Class ArrayDeque
java.util
java.lang.Object
  java.util.AbstractCollection<E>
      java.util.ArrayDeque
Type Parameters:
E - the type of elements held in this collection
All Implemented Interfaces:
Cloneable, Collection<E>, Deque<E>, Iterable<E>, Queue<E>, java.io.Serializable
public class ArrayDeque
extends AbstractCollection<E>
implements Deque<E>, Cloneable, java.io.Serializable
Resizable-array implementation of the 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 and Iterator interfaces.

This class is a member of the Java Collections Framework.

Since:
1.6
Author:
Josh Bloch and Doug Lea
Constructor Summary
ArrayDeque()
Constructs 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.
Method Summary
boolean
 add(E e)
Inserts the specified element at the end of this deque.
void
 addFirst(E e)
Inserts the specified element at the front of this deque.
void
 addLast(E e)
Inserts the specified element at the end of this deque.
void
 clear()
Removes all of the elements from this deque.
ArrayDeque<E>
 clone()
Returns a copy of this deque.
boolean
 contains(Object o)
Returns true if this deque contains the specified element.
Iterator<E>
 descendingIterator()
Returns an iterator over the elements in this deque in reverse sequential order.
E
 element()
Retrieves, but does not remove, the head of the queue represented by this deque.
E
 getFirst()
Retrieves, but does not remove, the first element of this deque.
E
 getLast()
Retrieves, but does not remove, the last element of this deque.
boolean
 isEmpty()
Returns true if this deque contains no elements.
Iterator<E>
 iterator()
Returns an iterator over the elements in this deque.
boolean
 offer(E e)
Inserts the specified element at the end of this deque.
boolean
 offerFirst(E e)
Inserts the specified element at the front of this deque.
boolean
 offerLast(E e)
Inserts the specified element at the end of this deque.
E
 peek()
Retrieves, but does not remove, the head of the queue represented by this deque, or returns null if this deque is empty.
E
 peekFirst()
Retrieves, but does not remove, the first element of this deque, or returns null if this deque is empty.
E
 peekLast()
Retrieves, but does not remove, the last element of this deque, or returns null if this deque is empty.
E
 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.
E
 pollFirst()
Retrieves and removes the first element of this deque, or returns null if this deque is empty.
E
 pollLast()
Retrieves and removes the last element of this deque, or returns null if this deque is empty.
E
 pop()
Pops an element from the stack represented by this deque.
void
 push(E e)
Pushes an element onto the stack represented by this deque.
E
 remove()
Retrieves and removes the head of the queue represented by this deque.
boolean
 remove(Object o)
Removes a single instance of the specified element from this deque.
E
 removeFirst()
Retrieves and removes the first element of this deque.
boolean
 removeFirstOccurrence(Object o)
Removes the first occurrence of the specified element in this deque (when traversing the deque from head to tail).
E
 removeLast()
Retrieves and removes the last element of this deque.
boolean
 removeLastOccurrence(Object o)
Removes the last occurrence of the specified element in this deque (when traversing the deque from head to tail).
int
 size()
Returns the number of 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[]
 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 inherited from class java.util.AbstractCollection
addAll, containsAll, removeAll, retainAll, toString
Methods inherited from class java.lang.Object
equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait
Methods inherited from interface java.util.Collection
equals, hashCode
Constructor Detail
ArrayDeque
public ArrayDeque()
Constructs an empty array deque with an initial capacity sufficient to hold 16 elements.
ArrayDeque
public 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
ArrayDeque
public 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. (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
Method Detail
addFirst
public void addFirst(E e)
Inserts the specified element at the front of this deque.
Specified by:
addFirst in interface Deque
Parameters:
e - the element to add
Throws:
NullPointerException - if the specified element is null
addLast
public void addLast(E e)
Inserts the specified element at the end of this deque.

This method is equivalent to add().

Specified by:
addLast in interface Deque
Parameters:
e - the element to add
Throws:
NullPointerException - if the specified element is null
offerFirst
public boolean offerFirst(E e)
Inserts the specified element at the front of this deque.
Specified by:
offerFirst in interface Deque
Parameters:
e - the element to add
Returns:
true (as specified by Deque.offerFirst())
Throws:
NullPointerException - if the specified element is null
offerLast
public boolean offerLast(E e)
Inserts the specified element at the end of this deque.
Specified by:
offerLast in interface Deque
Parameters:
e - the element to add
Returns:
true (as specified by Deque.offerLast())
Throws:
NullPointerException - if the specified element is null
removeFirst
public E removeFirst()
Description copied from interface: Deque
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.
Specified by:
removeFirst in interface Deque
Returns:
the head of this deque
Throws:
NoSuchElementException - if this deque is empty
removeLast
public E removeLast()
Description copied from interface: Deque
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.
Specified by:
removeLast in interface Deque
Returns:
the tail of this deque
Throws:
NoSuchElementException - if this deque is empty
pollFirst
public E pollFirst()
Description copied from interface: Deque
Retrieves and removes the first element of this deque, or returns null if this deque is empty.
Specified by:
pollFirst in interface Deque
Returns:
the head of this deque, or null if this deque is empty
pollLast
public E pollLast()
Description copied from interface: Deque
Retrieves and removes the last element of this deque, or returns null if this deque is empty.
Specified by:
pollLast in interface Deque
Returns:
the tail of this deque, or null if this deque is empty
getFirst
public E getFirst()
Description copied from interface: Deque
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.
Specified by:
getFirst in interface Deque
Returns:
the head of this deque
Throws:
NoSuchElementException - if this deque is empty
getLast
public E getLast()
Description copied from interface: Deque
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.
Specified by:
getLast in interface Deque
Returns:
the tail of this deque
Throws:
NoSuchElementException - if this deque is empty
peekFirst
public E peekFirst()
Description copied from interface: Deque
Retrieves, but does not remove, the first element of this deque, or returns null if this deque is empty.
Specified by:
peekFirst in interface Deque
Returns:
the head of this deque, or null if this deque is empty
peekLast
public E peekLast()
Description copied from interface: Deque
Retrieves, but does not remove, the last element of this deque, or returns null if this deque is empty.
Specified by:
peekLast in interface Deque
Returns:
the tail of this deque, or null if this deque is empty
removeFirstOccurrence
public boolean removeFirstOccurrence(Object o)
Removes 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 element e such that 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).
Specified by:
Parameters:
o - element to be removed from this deque, if present
Returns:
true if the deque contained the specified element
removeLastOccurrence
public boolean removeLastOccurrence(Object o)
Removes 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 element e such that 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).
Specified by:
Parameters:
o - element to be removed from this deque, if present
Returns:
true if the deque contained the specified element
add
public boolean add(E e)
Inserts the specified element at the end of this deque.

This method is equivalent to addLast().

Specified by:
add in interface Collection
add in interface Queue
add in interface Deque
Overrides:
Parameters:
e - the element to add
Returns:
true (as specified by Collection.add())
Throws:
NullPointerException - if the specified element is null
offer
public boolean offer(E e)
Inserts the specified element at the end of this deque.

This method is equivalent to offerLast().

Specified by:
offer in interface Queue
offer in interface Deque
Parameters:
e - the element to add
Returns:
true (as specified by Queue.offer())
Throws:
NullPointerException - if the specified element is null
remove
public E remove()
Retrieves and removes the head of the queue represented by 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().

Specified by:
remove in interface Queue
remove in interface Deque
Returns:
the head of the queue represented by this deque
Throws:
NoSuchElementException - if this queue is empty
poll
public E 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().

Specified by:
poll in interface Queue
poll in interface Deque
Returns:
the head of the queue represented by this deque, or null if this deque is empty
element
public E element()
Retrieves, but does not remove, the head of the queue represented by 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().

Specified by:
element in interface Queue
element in interface Deque
Returns:
the head of the queue represented by this deque
Throws:
NoSuchElementException - if this queue is empty
peek
public E peek()
Retrieves, but does not remove, the head of the queue represented by this deque, or returns null if this deque is empty.

This method is equivalent to peekFirst().

Specified by:
peek in interface Queue
peek in interface Deque
Returns:
the head of the queue represented by this deque, or null if this deque is empty
push
public void push(E e)
Pushes 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().

Specified by:
push in interface Deque
Parameters:
e - the element to push
Throws:
NullPointerException - if the specified element is null
pop
public 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().

Specified by:
pop in interface Deque
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
size
public int size()
Returns the number of elements in this deque.
Specified by:
size in interface Collection
size in interface Deque
Overrides:
Returns:
the number of elements in this deque
isEmpty
public boolean isEmpty()
Returns true if this deque contains no elements.
Specified by:
isEmpty in interface Collection
Overrides:
Returns:
true if this deque contains no elements
iterator
public Iterator<E> iterator()
Returns an iterator over the elements in this deque. The elements will be ordered from first (head) to last (tail). This is the same order that elements would be dequeued (via successive calls to remove() or popped (via successive calls to pop()).
Specified by:
iterator in interface Iterable
iterator in interface Collection
iterator in interface Deque
Overrides:
Returns:
an iterator over the elements in this deque
descendingIterator
public Iterator<E> descendingIterator()
Description copied from interface: Deque
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).
Specified by:
Returns:
an iterator over the elements in this deque in reverse sequence
contains
public 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.equals(e).
Specified by:
contains in interface Collection
contains in interface Deque
Overrides:
Parameters:
o - object to be checked for containment in this deque
Returns:
true if this deque contains the specified element
remove
public boolean remove(Object o)
Removes 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 element e such that 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().

Specified by:
remove in interface Collection
remove in interface Deque
Overrides:
Parameters:
o - element to be removed from this deque, if present
Returns:
true if this deque contained the specified element
clear
public void clear()
Removes all of the elements from this deque. The deque will be empty after this call returns.
Specified by:
clear in interface Collection
Overrides:
toArray
public Object[] toArray()
Returns 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:
toArray in interface Collection
Overrides:
Returns:
an array containing all of the elements in this deque
toArray
public 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 x is a deque known to contain only strings. The following code can be used to dump the deque into a newly allocated array of String: 

     String[] y = x.toArray(new String[0]);
Note that toArray(new Object[0]) is identical in function to toArray().
Specified by:
toArray in interface Collection
Overrides:
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
clone
public ArrayDeque<E> clone()
Returns a copy of this deque.
Overrides:
clone in class Object
Returns:
a copy of this deque
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