question: public int addAllAbsent ( Collection < ? extends E > c ) { Object [ ] cs = c . toArray ( ) ; if ( cs . length == 0 ) return 0 ; final ReentrantLock lock = this . lock ; lock . lock ( ) ; try { Object [ ] elements = getArray ( ) ; int len = elements . length ; int added = 0 ; for ( int i = 0 ; i < cs . length ; ++ i ) { Object e = cs [ i ] ; if ( indexOf ( e , elements , 0 , len ) < 0 && indexOf ( e , cs , 0 , added ) < 0 ) cs [ added ++ ] = e ; } if ( added > 0 ) { Object [ ] newElements = Arrays . copyOf ( elements , len + added ) ; System . arraycopy ( cs , 0 , newElements , len , added ) ; setArray ( newElements ) ; } return added ; } finally { lock . unlock ( ) ; } }
target:  Appends all of the elements in the specified collection that are not already contained in this list, to the end of this list, in the order that they are returned by the specified collection's iterator.