question: @ Override public synchronized CacheObject < V > put ( K key , CacheObject < V > value ) { if ( cache . size ( ) >= maxSize ) { if ( removeExpired ( ) <= 0 ) { if ( cacheFullRemoveType instanceof RemoveTypeNotRemove ) { return null ; } if ( fullRemoveOne ( ) == null ) { return null ; } } } value . setEnterTime ( System . currentTimeMillis ( ) ) ; cache . put ( key , value ) ; return value ; }
target:  put element, key and value both not allowed to be null