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    <p>As per the javadoc for <a href="http://docs.oracle.com/javase/1.5.0/docs/api/java/lang/Object.html#wait%28%29" rel="nofollow">Object#wait()</a></p> <blockquote> <p>The <strong>current thread must own this object's monitor</strong>. The thread releases ownership of this monitor and waits until another thread notifies threads waiting on this object's monitor to wake up either through a call to the notify method or the notifyAll method. The thread then waits until it can re-obtain ownership of the monitor and resumes execution.</p> </blockquote> <p>To use wait/notify the thread must have the lock, otherwise <a href="http://docs.oracle.com/javase/1.5.0/docs/api/java/lang/IllegalMonitorStateException.html" rel="nofollow">IllegalMonitorStateException</a> is thrown</p> <blockquote> <p>Throws: <strong>IllegalMonitorStateException</strong> - if the current thread is not the owner of the object's monitor.</p> </blockquote> <h2>Why</h2> <p>So, <code>wait()</code> makes current thread <em>release</em> the lock</p> <p><code>notify()</code> signals other waiting thread(s) which then <em>try to acquire</em> the lock.</p> <p>For doing either, the current thread <strong>must</strong> have the lock. And it makes sense!</p> <h2><em>Edit</em></h2> <ul> <li><p>Why must thread calling wait() hold the lock is pretty obvious now.</p></li> <li><p>But why must thread calling notify() hold the lock? Well, for one, to prove its <code>authenticity</code>. Otherwise any thread could keep firing <code>false notifications</code> and the <code>waiting</code> threads would keep getting interrupted. <em>Thankfully that is not the case.</em></p></li> </ul>
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    1. POWhy do we need to synchronize on the same object for notify() to work
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    1. COBut why is it necessary to hold the lock to call notify? Saying "because the spec says so" is true but doesn't really answert the question.
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    2. COHope the edit helps.
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    3. COYou write "current thread must have the lock": why does it have to have the lock to signal other threads? They don't care who is holding the lock; they will attempt to acquire it anyway.
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