public Set<K> keySet() {
        if (keySet == null) {
            keySet = new ReferenceKeySet<K, V>(this);
        }
        return keySet;
    }
public Set < K > keySet ( ) { if ( keySet == null ) { keySet = new ReferenceKeySet < K , V > ( this ) ; } return keySet ; }
Returns a set view of this map's keys.

@return a set view of this map's keys