code: @Override
      public boolean containsKey(Object key) {
          if (fast) {
              return (map.containsKey(key));
          } else {
              synchronized (map) {
                  return (map.containsKey(key));
              }
          }
      }
docs: Return <code>true</code> if this map contains a mapping for the
  specified key.
  
  @param key the key to be searched for
  @return true if the map contains the key