public NavigableSet<K> navigableKeySet() {
        KeySet<K> nks = navigableKeySet;
        return (nks != null) ? nks : (navigableKeySet = new KeySet<>(this));
    }
public NavigableSet < K > navigableKeySet ( ) { KeySet < K > nks = navigableKeySet ; return ( nks != null ) ? nks : ( navigableKeySet = new KeySet < > ( this ) ) ; }
@since 1.6