question: public Set keySet ( ) { if ( keySet == null ) keySet = new SynchronizedSet ( new KeySet ( ) , this ) ; return keySet ; }
target:  Returns a Set view of the keys contained in this Cache. The Set is backed by the Cache, so changes to the Cache are reflected in the Set, and vice-versa. The Set supports element removal (which removes the corresponding entry from the Cache), but not element addition.