question: public synchronized boolean retainAll ( Collection c ) { Object [ ] elements = getArray ( ) ; int len = elements . length ; if ( len != 0 ) { int newlen = 0 ; Object [ ] temp = new Object [ len ] ; for ( int i = 0 ; i < len ; ++ i ) { Object element = elements [ i ] ; if ( c . contains ( element ) ) temp [ newlen ++ ] = element ; } if ( newlen != len ) { setArray ( copyOfRange ( temp , 0 , newlen , Object [ ] . class ) ) ; return true ; } } return false ; }
target:  Retains only the elements in this list that are contained in the specified collection.  In other words, removes from this list all of its elements that are not contained in the specified collection.