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    <p>I'd suggest you not synchronize on entire method <code>put</code>. But <strong>synchronize only on on specific cell</strong>. In code below, I'll describe how to do that:</p> <pre><code>class ConcurrMattr { private ConcurrentHashMap&lt;Integer, Lock&gt; locks = new ConcurrentHashMap&lt;Integer, Lock&gt;(); public Cell put( CellCoords key, Cell arg1 ) { // get or create lock for specific cell (guarantee its uniqueness) Lock lock = this.locks.putIfAbsent( coords.hash % 64, new ReentrantLock() ); // 64 threads may concurrently modify different cells of matrix try { // lock only specific cell lock.lock(); // do all you need with cell return ...; } finally { // unlock cell lock.unlock(); } } } // Immutable class to represent cell coordinates class CellCoords { public final int x; public final int y; public final int hash; public CellCoords( int x, int y ) { this.x = x; this.y = y; this.hash = this.calcHash(); } private int calcHash() { int result = 31 + this.x; return 31 * result + this.y; } } </code></pre> <p>So you may synchronize <code>read/write</code> methods on specific cell, while <strong>other part of matrix will be accessible to other threads</strong>.</p> <p>Look at javadoc for <a href="http://docs.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/ConcurrentHashMap.html" rel="nofollow"><code>ConcurrentHashMap</code></a> and for <a href="http://docs.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/locks/Lock.html" rel="nofollow"><code>Lock</code></a></p> <p><strong>P.S.</strong> <br/> You may notice, that <code>CellCoords</code> field <code>hash</code> has type <code>int</code>. To avoid growth of locks map size till 2^31, you have to <strong>limit range of hashes</strong>. For example: <code>(coords.hash % 64)</code> - allows only 64 concurrent threads to work with entire matrix.</p> <p>P.P.S. Might be interesting article for you: <a href="http://www.ibm.com/developerworks/java/library/j-jtp08223/" rel="nofollow">http://www.ibm.com/developerworks/java/library/j-jtp08223/</a></p>
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    1. COReading around a little I saw the interface for ReadWriteLock. This may be the exact solution I'm looking for! It does mean I need locks in my get functions, but at least they are clean and concurrent.
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    2. CO@K.Barad, you may use read and write locks separately, only if you can guarantee that there won't be a race condition: when one thread write data, and other thread read data from the same cell. Otherwise read-thread can get inconsistent data
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    3. COfrom what I read the ReadWriteLock allows multiple read locks to be acquired, but the write lock is exclusive. While the write lock is held no one else can read, meaning that the write and read locks are in a master-slave style relationship. To me this sounds like the perfect behaviour to achieve what I want unless you see further risks? I will have the write lock all going through the put and remove functions anyway so 90% of the work happens outside the locks, only the 4 modification lines are synched.
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