How to use synchronized(obj) ?

Question

Hi, dear all:
It may be a basic lesson, but I need to know how synchronized(obj) exactly work.
Code as below :
Boolean flag = new Boolean(false);
......// some other codes
synchronized(flag)
  flag = Boolean.TRUE;
  flag.notify(); // or flag.wait();
}Why the notify() or wait() will cause IllegalMonitorStateExcption ?
If I can't change flag object in synchronized block, how can I change its value ?
In JDK documents, there seems no method of Boolean class can do this job ....
Or, It must be written as below ?
boolean bflag = false;
Boolean flag = new Boolean(false);
synchronized(flag)
  bflag = true;
  flag.notify();
} Thank you for help !
Edited by: jesvh on Dec 2, 2008 11:54 PM

Answer 1

jesvh wrote:
If I can't change flag object in synchronized block, how can I change its value ?You need to synchronize on something else.
  Boolean flag = new Boolean(false);
  Object lockObject = new Object();
  synchronized(lockObject )
     flag = Boolean.TRUE;
    lockObject.notify();
  }