@Override
    public V replace(K key, V value) {
        return rMap.replace(key, value);
    }
@ Override public V replace ( K key , V value ) { return rMap . replace ( key , value ) ; }
Replaces the entry for a key only if currently mapped to some value.
This is equivalent to
<pre>
if (map.containsKey(key)) {
return map.put(key, value);
} else return null;</pre>
except that the action is performed atomically.

@param key key with which the specified value is associated
@param value value to be associated with the specified key
@return the previous value associated with the specified key, or
<tt>null</tt> if there was no mapping for the key.
(A <tt>null</tt> return can also indicate that the map
previously associated <tt>null</tt> with the key,
if the implementation supports null values.)
@throws UnsupportedOperationException if the <tt>put</tt> operation
is not supported by this map
@throws ClassCastException if the class of the specified key or value
prevents it from being stored in this map
@throws NullPointerException if the specified key or value is null,
and this map does not permit null keys or values
@throws IllegalArgumentException if some property of the specified key
or value prevents it from being stored in this map