E
- the type of elements held in this collectionpublic interface Deque<E> extends Queue<E>
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 | Returns special value | Throws exception | Returns 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 to 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 |
offer(e) |
offerLast(e) |
add(e) |
addLast(e) |
poll() |
pollFirst() |
remove() |
removeFirst() |
peek() |
peekFirst() |
element() |
getFirst() |
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 dequeue 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.
Modifier and Type | Method and Description |
---|---|
boolean |
add(E e)
Inserts the specified element into the queue represented by this
deque unless it would violate capacity restrictions.
|
void |
addFirst(E e)
Inserts the specified element to the front of this deque unless it
would violate capacity restrictions.
|
void |
addLast(E e)
Inserts the specified element to the end of this deque unless it would
violate capacity restrictions.
|
E |
element()
Retrieves, but does not remove, the head of the queue represented by
this deque.
|
E |
getFirst()
Retrieves, but does not remove, the first element of this
deque.
|
E |
getLast()
Retrieves, but does not remove, the last element of this
deque.
|
Iterator<E> |
iterator()
Returns an iterator over the elements in this deque.
|
boolean |
offer(E e)
Inserts the specified element into the queue represented by this deque
unless it would violate capacity restrictions.
|
boolean |
offerFirst(E e)
Inserts the specified element to the front this deque unless it would
violate capacity restrictions.
|
boolean |
offerLast(E e)
Inserts the specified element to the end of this deque unless it would
violate capacity restrictions.
|
E |
peek()
Retrieves, but does not remove, the head of the queue represented by
this deque, returning null if this deque is empty.
|
E |
peekFirst()
Retrieves, but does not remove, the first element of this deque,
returning null if this deque is empty.
|
E |
peekLast()
Retrieves, but does not remove, the last element of this deque,
returning null if this deque is empty.
|
E |
poll()
Retrieves and removes the head of the queue represented by
this deque, or null if this deque is empty.
|
E |
pollFirst()
Retrieves and removes the first element of this deque, or
null if this deque is empty.
|
E |
pollLast()
Retrieves and removes the last element of this deque, or
null if this deque is empty.
|
E |
pop()
Pops an element from the stack represented by this deque.
|
void |
push(E e)
Pushes an element onto the stack represented by this deque.
|
E |
remove()
Retrieves and removes the head of the queue represented by this deque.
|
E |
removeFirst()
Removes and returns the first element of this deque.
|
boolean |
removeFirstOccurrence(Object e)
Removes the first occurrence of the specified element in this
deque.
|
E |
removeLast()
Retrieves and removes the last element of this deque.
|
boolean |
removeLastOccurrence(Object e)
Removes the last occurrence of the specified element in this
deque.
|
addAll, clear, contains, containsAll, equals, hashCode, isEmpty, parallelStream, remove, removeAll, removeIf, retainAll, size, spliterator, stream, toArray, toArray
boolean offerFirst(E e)
e
- the element to insertNullPointerException
- if e is null and this
deque does not permit null elementsboolean offerLast(E e)
e
- the element to insertNullPointerException
- if e is null and this
deque does not permit null elementsvoid addFirst(E e)
e
- the element to insertIllegalStateException
- if it was not possible to insert
the element due to capacity restrictionsNullPointerException
- if e is null and this
deque does not permit null elementsvoid addLast(E e)
e
- the element to insertIllegalStateException
- if it was not possible to insert
the element due to capacity restrictionsNullPointerException
- if e is null and this
deque does not permit null elementsE pollFirst()
E pollLast()
E removeFirst()
NoSuchElementException
- if this deque is emptyE removeLast()
NoSuchElementException
- if this deque is emptyE peekFirst()
E peekLast()
E getFirst()
NoSuchElementException
- if this deque is emptyE getLast()
NoSuchElementException
- if this deque is emptyboolean removeFirstOccurrence(Object e)
e
- element to be removed from this deque, if presentNullPointerException
- if the specified element is nullboolean removeLastOccurrence(Object e)
e
- element to be removed from this deque, if presentNullPointerException
- if the specified element is nullboolean offer(E e)
add(E)
method, which can fail to insert an element only by
throwing an exception.
This method is equivalent to offerLast(E)
.
offer
in interface Queue<E>
e
- the element to insertNullPointerException
- if e is null and this
deque does not permit null elementsboolean add(E e)
This method is equivalent to addLast(E)
.
add
in interface Collection<E>
add
in interface Queue<E>
e
- the element to insertCollection.add(E)
)IllegalStateException
- if it was not possible to insert
the element due to capacity restrictionsNullPointerException
- if e is null and this
deque does not permit null elementsE poll()
This method is equivalent to pollFirst()
.
E remove()
This method is equivalent to removeFirst()
.
remove
in interface Queue<E>
NoSuchElementException
- if this deque is emptyE peek()
This method is equivalent to peekFirst()
E element()
This method is equivalent to getFirst()
element
in interface Queue<E>
NoSuchElementException
- if this deque is emptyvoid push(E e)
This method is equivalent to addFirst(E)
.
IllegalStateException
- if it was not possible to insert
the element due to capacity restrictionsNullPointerException
- if e is null and this
deque does not permit null elementsE pop()
This method is equivalent to removeFirst()
.
NoSuchElementException
- if this deque is empty