A linear collection that supports element insertion and removal at
both ends. The name deque is short for "double ended queue"
and is usually pronounced "deck". Most Deque
implementations place no fixed limits on the number of elements
they may contain, but this interface supports capacity-restricted
deques as well as those with no fixed size limit.
This interface defines methods to access the elements at both
ends of the deque. Methods are provided to insert, remove, and
examine the element. Each of these methods exists in two forms:
one throws an exception if the operation fails, the other returns a
special value (either null or false, depending on
the operation). The latter form of the insert operation is
designed specifically for use with capacity-restricted
Deque implementations; in most implementations, insert
operations cannot fail.
The twelve methods described above are summarized in the following table:
| First Element (Head) | Last Element (Tail) | |||
| Throws exception | Special value | Throws exception | Special value | |
| Insert | addFirst(e) |
offerFirst(e) |
addLast(e) |
offerLast(e) |
| Remove | removeFirst() |
pollFirst() |
removeLast() |
pollLast() |
| Examine | getFirst() |
peekFirst() |
getLast() |
peekLast() |
This interface extends the Queue interface. When a deque is
used as a queue, FIFO (First-In-First-Out) behavior results. Elements are
added at the end of the deque and removed from the beginning. The methods
inherited from the Queue interface are precisely equivalent to
Deque methods as indicated in the following table:
Queue Method |
Equivalent Deque Method |
add(e) |
addLast(e) |
offer(e) |
offerLast(e) |
remove() |
removeFirst() |
poll() |
pollFirst() |
element() |
getFirst() |
peek() |
peekFirst() |
Deques can also be used as LIFO (Last-In-First-Out) stacks. This
interface should be used in preference to the legacy Stack class.
When a deque is used as a stack, elements are pushed and popped from the
beginning of the deque. Stack methods are precisely equivalent to
Deque methods as indicated in the table below:
| Stack Method | Equivalent Deque Method |
push(e) |
addFirst(e) |
pop() |
removeFirst() |
peek() |
peekFirst() |
Note that the peek method works equally well when
a deque is used as a queue or a stack; in either case, elements are
drawn from the beginning of the deque.
This interface provides two methods to remove interior
elements, removeFirstOccurrence and
removeLastOccurrence.
Unlike the List interface, this interface does not
provide support for indexed access to elements.
While Deque implementations are not strictly required
to prohibit the insertion of null elements, they are strongly
encouraged to do so. Users of any Deque implementations
that do allow null elements are strongly encouraged not to
take advantage of the ability to insert nulls. This is so because
null is used as a special return value by various methods
to indicated that the deque is empty.
Deque implementations generally do not define
element-based versions of the equals and hashCode
methods, but instead inherit the identity-based versions from class
Object.
This interface is a member of the Java Collections Framework.
extends
<E> | the type of elements held in this collection |