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copied!<p>I think your problem is that you call <code>swap(.., ..)</code> from multiple threads and the <code>swap</code> (and <code>lockedIndex</code>) method is not thread safe since the member (<code>lock1</code> and <code>lock2</code>) variables could be set by another thread - and you would <code>unlock</code> some other index that you did not lock.</p> <hr> <p>Using <code>tryLock</code> could be tricky, what would you do if you cannot get the lock?</p> <p>I would go for the solution the guys over at the forum described - locking the indexes in a perticular order for more information about the problem you have run into see <a href="http://en.wikipedia.org/wiki/Dining_philosophers_problem" rel="nofollow">Dining philosophers problem</a>.</p> <p>Here is your code re-written to use <code>lock()</code> instead:</p> <pre><code>class ShuffleBase { private int[] data; private ReentrantLock[] locker; public ShuffleBase(int[] data) { this.data = data; this.locker = new ReentrantLock[data.length]; for (int x = 0; x < data.length; x++) this.locker[x] = new ReentrantLock(); } public void swap(int a, int b) { // check if we have to do sothing if (a == b) return; // set the order (we want to lock lower indexes first!) if (b < a) { int tmp = b; b = a; a = tmp; } // lock them in sequence (first the lower, then the higher index) locker[a].lock(); locker[b].lock(); // do the swap int tmp = data[b]; data[b] = data[a]; data[a] = tmp; // unlock locker[b].unlock(); locker[a].unlock(); } public void display() { for (int i = 0; i < locker.length; i++) locker[i].lock(); System.out.println("Swapped array: " + Arrays.toString(data)); for (int i = 0; i < locker.length; i++) locker[i].unlock(); } } </code></pre>

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