| java.util.Deque<E> |
 Known Indirect Subclasses
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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.
| Public Methods | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
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.
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Inserts the specified element at the front of this deque if it is
possible to do so immediately without violating capacity restrictions.
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Inserts the specified element at the end of this deque if it is
possible to do so immediately without violating capacity restrictions.
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Returns true if this deque contains the specified element.
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Returns an iterator over the elements in this deque in reverse
sequential order.
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Retrieves, but does not remove, the head of the queue represented by
this deque (in other words, the first element of this deque).
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Retrieves, but does not remove, the first element of this deque.
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Retrieves, but does not remove, the last element of this deque.
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Returns an iterator over the elements in this deque in proper sequence.
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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.
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Inserts the specified element at the front of this deque unless it would
violate capacity restrictions.
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Inserts the specified element at the end of this deque unless it would
violate capacity restrictions.
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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.
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Retrieves, but does not remove, the first element of this deque,
or returns null if this deque is empty.
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Retrieves, but does not remove, the last element of this deque,
or returns null if this deque is empty.
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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.
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Retrieves and removes the first element of this deque,
or returns null if this deque is empty.
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Retrieves and removes the last element of this deque,
or returns null if this deque is empty.
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Pops an element from the stack represented by this deque.
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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, returning
true upon success and throwing an
IllegalStateException if no space is currently available.
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Retrieves and removes the head of the queue represented by this deque
(in other words, the first element of this deque).
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Removes the first occurrence of the specified element from this deque.
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Retrieves and removes the first element of this deque.
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Removes the first occurrence of the specified element from this deque.
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Retrieves and removes the last element of this deque.
| |||||||||||
Removes the last occurrence of the specified element from this deque.
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Returns the number of elements in this deque.
| |||||||||||
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[Expand]
Inherited Methods | |||||||||||
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From interface
java.lang.Iterable
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From interface
java.util.Collection
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From interface
java.util.Queue
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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).
| e | the element to add |
|---|
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 |
Inserts the specified element at the front of this deque if it is
possible to do so immediately without violating capacity restrictions.
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 |
Inserts the specified element at the end of this deque if it is
possible to do so immediately without violating capacity restrictions.
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 |
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)).
| o | element whose presence in this deque is to be tested |
|---|
| 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) |
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).
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().
| NoSuchElementException | if this deque is empty |
|---|
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 empty |
|---|
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 empty |
|---|
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).
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).
| e | the element to add |
|---|
| 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 |
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 add |
|---|
| 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 |
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 add |
|---|
| 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 |
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().
Retrieves, but does not remove, the first element of this deque, or returns null if this deque is empty.
Retrieves, but does not remove, the last element of this deque, or returns null if this deque is empty.
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().
Retrieves and removes the first element of this deque, or returns null if this deque is empty.
Retrieves and removes the last element of this deque, or returns null if this deque is empty.
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 empty |
|---|
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, returning true upon success and throwing an 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 |
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().
| NoSuchElementException | if this deque is empty |
|---|
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).
| o | element to be removed from this deque, if present |
|---|
| 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) |
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 empty |
|---|
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 present |
|---|
| 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) |
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 empty |
|---|
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 present |
|---|
| 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) |
Returns the number of elements in this deque.