code: @Override
      public boolean replace(K key, V oldValue, V newValue) {
          return rMap.replace(key, oldValue, newValue);
      }
docs: Replaces the entry for a key only if currently mapped to a given value.
  This is equivalent to
  <pre>
  if (map.containsKey(key) &amp;&amp; map.get(key).equals(oldValue)) {
  map.put(key, newValue);
  return true;
  } else return false;</pre>
  except that the action is performed atomically.
  
  @param key key with which the specified value is associated
  @param oldValue value expected to be associated with the specified key
  @param newValue value to be associated with the specified key
  @return <tt>true</tt> if the value was replaced
  @throws UnsupportedOperationException if the <tt>put</tt> operation
  is not supported by this map
  @throws ClassCastException if the class of a specified key or value
  prevents it from being stored in this map
  @throws NullPointerException if a specified key or value is null,
  and this map does not permit null keys or values
  @throws IllegalArgumentException if some property of a specified key
  or value prevents it from being stored in this map