is equivalent to

private static Integer state = Integer.valueOf(0);

The invocation of valueOf(0) returns a reference to an Integer object, call it A.

You then do

synchronized(state) {

your thread acquires the lock on the object referenced by state, currently that is A.

You then do

state = 1;

which is equivalent to

state = Integer.valueOf(1);

which gives you a different reference to an Integer object, call it B, and assigns it to state. When you then call

state.notify();

you're invoking notify() on an object, B, for which your thread doesn't own the monitor. You can't call notify or wait on objects for which your thread doesn't own the monitor.