E
- the type of elements held in this collectionpublic interface Deque<E> extends Queue<E>
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.
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. 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)
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. 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)
Inserts the specified element at the front of this deque unless it would violate capacity restrictions. When using a capacity-restricted deque, this method is generally preferable to the 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 false
ClassCastException
- 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)
Inserts the specified element at the end of this deque unless it would violate capacity restrictions. When using a capacity-restricted deque, this method is generally preferable to the 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 false
ClassCastException
- 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()
Retrieves and removes the first element of this deque. This method differs from pollFirst
only in that it throws an exception if this deque is empty.
NoSuchElementException
- if this deque is emptyE removeLast()
Retrieves and removes the last element of this deque. This method differs from pollLast
only in that it throws an exception if this deque is empty.
NoSuchElementException
- if this deque is emptyE pollFirst()
Retrieves and removes the first element of this deque, or returns null
if this deque is empty.
null
if this deque is emptyE pollLast()
Retrieves and removes the last element of this deque, or returns null
if this deque is empty.
null
if this deque is emptyE getFirst()
Retrieves, but does not remove, the first element of this deque. This method differs from peekFirst
only in that it throws an exception if this deque is empty.
NoSuchElementException
- if this deque is emptyE getLast()
Retrieves, but does not remove, the last element of this deque. This method differs from peekLast
only in that it throws an exception if this deque is empty.
NoSuchElementException
- if this deque is emptyE peekFirst()
Retrieves, but does not remove, the first element of this deque, or returns null
if this deque is empty.
null
if this deque is emptyE peekLast()
Retrieves, but does not remove, the last element of this deque, or returns null
if this deque is empty.
null
if this deque is emptyboolean removeFirstOccurrence(Object o)
Removes the first occurrence of the specified element from this deque. If the deque does not contain the element, it is unchanged. More formally, removes the first element 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)
Removes the last occurrence of the specified element from this deque. If the deque does not contain the element, it is unchanged. More formally, removes the last element 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)
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. 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)
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. 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 false
ClassCastException
- 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()
Retrieves and removes the head of the queue represented by this deque (in other words, the first element of this deque). This method differs from 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()
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.
This method is equivalent to pollFirst()
.
poll
in interface Queue<E>
null
if this deque is emptyE element()
Retrieves, but does not remove, the head of the queue represented by this deque (in other words, the first element of this deque). This method differs from 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()
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.
This method is equivalent to peekFirst()
.
peek
in interface Queue<E>
null
if this deque is emptyvoid 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.
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()
Pops an element from the stack represented by this deque. In other words, removes and returns the first element of this deque.
This method is equivalent to removeFirst()
.
NoSuchElementException
- if this deque is emptyboolean remove(Object o)
Removes the first occurrence of the specified element from this deque. If the deque does not contain the element, it is unchanged. More formally, removes the first element 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)
Returns 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()
Returns the number of elements in this deque.
size
in interface Collection<E>
Iterator<E> iterator()
Returns an iterator over the elements in this deque in proper sequence. The elements will be returned in order from first (head) to last (tail).
iterator
in interface Collection<E>
iterator
in interface Iterable<E>
Iterator<E> descendingIterator()
Returns an iterator over the elements in this deque in reverse sequential order. The elements will be returned in order from last (tail) to first (head).
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