java - Why does notifyAll() raise IllegalMonitorStateException when synchronized on Integer? -

why test program result in java.lang.illegalmonitorstateexception?

public class test {     static integer foo = new integer(1);     public static void main(string[] args) {         synchronized(foo) {             foo++;             foo.notifyall();         }         system.err.println("success");     } } 

result:

exception in thread "main" java.lang.illegalmonitorstateexception         @ java.lang.object.notifyall(native method)         @ test.main(test.java:6) 

you have noted correctly notifyall must called synchronized block.

however, in case, because of auto-boxing, object synchronized on not same instance invoked notifyall on. in fact, new, incremented foo instance still confined stack, , no other threads possibly blocked on wait call.

you implement own, mutable counter on synchronization performed. depending on application, might find atomicinteger meets needs.