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- Type Parameters:
- E- the type of elements held in this deque
 - All Superinterfaces:
- Collection<E>,- Iterable<E>,- Queue<E>
 - All Known Subinterfaces:
- BlockingDeque<E>
 - All Known Implementing Classes:
- ArrayDeque,- ConcurrentLinkedDeque,- LinkedBlockingDeque,- LinkedList
 
 public 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". MostDequeimplementations 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 nullorfalse, depending on the operation). The latter form of the insert operation is designed specifically for use with capacity-restrictedDequeimplementations; in most implementations, insert operations cannot fail.The twelve methods described above are summarized in the following table: Summary of Deque methods 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 Queueinterface. 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 theQueueinterface are precisely equivalent toDequemethods as indicated in the following table:Comparison of Queue and Deque methods QueueMethodEquivalent DequeMethodadd(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 Stackclass. When a deque is used as a stack, elements are pushed and popped from the beginning of the deque. Stack methods are equivalent toDequemethods as indicated in the table below:Comparison of Stack and Deque methods Stack Method Equivalent DequeMethodpush(e)addFirst(e)pop()removeFirst()peek()getFirst()Note that the peekmethod 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, removeFirstOccurrenceandremoveLastOccurrence.Unlike the Listinterface, this interface does not provide support for indexed access to elements.While Dequeimplementations are not strictly required to prohibit the insertion of null elements, they are strongly encouraged to do so. Users of anyDequeimplementations that do allow null elements are strongly encouraged not to take advantage of the ability to insert nulls. This is so becausenullis used as a special return value by various methods to indicate that the deque is empty.Dequeimplementations generally do not define element-based versions of theequalsandhashCodemethods, but instead inherit the identity-based versions from classObject.This interface is a member of the Java Collections Framework. - Since:
- 1.6
 
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Method SummaryAll Methods Instance Methods Abstract Methods Modifier and Type Method Description booleanadd(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, 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.voidaddFirst(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 anIllegalStateExceptionif no space is currently available.voidaddLast(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 anIllegalStateExceptionif no space is currently available.booleancontains(Object o)Returnstrueif this deque contains the specified element.Iterator<E>descendingIterator()Returns an iterator over the elements in this deque in reverse sequential order.Eelement()Retrieves, but does not remove, the head of the queue represented by this deque (in other words, the first element of this deque).EgetFirst()Retrieves, but does not remove, the first element of this deque.EgetLast()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.booleanoffer(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, 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.booleanofferLast(E e)Inserts the specified element at the end of this deque unless it would violate capacity restrictions.Epeek()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.EpeekFirst()Retrieves, but does not remove, the first element of this deque, or returnsnullif this deque is empty.EpeekLast()Retrieves, but does not remove, the last element of this deque, or returnsnullif this deque is empty.Epoll()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.EpollFirst()Retrieves and removes the first element of this deque, or returnsnullif this deque is empty.EpollLast()Retrieves and removes the last element of this deque, or returnsnullif this deque is empty.Epop()Pops an element from the stack represented by this deque.voidpush(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 anIllegalStateExceptionif no space is currently available.Eremove()Retrieves and removes the head of the queue represented by this deque (in other words, the first element of this deque).booleanremove(Object o)Removes the first occurrence of the specified element from this deque.EremoveFirst()Retrieves and removes the first element of this deque.booleanremoveFirstOccurrence(Object o)Removes the first occurrence of the specified element from this deque.EremoveLast()Retrieves and removes the last element of this deque.booleanremoveLastOccurrence(Object o)Removes the last occurrence of the specified element from this deque.intsize()Returns the number of elements in this deque.- 
Methods declared in interface java.util.Collectionclear, containsAll, equals, hashCode, isEmpty, parallelStream, removeAll, removeIf, retainAll, spliterator, stream, toArray, toArray, toArray
 
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Method Detail- 
addFirstvoid 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 anIllegalStateExceptionif no space is currently available. When using a capacity-restricted deque, it is generally preferable to use methodofferFirst(E).- Parameters:
- e- the element to add
- Throws:
- 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
 
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addLastvoid 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 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).- Parameters:
- e- the element to add
- Throws:
- 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
 
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offerFirstboolean 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 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, else- false
- Throws:
- 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
 
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offerLastboolean 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 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, else- false
- Throws:
- 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
 
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removeFirstE 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.- Returns:
- the head of this deque
- Throws:
- NoSuchElementException- if this deque is empty
 
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removeLastE 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.- Returns:
- the tail of this deque
- Throws:
- NoSuchElementException- if this deque is empty
 
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pollFirstE 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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pollLastE 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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getFirstE 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.- Returns:
- the head of this deque
- Throws:
- NoSuchElementException- if this deque is empty
 
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getLastE 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.- Returns:
- the tail of this deque
- Throws:
- NoSuchElementException- if this deque is empty
 
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peekFirstE 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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peekLastE 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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removeFirstOccurrenceboolean 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 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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removeLastOccurrenceboolean 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 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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addboolean 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, 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 interface- Collection<E>
- Specified by:
- addin interface- Queue<E>
- Parameters:
- e- the element to add
- Returns:
- true(as specified by- Collection.add(E))
- Throws:
- 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
 
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offerboolean 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, 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 interface- Queue<E>
- Parameters:
- e- the element to add
- Returns:
- trueif the element was added to this deque, else- false
- Throws:
- 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
 
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removeE 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 interface- Queue<E>
- Returns:
- the head of the queue represented by this deque
- Throws:
- NoSuchElementException- if this deque is empty
 
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pollE 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().
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elementE 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 interface- Queue<E>
- Returns:
- the head of the queue represented by this deque
- Throws:
- NoSuchElementException- if this deque is empty
 
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peekE 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().
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addAllboolean addAll(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.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 interface- Collection<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 restrictions
- ClassCastException- if the class of an element of the specified collection prevents it from being added to this deque
- NullPointerException- if the specified collection contains a null element and this deque does not permit null elements, or if the specified collection is null
- IllegalArgumentException- if some property of an element of the specified collection prevents it from being added to this deque
- See Also:
- Collection.add(Object)
 
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pushvoid 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 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 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
 
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popE 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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removeboolean 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 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 interface- Collection<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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containsboolean contains(Object o) 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 interface- Collection<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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sizeint size() Returns the number of elements in this deque.- Specified by:
- sizein interface- Collection<E>
- Returns:
- the number of elements in this deque
 
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iteratorIterator<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).
 
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