code: public final Object putIfAbsent(Object key,
                                      Object value)
      {
          Object foundObject = null;
          java.util.Map subMap = getSubMap(key);
          synchronized (subMap) {
              foundObject = subMap.get(key);
              if (foundObject == null)
                  subMap.put(key,
                             value);
  //      // Assume that most of the time the key is vacant, but be prepared to put back the original 
  //      // if it is not.
  //      foundObject = subMap.put(key,
  //                               value);
  //      // If there was already something there put back the original.
  //      if (foundObject != null && foundObject != value)
  //        subMap.put(key,
  //                   foundObject);
          } // synchronized (subMap).
          return foundObject;
      }
docs: Add an object to the subset of the Map, if there is no Object
  represented by the key.
  
  @param key of the Object to be inserted into the map.
  @param value to b einserted if there is no Object alreay matching the key.
  
  @return the Object occupying the key or null if there is none.
  @see java.util.concurrent.ConcurrentHashMap#putIfabsent(K, V)