title: ConcurrentHashMap locking
body: <p>I have read somewhere that in <code>ConcurrentHashMap</code>, the whole map object is not locked and instead a lock is made on a portion of the Map. </p>
  
  <p>Can somebody elaborate when does locking come into the picture?</p>
  
  <p>Is it right that while reading the Map there is no locking involved in it but while updating it only locking is used?</p>