E - the type of elements held in this collectionpublic interface Deque<E> extends Queue<E>
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.
| Modifier and Type | Method and Description |
|---|---|
boolean |
add(E e)
Inserts the specified element into the queue represented by this deque
(in other words, at the tail of this deque) if it is possible to do so
immediately without violating capacity restrictions, returning
true upon success and throwing an
IllegalStateException if no space is currently available. |
void |
addFirst(E e)
Inserts the specified element at the front of this deque if it is
possible to do so immediately without violating capacity restrictions,
throwing an
IllegalStateException if no space is currently
available. |
void |
addLast(E e)
Inserts the specified element at the end of this deque if it is
possible to do so immediately without violating capacity restrictions,
throwing an
IllegalStateException if no space is currently
available. |
boolean |
contains(Object o)
Returns
true if this deque contains the specified element. |
Iterator<E> |
descendingIterator()
Returns an iterator over the elements in this deque in reverse
sequential order.
|
E |
element()
Retrieves, but does not remove, the head of the queue represented by
this deque (in other words, the first element of 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 in proper sequence.
|
boolean |
offer(E e)
Inserts the specified element into the queue represented by this deque
(in other words, at the tail of this deque) if it is possible to do so
immediately without violating capacity restrictions, returning
true upon success and false if no space is currently
available. |
boolean |
offerFirst(E e)
Inserts the specified element at the front of this deque unless it would
violate capacity restrictions.
|
boolean |
offerLast(E e)
Inserts the specified element at 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 (in other words, the first element of this deque), or
returns
null if this deque is empty. |
E |
peekFirst()
Retrieves, but does not remove, the first element of this deque,
or returns
null if this deque is empty. |
E |
peekLast()
Retrieves, but does not remove, the last element of this deque,
or returns
null if this deque is empty. |
E |
poll()
Retrieves and removes the head of the queue represented by this deque
(in other words, the first element of this deque), or returns
null if this deque is empty. |
E |
pollFirst()
Retrieves and removes the first element of this deque,
or returns
null if this deque is empty. |
E |
pollLast()
Retrieves and removes the last element of this deque,
or returns
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 (in other
words, at the head of this deque) if it is possible to do so
immediately without violating capacity restrictions, throwing an
IllegalStateException if no space is currently available. |
E |
remove()
Retrieves and removes the head of the queue represented by this deque
(in other words, the first element of this deque).
|
boolean |
remove(Object o)
Removes the first occurrence of the specified element from this deque.
|
E |
removeFirst()
Retrieves and removes the first element of this deque.
|
boolean |
removeFirstOccurrence(Object o)
Removes the first occurrence of the specified element from this deque.
|
E |
removeLast()
Retrieves and removes the last element of this deque.
|
boolean |
removeLastOccurrence(Object o)
Removes the last occurrence of the specified element from this deque.
|
int |
size()
Returns the number of elements in this deque.
|
addAll, clear, containsAll, equals, hashCode, isEmpty, parallelStream, removeAll, removeIf, retainAll, spliterator, stream, toArray, toArrayvoid addFirst(E e)
IllegalStateException if no space is currently
available. When using a capacity-restricted deque, it is generally
preferable to use method offerFirst(E).e - the element to addIllegalStateException - if the element cannot be added at this
time due to capacity restrictionsClassCastException - if the class of the specified element
prevents it from being added to this dequeNullPointerException - if the specified element is null and this
deque does not permit null elementsIllegalArgumentException - if some property of the specified
element prevents it from being added to this dequevoid addLast(E e)
IllegalStateException if no space is currently
available. When using a capacity-restricted deque, it is generally
preferable to use method offerLast(E).
This method is equivalent to add(E).
e - the element to addIllegalStateException - if the element cannot be added at this
time due to capacity restrictionsClassCastException - if the class of the specified element
prevents it from being added to this dequeNullPointerException - if the specified element is null and this
deque does not permit null elementsIllegalArgumentException - if some property of the specified
element prevents it from being added to this dequeboolean offerFirst(E e)
addFirst(E) method,
which can fail to insert an element only by throwing an exception.e - the element to addtrue if the element was added to this deque, else
falseClassCastException - if the class of the specified element
prevents it from being added to this dequeNullPointerException - if the specified element is null and this
deque does not permit null elementsIllegalArgumentException - if some property of the specified
element prevents it from being added to this dequeboolean offerLast(E e)
addLast(E) method,
which can fail to insert an element only by throwing an exception.e - the element to addtrue if the element was added to this deque, else
falseClassCastException - if the class of the specified element
prevents it from being added to this dequeNullPointerException - if the specified element is null and this
deque does not permit null elementsIllegalArgumentException - if some property of the specified
element prevents it from being added to this dequeE removeFirst()
pollFirst only in that it throws an
exception if this deque is empty.NoSuchElementException - if this deque is emptyE removeLast()
pollLast only in that it throws an
exception if this deque is empty.NoSuchElementException - if this deque is emptyE pollFirst()
null if this deque is empty.null if this deque is emptyE pollLast()
null if this deque is empty.null if this deque is emptyE getFirst()
peekFirst only in that it
throws an exception if this deque is empty.NoSuchElementException - if this deque is emptyE getLast()
peekLast only in that it
throws an exception if this deque is empty.NoSuchElementException - if this deque is emptyE peekFirst()
null if this deque is empty.null if this deque is emptyE peekLast()
null if this deque is empty.null if this deque is emptyboolean removeFirstOccurrence(Object o)
e such that
(o==null ? e==null : o.equals(e))
(if such an element exists).
Returns true if this deque contained the specified element
(or equivalently, if this deque changed as a result of the call).o - element to be removed from this deque, if presenttrue if an element was removed as a result of this callClassCastException - if the class of the specified element
is incompatible with this deque
(optional)NullPointerException - if the specified element is null and this
deque does not permit null elements
(optional)boolean removeLastOccurrence(Object o)
e such that
(o==null ? e==null : o.equals(e))
(if such an element exists).
Returns true if this deque contained the specified element
(or equivalently, if this deque changed as a result of the call).o - element to be removed from this deque, if presenttrue if an element was removed as a result of this callClassCastException - if the class of the specified element
is incompatible with this deque
(optional)NullPointerException - if the specified element is null and this
deque does not permit null elements
(optional)boolean add(E e)
true upon success and throwing an
IllegalStateException if no space is currently available.
When using a capacity-restricted deque, it is generally preferable to
use offer.
This method is equivalent to addLast(E).
add in interface Collection<E>add in interface Queue<E>e - the element to addtrue (as specified by Collection.add(E))IllegalStateException - if the element cannot be added at this
time due to capacity restrictionsClassCastException - if the class of the specified element
prevents it from being added to this dequeNullPointerException - if the specified element is null and this
deque does not permit null elementsIllegalArgumentException - if some property of the specified
element prevents it from being added to this dequeboolean offer(E e)
true upon success and false if no space is currently
available. When using a capacity-restricted deque, this method is
generally preferable to the 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 addtrue if the element was added to this deque, else
falseClassCastException - if the class of the specified element
prevents it from being added to this dequeNullPointerException - if the specified element is null and this
deque does not permit null elementsIllegalArgumentException - if some property of the specified
element prevents it from being added to this dequeE remove()
poll only in that it throws an
exception if this deque is empty.
This method is equivalent to removeFirst().
remove in interface Queue<E>NoSuchElementException - if this deque is emptyE poll()
null if this deque is empty.
This method is equivalent to pollFirst().
E element()
peek only in that it throws an
exception if this deque is empty.
This method is equivalent to getFirst().
element in interface Queue<E>NoSuchElementException - if this deque is emptyE peek()
null if this deque is empty.
This method is equivalent to peekFirst().
void push(E e)
IllegalStateException if no space is currently available.
This method is equivalent to addFirst(E).
e - the element to pushIllegalStateException - if the element cannot be added at this
time due to capacity restrictionsClassCastException - if the class of the specified element
prevents it from being added to this dequeNullPointerException - if the specified element is null and this
deque does not permit null elementsIllegalArgumentException - if some property of the specified
element prevents it from being added to this dequeE pop()
This method is equivalent to removeFirst().
NoSuchElementException - if this deque is emptyboolean remove(Object o)
e such that
(o==null ? e==null : o.equals(e))
(if such an element exists).
Returns true if this deque contained the specified element
(or equivalently, if this deque changed as a result of the call).
This method is equivalent to removeFirstOccurrence(Object).
remove in interface Collection<E>o - element to be removed from this deque, if presenttrue if an element was removed as a result of this callClassCastException - if the class of the specified element
is incompatible with this deque
(optional)NullPointerException - if the specified element is null and this
deque does not permit null elements
(optional)boolean contains(Object o)
true if this deque contains the specified element.
More formally, returns true if and only if this deque contains
at least one element e such that
(o==null ? e==null : o.equals(e)).contains in interface Collection<E>o - element whose presence in this deque is to be testedtrue if this deque contains the specified elementClassCastException - if the type of the specified element
is incompatible with this deque
(optional)NullPointerException - if the specified element is null and this
deque does not permit null elements
(optional)int size()
size in interface Collection<E>Iterator<E> iterator()
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