E
- the type of elements held in this collection
public 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
Returns
true if this deque contains the specified element.
|
Iterator |
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 |
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
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
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
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, toArray
void 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 add
IllegalStateException
- if the element cannot be added at this time due to capacity restrictions
ClassCastException
- if the class of the specified element prevents it from being added to this deque
NullPointerException
- if the specified element is null and this deque does not permit null elements
IllegalArgumentException
- if some property of the specified element prevents it from being added to this deque
void 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 add
IllegalStateException
- if the element cannot be added at this time due to capacity restrictions
ClassCastException
- if the class of the specified element prevents it from being added to this deque
NullPointerException
- if the specified element is null and this deque does not permit null elements
IllegalArgumentException
- if some property of the specified element prevents it from being added to this deque
boolean offerFirst(E e)
addFirst(E)
method, which can fail to insert an element only by throwing an exception.
e
- the element to add
true
if the element was added to this deque, else
false
ClassCastException
- if the class of the specified element prevents it from being added to this deque
NullPointerException
- if the specified element is null and this deque does not permit null elements
IllegalArgumentException
- if some property of the specified element prevents it from being added to this deque
boolean offerLast(E e)
addLast(E)
method, which can fail to insert an element only by throwing an exception.
e
- the element to add
true
if the element was added to this deque, else
false
ClassCastException
- if the class of the specified element prevents it from being added to this deque
NullPointerException
- if the specified element is null and this deque does not permit null elements
IllegalArgumentException
- if some property of the specified element prevents it from being added to this deque
E removeFirst()
pollFirst
only in that it throws an exception if this deque is empty.
NoSuchElementException
- if this deque is empty
E removeLast()
pollLast
only in that it throws an exception if this deque is empty.
NoSuchElementException
- if this deque is empty
E pollFirst()
null
if this deque is empty.
null
if this deque is empty
E pollLast()
null
if this deque is empty.
null
if this deque is empty
E getFirst()
peekFirst
only in that it throws an exception if this deque is empty.
NoSuchElementException
- if this deque is empty
E getLast()
peekLast
only in that it throws an exception if this deque is empty.
NoSuchElementException
- if this deque is empty
E peekFirst()
null
if this deque is empty.
null
if this deque is empty
E peekLast()
null
if this deque is empty.
null
if this deque is empty
boolean removeFirstOccurrence(Objecto)
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 present
true
if an element was removed as a result of this call
ClassCastException
- 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(Objecto)
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 present
true
if an element was removed as a result of this call
ClassCastException
- 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 add
true
(as specified by
Collection.add(E)
)
IllegalStateException
- if the element cannot be added at this time due to capacity restrictions
ClassCastException
- if the class of the specified element prevents it from being added to this deque
NullPointerException
- if the specified element is null and this deque does not permit null elements
IllegalArgumentException
- if some property of the specified element prevents it from being added to this deque
boolean 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 add
true
if the element was added to this deque, else
false
ClassCastException
- if the class of the specified element prevents it from being added to this deque
NullPointerException
- if the specified element is null and this deque does not permit null elements
IllegalArgumentException
- if some property of the specified element prevents it from being added to this deque
E 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 empty
E 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 empty
E 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 push
IllegalStateException
- if the element cannot be added at this time due to capacity restrictions
ClassCastException
- if the class of the specified element prevents it from being added to this deque
NullPointerException
- if the specified element is null and this deque does not permit null elements
IllegalArgumentException
- if some property of the specified element prevents it from being added to this deque
E pop()
This method is equivalent to removeFirst()
.
NoSuchElementException
- if this deque is empty
boolean remove(Objecto)
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 present
true
if an element was removed as a result of this call
ClassCastException
- 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(Objecto)
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 tested
true
if this deque contains the specified element
ClassCastException
- 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()