Interface Deque<E>
- Type Parameters:
E- the type of elements held in this deque
- All Superinterfaces:
Collection<E>,Iterable<E>,Queue<E>,SequencedCollection<E>
- All Known Subinterfaces:
BlockingDeque<E>
- All Known Implementing Classes:
ArrayDeque,ConcurrentLinkedDeque,LinkedBlockingDeque,LinkedList
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 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 indicate 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.
- Since:
- 1.6
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Method Summary
Modifier and TypeMethodDescriptionbooleanInserts 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, returningtrueupon success and throwing anIllegalStateExceptionif no space is currently available.booleanaddAll(Collection<? extends E> c) Adds all of the elements in the specified collection at the end of this deque, as if by callingaddLast(E)on each one, in the order that they are returned by the collection's iterator.voidInserts the specified element at the front of this deque if it is possible to do so immediately without violating capacity restrictions, throwing anIllegalStateExceptionif no space is currently available.voidInserts the specified element at the end of this deque if it is possible to do so immediately without violating capacity restrictions, throwing anIllegalStateExceptionif no space is currently available.booleanReturnstrueif this deque contains the specified element.Returns an iterator over the elements in this deque in reverse sequential order.element()Retrieves, but does not remove, the head of the queue represented by this deque (in other words, the first element of this deque).getFirst()Retrieves, but does not remove, the first element of this deque.getLast()Retrieves, but does not remove, the last element of this deque.iterator()Returns an iterator over the elements in this deque in proper sequence.booleanInserts 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, returningtrueupon success andfalseif no space is currently available.booleanofferFirst(E e) Inserts the specified element at the front of this deque unless it would violate capacity restrictions.booleanInserts the specified element at the end of this deque unless it would violate capacity restrictions.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 returnsnullif this deque is empty.Retrieves, but does not remove, the first element of this deque, or returnsnullif this deque is empty.peekLast()Retrieves, but does not remove, the last element of this deque, or returnsnullif this deque is empty.poll()Retrieves and removes the head of the queue represented by this deque (in other words, the first element of this deque), or returnsnullif this deque is empty.Retrieves and removes the first element of this deque, or returnsnullif this deque is empty.pollLast()Retrieves and removes the last element of this deque, or returnsnullif this deque is empty.pop()Pops an element from the stack represented by this deque.voidPushes 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 anIllegalStateExceptionif no space is currently available.remove()Retrieves and removes the head of the queue represented by this deque (in other words, the first element of this deque).booleanRemoves the first occurrence of the specified element from this deque.Retrieves and removes the first element of this deque.booleanRemoves the first occurrence of the specified element from this deque.Retrieves and removes the last element of this deque.booleanRemoves the last occurrence of the specified element from this deque.reversed()Returns a reverse-ordered view of this collection.intsize()Returns the number of elements in this deque.Methods declared in interface java.util.Collection
clear, containsAll, equals, hashCode, isEmpty, parallelStream, removeAll, removeIf, retainAll, spliterator, stream, toArray, toArray, toArray
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Method Details
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addFirst
Inserts the specified element at the front of this deque if it is possible to do so immediately without violating capacity restrictions, throwing anIllegalStateExceptionif no space is currently available. When using a capacity-restricted deque, it is generally preferable to use methodofferFirst(E).- Specified by:
addFirstin interfaceSequencedCollection<E>- Parameters:
e- the element to add- Throws:
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 deque
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addLast
Inserts the specified element at the end of this deque if it is possible to do so immediately without violating capacity restrictions, throwing anIllegalStateExceptionif no space is currently available. When using a capacity-restricted deque, it is generally preferable to use methodofferLast(E).This method is equivalent to
add(E).- Specified by:
addLastin interfaceSequencedCollection<E>- Parameters:
e- the element to add- Throws:
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 deque
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offerFirst
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 theaddFirst(E)method, which can fail to insert an element only by throwing an exception.- Parameters:
e- the element to add- Returns:
trueif the element was added to this deque, elsefalse- Throws:
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 deque
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offerLast
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 theaddLast(E)method, which can fail to insert an element only by throwing an exception.- Parameters:
e- the element to add- Returns:
trueif the element was added to this deque, elsefalse- Throws:
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 deque
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removeFirst
E removeFirst()Retrieves and removes the first element of this deque. This method differs frompollFirstonly in that it throws an exception if this deque is empty.- Specified by:
removeFirstin interfaceSequencedCollection<E>- Returns:
- the head of this deque
- Throws:
NoSuchElementException- if this deque is empty
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removeLast
E removeLast()Retrieves and removes the last element of this deque. This method differs frompollLastonly in that it throws an exception if this deque is empty.- Specified by:
removeLastin interfaceSequencedCollection<E>- Returns:
- the tail of this deque
- Throws:
NoSuchElementException- if this deque is empty
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pollFirst
E pollFirst()Retrieves and removes the first element of this deque, or returnsnullif this deque is empty.- Returns:
- the head of this deque, or
nullif this deque is empty
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pollLast
E pollLast()Retrieves and removes the last element of this deque, or returnsnullif this deque is empty.- Returns:
- the tail of this deque, or
nullif this deque is empty
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getFirst
E getFirst()Retrieves, but does not remove, the first element of this deque. This method differs frompeekFirstonly in that it throws an exception if this deque is empty.- Specified by:
getFirstin interfaceSequencedCollection<E>- Returns:
- the head of this deque
- Throws:
NoSuchElementException- if this deque is empty
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getLast
E getLast()Retrieves, but does not remove, the last element of this deque. This method differs frompeekLastonly in that it throws an exception if this deque is empty.- Specified by:
getLastin interfaceSequencedCollection<E>- Returns:
- the tail of this deque
- Throws:
NoSuchElementException- if this deque is empty
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peekFirst
E peekFirst()Retrieves, but does not remove, the first element of this deque, or returnsnullif this deque is empty.- Returns:
- the head of this deque, or
nullif this deque is empty
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peekLast
E peekLast()Retrieves, but does not remove, the last element of this deque, or returnsnullif this deque is empty.- Returns:
- the tail of this deque, or
nullif this deque is empty
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removeFirstOccurrence
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 elementesuch thatObjects.equals(o, e)(if such an element exists). Returnstrueif this deque contained the specified element (or equivalently, if this deque changed as a result of the call).- Parameters:
o- element to be removed from this deque, if present- Returns:
trueif an element was removed as a result of this call- Throws:
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)
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removeLastOccurrence
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 elementesuch thatObjects.equals(o, e)(if such an element exists). Returnstrueif this deque contained the specified element (or equivalently, if this deque changed as a result of the call).- Parameters:
o- element to be removed from this deque, if present- Returns:
trueif an element was removed as a result of this call- Throws:
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)
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add
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, returningtrueupon success and throwing anIllegalStateExceptionif no space is currently available. When using a capacity-restricted deque, it is generally preferable to useoffer.This method is equivalent to
addLast(E).- Specified by:
addin interfaceCollection<E>- Specified by:
addin interfaceQueue<E>- Parameters:
e- the element to add- Returns:
true(as specified byCollection.add(E))- Throws:
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 deque
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offer
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, returningtrueupon success andfalseif no space is currently available. When using a capacity-restricted deque, this method is generally preferable to theadd(E)method, which can fail to insert an element only by throwing an exception.This method is equivalent to
offerLast(E).- Specified by:
offerin interfaceQueue<E>- Parameters:
e- the element to add- Returns:
trueif the element was added to this deque, elsefalse- Throws:
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 deque
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remove
E 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 frompoll()only in that it throws an exception if this deque is empty.This method is equivalent to
removeFirst().- Specified by:
removein interfaceQueue<E>- Returns:
- the head of the queue represented by this deque
- Throws:
NoSuchElementException- if this deque is empty
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poll
E poll()Retrieves and removes the head of the queue represented by this deque (in other words, the first element of this deque), or returnsnullif this deque is empty.This method is equivalent to
pollFirst(). -
element
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). This method differs frompeekonly in that it throws an exception if this deque is empty.This method is equivalent to
getFirst().- Specified by:
elementin interfaceQueue<E>- Returns:
- the head of the queue represented by this deque
- Throws:
NoSuchElementException- if this deque is empty
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peek
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 returnsnullif this deque is empty.This method is equivalent to
peekFirst(). -
addAll
Adds all of the elements in the specified collection at the end of this deque, as if by callingaddLast(E)on each one, in the order that they are returned by the collection's iterator.When using a capacity-restricted deque, it is generally preferable to call
offerseparately on each element.An exception encountered while trying to add an element may result in only some of the elements having been successfully added when the associated exception is thrown.
- Specified by:
addAllin interfaceCollection<E>- Parameters:
c- the elements to be inserted into this deque- Returns:
trueif this deque changed as a result of the call- Throws:
IllegalStateException- if not all the elements can be added at this time due to insertion restrictionsClassCastException- if the class of an element of the specified collection prevents it from being added to this dequeNullPointerException- if the specified collection contains a null element and this deque does not permit null elements, or if the specified collection is nullIllegalArgumentException- if some property of an element of the specified collection prevents it from being added to this deque- See Also:
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push
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 anIllegalStateExceptionif no space is currently available.This method is equivalent to
addFirst(E).- Parameters:
e- the element to push- Throws:
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 deque
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pop
E 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().- Returns:
- the element at the front of this deque (which is the top of the stack represented by this deque)
- Throws:
NoSuchElementException- if this deque is empty
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remove
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 elementesuch thatObjects.equals(o, e)(if such an element exists). Returnstrueif 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).- Specified by:
removein interfaceCollection<E>- Parameters:
o- element to be removed from this deque, if present- Returns:
trueif an element was removed as a result of this call- Throws:
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)
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contains
Returnstrueif this deque contains the specified element. More formally, returnstrueif and only if this deque contains at least one elementesuch thatObjects.equals(o, e).- Specified by:
containsin interfaceCollection<E>- Parameters:
o- element whose presence in this deque is to be tested- Returns:
trueif this deque contains the specified element- Throws:
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)
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size
int size()Returns the number of elements in this deque.- Specified by:
sizein interfaceCollection<E>- Returns:
- the number of elements in this deque
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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). -
descendingIterator
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reversed
Returns a reverse-ordered view of this collection. The encounter order of elements in the returned view is the inverse of the encounter order of elements in this collection. The reverse ordering affects all order-sensitive operations, including those on the view collections of the returned view. If the collection implementation permits modifications to this view, the modifications "write through" to the underlying collection. Changes to the underlying collection might or might not be visible in this reversed view, depending upon the implementation.- Specified by:
reversedin interfaceSequencedCollection<E>- Implementation Requirements:
- The implementation in this interface returns a reverse-ordered Deque
view. The
reversed()method of the view returns a reference to this Deque. Other operations on the view are implemented via calls to public methods on this Deque. The exact relationship between calls on the view and calls on this Deque is unspecified. However, order-sensitive operations generally behave as if they delegate to the appropriate method with the opposite orientation. For example, callinggetFirston the view might result in a call togetLaston this Deque. - Returns:
- a reverse-ordered view of this collection, as a
Deque - Since:
- 21
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