无需删除即可检索元素

//Retrieves, but does not remove, the head of the queue represented by this deque
Object headItem = deque.element();

//Retrieves, but does not remove, the first element of this deque.
Object firstItem = deque.getFirst();

//Retrieves, but does not remove, the last element of this deque.    
Object lastItem  = deque.getLast();