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    <p>See <a href="https://stackoverflow.com/questions/12023056/java-wait-and-notify-in-a-non-synchronized-block#comment16045203_12023056">my comment</a>. Since <code>IllegalMonitorStateException</code> is never thrown, we know that <code>wait</code> is never being called.</p> <p>Notice you have two instances of <code>counthrd</code>...</p> <pre><code>counthrd cnt1 = new counthrd(); counthrd cnthrd1 = new counthrd("thrd 1", cnt1); </code></pre> <p>See which instance you're calling <code>suspendme</code> and <code>resumeme</code> on?</p> <pre><code>Thread.sleep(1000); System.out.println("going to wait mode"); cnt1.suspendme(); Thread.sleep(1000); System.out.println("resuming"); cnt1.resumeme(); Thread.sleep(1000); </code></pre> <p><code>cnt1</code> is initialized using your no-arg constructor, seen here:</p> <pre><code>counthrd() { } </code></pre> <p>The point is that <code>cnt1</code> never actually starts its own thread. It never does anything, really. <code>cnthrd1</code> is the one that starts a thread, as seen here:</p> <pre><code>counthrd(String s, counthrd cnt1) { thrd = new Thread(this, s); this.cnt1 = cnt1; thrd.start(); x = s; } </code></pre> <p>The point to make is that <code>suspended</code> is an instance field, and not shared between <code>cnt1</code> and <code>cnthrd1</code>. Modifying <code>cnt1.suspended</code> will not cause <code>cnthrd1</code> to go into "wait mode". <code>wait</code> is never called, and thus the exception is never thrown.</p> <p>To demonstrate, try calling <code>suspendme</code> and <code>resumeme</code> on <code>cnthrd1</code>, instead... :-)</p> <pre><code>C:\dev\scrap&gt;javac counter.java C:\dev\scrap&gt;java counter Starting thrd 1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 going to wait mode going to wait mode java.lang.IllegalMonitorStateException resuming </code></pre> <hr> <p>That being said, I figured I'd suggest you do some stuff that your code should be doing.</p> <ol> <li>Declare <code>suspended</code> as <code>volatile</code>. Without some explicit memory ordering guarantees, there's no guarantee when or even <em>if</em> <code>cnthrd1</code> reads the updated value of <code>suspended</code>.</li> <li>Ditch the <code>cnt1</code> field and instance; there's no reason for them. Get rid of that empty constructor, too.</li> <li><code>Thread.currentThread</code> is a static method; you don't need to use an instance for it. That all aside, <code>thrd</code> is guaranteed to equal <code>Thread.currentThread</code> here.</li> <li><code>counthrd.x</code> is equal to <code>thrd.getName</code>; why not just use <code>x</code> instead?</li> <li>Use some better, more descriptive names. For example, instead of <code>x</code>, why not <code>name</code>? Instead of <code>thrd</code>, why not <code>thread</code>? Instead of <code>counthrd</code>, why not <code>CountingThread</code>?</li> <li>You only need to call <code>notify</code> in <code>resumeme</code>, not <code>suspendme</code>. (in fact, calling <code>notify</code> in <code>suspendme</code> could accidentally trigger an <code>InterruptedException</code> if the thread is sleeping i.e. when <code>(i % 10) == 0</code>)</li> <li>You also don't want <code>notify</code> in the <code>while (suspended)</code> loop. Your <code>while</code> loop can actually be turned into an <code>if</code> statement, too, now.</li> <li>As previously stated, you need <code>synchronized (this)</code> around your code that calls <code>while</code>.</li> <li>Avoid doing real logic in the constructor, e.g. <code>thrd.start()</code>. </li> <li><code>suspend</code> doesn't need to be <code>synchronized</code>. <code>resume</code> doesn't need to be <code>synchronized</code>, either; only the <code>wait</code> and <code>notify</code> calls require it.</li> </ol> <p>You can find a modified version of your example that works properly <a href="http://ideone.com/LLjey" rel="nofollow noreferrer">here</a>.</p>
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    1. CO@veer...Thank you so much for taking your time and explaining in detail. I actually wanted to create 2 threads on `counthrd` to interleave the output and I wanted a common reference for these 2 threads to use. So that is why I was trying to create `cnt1` instance. Also in the synchornized block I want to synchronize on this common reference `cnt1` and not on `this` an instance for each thread. Anyways, I found that whatever crazy thing I was trying to do, I still need to use `this` in the synchornized block. I also looked at the modified version. Thanks again.
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