- Type Parameters:
- E- the type of elements held in this deque
- All Implemented Interfaces:
- Serializable,- Cloneable,- Iterable<E>,- Collection<E>,- Deque<E>,- Queue<E>,- SequencedCollection<E>
Deque interface.  Array
 deques have no capacity restrictions; they grow as necessary to support
 usage.  They are not thread-safe; in the absence of external
 synchronization, they do not support concurrent access by multiple threads.
 Null elements are prohibited.  This class is likely to be faster than
 Stack when used as a stack, and faster than LinkedList
 when used as a queue.
 Most ArrayDeque operations run in amortized constant time.
 Exceptions include
 remove,
 removeFirstOccurrence,
 removeLastOccurrence,
 contains,
 iterator.remove(),
 and the bulk operations, all of which run in linear time.
 
The iterators returned by this class's iterator
 method are fail-fast: If the deque is modified at any time after
 the iterator is created, in any way except through the iterator's own
 remove method, the iterator will generally throw a ConcurrentModificationException.  Thus, in the face of concurrent
 modification, the iterator fails quickly and cleanly, rather than risking
 arbitrary, non-deterministic behavior at an undetermined time in the
 future.
 
Note that the fail-fast behavior of an iterator cannot be guaranteed
 as it is, generally speaking, impossible to make any hard guarantees in the
 presence of unsynchronized concurrent modification.  Fail-fast iterators
 throw ConcurrentModificationException on a best-effort basis.
 Therefore, it would be wrong to write a program that depended on this
 exception for its correctness: the fail-fast behavior of iterators
 should be used only to detect bugs.
 
This class and its iterator implement all of the optional methods of the
 Collection, SequencedCollection, and Iterator interfaces.
 
This class is a member of the Java Collections Framework.
- Since:
- 1.6
- See Also:
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Constructor SummaryConstructorsConstructorDescriptionConstructs an empty array deque with an initial capacity sufficient to hold 16 elements.ArrayDeque(int numElements) Constructs an empty array deque with an initial capacity sufficient to hold the specified number of elements.ArrayDeque(Collection<? extends E> c) Constructs a deque containing the elements of the specified collection, in the order they are returned by the collection's iterator.
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Method SummaryModifier and TypeMethodDescriptionbooleanInserts the specified element at the end of this deque.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.voidInserts the specified element at the end of this deque.voidclear()Removes all of the elements from this deque.clone()Returns a copy of this deque.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.voidPerforms the given action for each element of theIterableuntil all elements have been processed or the action throws an exception.getFirst()Retrieves, but does not remove, the first element of this deque.getLast()Retrieves, but does not remove, the last element of this deque.booleanisEmpty()Returnstrueif this deque contains no elements.iterator()Returns an iterator over the elements in this deque.booleanInserts the specified element at the end of this deque.booleanofferFirst(E e) Inserts the specified element at the front of this deque.booleanInserts the specified element at the end of this deque.peek()Retrieves, but does not remove, the head of the queue represented by 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.remove()Retrieves and removes the head of the queue represented by this deque.booleanRemoves a single instance of the specified element from this deque.booleanremoveAll(Collection<?> c) Removes all of this collection's elements that are also contained in the specified collection (optional operation).Retrieves and removes the first element of this deque.booleanRemoves the first occurrence of the specified element in this deque (when traversing the deque from head to tail).booleanRemoves all of the elements of this collection that satisfy the given predicate.Retrieves and removes the last element of this deque.booleanRemoves the last occurrence of the specified element in this deque (when traversing the deque from head to tail).booleanretainAll(Collection<?> c) Retains only the elements in this collection that are contained in the specified collection (optional operation).intsize()Returns the number of elements in this deque.Creates a late-binding and fail-fastSpliteratorover the elements in this deque.Object[]toArray()Returns an array containing all of the elements in this deque in proper sequence (from first to last element).<T> T[]toArray(T[] a) Returns an array containing all of the elements in this deque in proper sequence (from first to last element); the runtime type of the returned array is that of the specified array.Methods declared in class java.util.AbstractCollectioncontainsAll, toStringMethods declared in class java.lang.Objectequals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, waitMethods declared in interface java.util.CollectioncontainsAll, equals, hashCode, parallelStream, stream, toArray
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Constructor Details- 
ArrayDequepublic ArrayDeque()Constructs an empty array deque with an initial capacity sufficient to hold 16 elements.
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ArrayDequepublic ArrayDeque(int numElements) Constructs an empty array deque with an initial capacity sufficient to hold the specified number of elements.- Parameters:
- numElements- lower bound on initial capacity of the deque
 
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ArrayDequeConstructs a deque containing the elements of the specified collection, in the order they are returned by the collection's iterator. (The first element returned by the collection's iterator becomes the first element, or front of the deque.)- Parameters:
- c- the collection whose elements are to be placed into the deque
- Throws:
- NullPointerException- if the specified collection is null
 
 
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Method Details- 
addFirstInserts the specified element at the front of this deque.- Specified by:
- addFirstin interface- Deque<E>
- Specified by:
- addFirstin interface- SequencedCollection<E>
- Parameters:
- e- the element to add
- Throws:
- NullPointerException- if the specified element is null
 
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addLastInserts the specified element at the end of this deque.This method is equivalent to add(E).- Specified by:
- addLastin interface- Deque<E>
- Specified by:
- addLastin interface- SequencedCollection<E>
- Parameters:
- e- the element to add
- Throws:
- NullPointerException- if the specified element is null
 
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addAllAdds 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.- Specified by:
- addAllin interface- Collection<E>
- Specified by:
- addAllin interface- Deque<E>
- Overrides:
- addAllin class- AbstractCollection<E>
- Parameters:
- c- the elements to be inserted into this deque
- Returns:
- trueif this deque changed as a result of the call
- Throws:
- NullPointerException- if the specified collection or any of its elements are null
- See Also:
 
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offerFirstInserts the specified element at the front of this deque.- Specified by:
- offerFirstin interface- Deque<E>
- Parameters:
- e- the element to add
- Returns:
- true(as specified by- Deque.offerFirst(E))
- Throws:
- NullPointerException- if the specified element is null
 
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offerLastInserts the specified element at the end of this deque.- Specified by:
- offerLastin interface- Deque<E>
- Parameters:
- e- the element to add
- Returns:
- true(as specified by- Deque.offerLast(E))
- Throws:
- NullPointerException- if the specified element is null
 
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removeFirstDescription copied from interface:DequeRetrieves 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 interface- Deque<E>
- Specified by:
- removeFirstin interface- SequencedCollection<E>
- Returns:
- the head of this deque
- Throws:
- NoSuchElementException- if this deque is empty
 
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removeLastDescription copied from interface:DequeRetrieves 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 interface- Deque<E>
- Specified by:
- removeLastin interface- SequencedCollection<E>
- Returns:
- the tail of this deque
- Throws:
- NoSuchElementException- if this deque is empty
 
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pollFirstDescription copied from interface:DequeRetrieves and removes the first element of this deque, or returnsnullif this deque is empty.
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pollLastDescription copied from interface:DequeRetrieves and removes the last element of this deque, or returnsnullif this deque is empty.
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getFirstDescription copied from interface:DequeRetrieves, 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 interface- Deque<E>
- Specified by:
- getFirstin interface- SequencedCollection<E>
- Returns:
- the head of this deque
- Throws:
- NoSuchElementException- if this deque is empty
 
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getLastDescription copied from interface:DequeRetrieves, 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 interface- Deque<E>
- Specified by:
- getLastin interface- SequencedCollection<E>
- Returns:
- the tail of this deque
- Throws:
- NoSuchElementException- if this deque is empty
 
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peekFirstDescription copied from interface:DequeRetrieves, but does not remove, the first element of this deque, or returnsnullif this deque is empty.
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peekLastDescription copied from interface:DequeRetrieves, but does not remove, the last element of this deque, or returnsnullif this deque is empty.
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removeFirstOccurrenceRemoves the first occurrence of the specified element in this deque (when traversing the deque from head to tail). If the deque does not contain the element, it is unchanged. More formally, removes the first elementesuch thato.equals(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).- Specified by:
- removeFirstOccurrencein interface- Deque<E>
- Parameters:
- o- element to be removed from this deque, if present
- Returns:
- trueif the deque contained the specified element
 
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removeLastOccurrenceRemoves the last occurrence of the specified element in this deque (when traversing the deque from head to tail). If the deque does not contain the element, it is unchanged. More formally, removes the last elementesuch thato.equals(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).- Specified by:
- removeLastOccurrencein interface- Deque<E>
- Parameters:
- o- element to be removed from this deque, if present
- Returns:
- trueif the deque contained the specified element
 
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addInserts the specified element at the end of this deque.This method is equivalent to addLast(E).- Specified by:
- addin interface- Collection<E>
- Specified by:
- addin interface- Deque<E>
- Specified by:
- addin interface- Queue<E>
- Overrides:
- addin class- AbstractCollection<E>
- Parameters:
- e- the element to add
- Returns:
- true(as specified by- Collection.add(E))
- Throws:
- NullPointerException- if the specified element is null
 
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offerInserts the specified element at the end of this deque.This method is equivalent to offerLast(E).- Specified by:
- offerin interface- Deque<E>
- Specified by:
- offerin interface- Queue<E>
- Parameters:
- e- the element to add
- Returns:
- true(as specified by- Queue.offer(E))
- Throws:
- NullPointerException- if the specified element is null
 
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removeRetrieves and removes the head of the queue represented by this deque. This method differs frompoll()only in that it throws an exception if this deque is empty.This method is equivalent to removeFirst().
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pollRetrieves 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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elementRetrieves, but does not remove, the head of the queue represented by this deque. This method differs frompeekonly in that it throws an exception if this deque is empty.This method is equivalent to getFirst().
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peekRetrieves, but does not remove, the head of the queue represented by this deque, or returnsnullif this deque is empty.This method is equivalent to peekFirst().
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pushPushes an element onto the stack represented by this deque. In other words, inserts the element at the front of this deque.This method is equivalent to addFirst(E).- Specified by:
- pushin interface- Deque<E>
- Parameters:
- e- the element to push
- Throws:
- NullPointerException- if the specified element is null
 
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popPops 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().- Specified by:
- popin interface- Deque<E>
- 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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sizepublic int size()Returns the number of elements in this deque.
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isEmptypublic boolean isEmpty()Returnstrueif this deque contains no elements.- Specified by:
- isEmptyin interface- Collection<E>
- Overrides:
- isEmptyin class- AbstractCollection<E>
- Returns:
- trueif this deque contains no elements
 
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iterator
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descendingIteratorDescription copied from interface:DequeReturns 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).- Specified by:
- descendingIteratorin interface- Deque<E>
- Returns:
- an iterator over the elements in this deque in reverse sequence
 
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spliteratorCreates a late-binding and fail-fastSpliteratorover the elements in this deque.The SpliteratorreportsSpliterator.SIZED,Spliterator.SUBSIZED,Spliterator.ORDERED, andSpliterator.NONNULL. Overriding implementations should document the reporting of additional characteristic values.- Specified by:
- spliteratorin interface- Collection<E>
- Specified by:
- spliteratorin interface- Iterable<E>
- Returns:
- a Spliteratorover the elements in this deque
- Since:
- 1.8
 
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forEachDescription copied from interface:IterablePerforms the given action for each element of theIterableuntil all elements have been processed or the action throws an exception. Actions are performed in the order of iteration, if that order is specified. Exceptions thrown by the action are relayed to the caller.The behavior of this method is unspecified if the action performs side-effects that modify the underlying source of elements, unless an overriding class has specified a concurrent modification policy. - Specified by:
- forEachin interface- Iterable<E>
- Parameters:
- action- The action to be performed for each element
- Throws:
- NullPointerException- if the specified action is null
 
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removeIfDescription copied from interface:CollectionRemoves all of the elements of this collection that satisfy the given predicate. Errors or runtime exceptions thrown during iteration or by the predicate are relayed to the caller.- Specified by:
- removeIfin interface- Collection<E>
- Parameters:
- filter- a predicate which returns- truefor elements to be removed
- Returns:
- trueif any elements were removed
- Throws:
- NullPointerException- if the specified filter is null
 
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removeAllDescription copied from class:AbstractCollectionRemoves all of this collection's elements that are also contained in the specified collection (optional operation). After this call returns, this collection will contain no elements in common with the specified collection.- Specified by:
- removeAllin interface- Collection<E>
- Overrides:
- removeAllin class- AbstractCollection<E>
- Parameters:
- c- collection containing elements to be removed from this collection
- Returns:
- trueif this collection changed as a result of the call
- Throws:
- NullPointerException- if this collection contains one or more null elements and the specified collection does not support null elements (optional) or if the specified collection is null
- See Also:
 
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retainAllDescription copied from class:AbstractCollectionRetains only the elements in this collection that are contained in the specified collection (optional operation). In other words, removes from this collection all of its elements that are not contained in the specified collection.- Specified by:
- retainAllin interface- Collection<E>
- Overrides:
- retainAllin class- AbstractCollection<E>
- Parameters:
- c- collection containing elements to be retained in this collection
- Returns:
- trueif this collection changed as a result of the call
- Throws:
- NullPointerException- if this collection contains one or more null elements and the specified collection does not permit null elements (optional) or if the specified collection is null
- See Also:
 
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containsReturnstrueif this deque contains the specified element. More formally, returnstrueif and only if this deque contains at least one elementesuch thato.equals(e).- Specified by:
- containsin interface- Collection<E>
- Specified by:
- containsin interface- Deque<E>
- Overrides:
- containsin class- AbstractCollection<E>
- Parameters:
- o- object to be checked for containment in this deque
- Returns:
- trueif this deque contains the specified element
 
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removeRemoves a single instance of the specified element from this deque. If the deque does not contain the element, it is unchanged. More formally, removes the first elementesuch thato.equals(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>
- Specified by:
- removein interface- Deque<E>
- Overrides:
- removein class- AbstractCollection<E>
- Parameters:
- o- element to be removed from this deque, if present
- Returns:
- trueif this deque contained the specified element
 
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clearpublic void clear()Removes all of the elements from this deque. The deque will be empty after this call returns.- Specified by:
- clearin interface- Collection<E>
- Overrides:
- clearin class- AbstractCollection<E>
 
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toArrayReturns an array containing all of the elements in this deque in proper sequence (from first to last element).The returned array will be "safe" in that no references to it are maintained by this deque. (In other words, this method must allocate a new array). The caller is thus free to modify the returned array. This method acts as bridge between array-based and collection-based APIs. - Specified by:
- toArrayin interface- Collection<E>
- Overrides:
- toArrayin class- AbstractCollection<E>
- Returns:
- an array containing all of the elements in this deque
 
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toArraypublic <T> T[] toArray(T[] a) Returns an array containing all of the elements in this deque in proper sequence (from first to last element); the runtime type of the returned array is that of the specified array. If the deque fits in the specified array, it is returned therein. Otherwise, a new array is allocated with the runtime type of the specified array and the size of this deque.If this deque fits in the specified array with room to spare (i.e., the array has more elements than this deque), the element in the array immediately following the end of the deque is set to null.Like the toArray()method, this method acts as bridge between array-based and collection-based APIs. Further, this method allows precise control over the runtime type of the output array, and may, under certain circumstances, be used to save allocation costs.Suppose xis a deque known to contain only strings. The following code can be used to dump the deque into a newly allocated array ofString:
 Note thatString[] y = x.toArray(new String[0]);toArray(new Object[0])is identical in function totoArray().- Specified by:
- toArrayin interface- Collection<E>
- Overrides:
- toArrayin class- AbstractCollection<E>
- Type Parameters:
- T- the component type of the array to contain the collection
- Parameters:
- a- the array into which the elements of the deque are to be stored, if it is big enough; otherwise, a new array of the same runtime type is allocated for this purpose
- Returns:
- an array containing all of the elements in this deque
- Throws:
- ArrayStoreException- if the runtime type of the specified array is not a supertype of the runtime type of every element in this deque
- NullPointerException- if the specified array is null
 
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cloneReturns a copy of this deque.
 
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