java.util

Class ArrayDeque<E>

Implemented Interfaces:
Cloneable, Collection<E>, Deque<E>, Iterable<E>, Queue<E>, Serializable

public class ArrayDeque<E>
extends AbstractCollection<E>
implements Deque<E>, Cloneable, 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 ../../../technotes/guides/collections/index.html"> Java Collections Framework.

Since:
1.6
See Also:
Serialized Form

Constructor Summary

ArrayDeque()
Constructs an empty array deque with an initial capacity sufficient to hold 16 elements.
ArrayDeque(E> c)
Constructs a deque containing the elements of the specified collection, in the order they are returned by the collection's iterator.
ArrayDeque(int numElements)
Constructs an empty array deque with an initial capacity sufficient to hold the specified number of elements.

Method Summary

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.
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
clone()
Returns a copy of this deque.
boolean
contains(Object o)
Returns true if this deque contains the specified element.
Iterator
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()
E
getLast()
boolean
isEmpty()
Returns true if this deque contains no elements.
Iterator
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()
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()
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).

Methods inherited from class java.util.AbstractCollection<E>

T[] toArray, add, addAll, clear, contains, containsAll, isEmpty, iterator, remove, removeAll, retainAll, size, toArray, toString

Methods inherited from class java.lang.Object

clone, equals, extends Object> getClass, finalize, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Details

ArrayDeque

public ArrayDeque()
Constructs an empty array deque with an initial capacity sufficient to hold 16 elements.

ArrayDeque

public ArrayDeque(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

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

Method Details

T[] 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:
T[] toArray in interface Collection<E>
Overrides:
T[] toArray in interface AbstractCollection<E>
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

add

public boolean add(E e)
Inserts the specified element at the end of this deque.

This method is equivalent to addLast(E).

Specified by:
add in interface Deque<E>
add in interface Queue<E>
add in interface Collection<E>
Overrides:
add in interface AbstractCollection<E>
Parameters:
e - the element to add
Returns:
true (as specified by Collection.add(E))
Throws:
NullPointerException - if the specified element is null

addFirst

public void addFirst(E e)
Inserts the specified element at the front of this deque.
Specified by:
addFirst in interface Deque<E>
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(E).

Specified by:
addLast in interface Deque<E>
Parameters:
e - the element to add
Throws:
NullPointerException - if the specified element is null

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<E>
Overrides:
clear in interface AbstractCollection<E>

clone

public ArrayDeque clone()
Returns a copy of this deque.
Overrides:
clone in interface Object
Returns:
a copy of this deque

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 Deque<E>
contains in interface Collection<E>
Overrides:
contains in interface AbstractCollection<E>
Parameters:
o - object to be checked for containment in this deque
Returns:
true if this deque contains the specified element

descendingIterator

public Iterator descendingIterator()
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:
descendingIterator in interface Deque<E>
Returns:
an iterator over the elements in this deque in reverse sequence

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 Deque<E>
element in interface Queue<E>
Returns:
the head of the queue represented by this deque

getFirst

public E getFirst()
Specified by:
getFirst in interface Deque<E>

getLast

public E getLast()
Specified by:
getLast in interface Deque<E>

isEmpty

public boolean isEmpty()
Returns true if this deque contains no elements.
Specified by:
isEmpty in interface Collection<E>
Overrides:
isEmpty in interface AbstractCollection<E>
Returns:
true if this deque contains no elements

iterator

public Iterator 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 Deque<E>
iterator in interface Collection<E>
iterator in interface Iterable<E>
Overrides:
iterator in interface AbstractCollection<E>
Returns:
an iterator over the elements in this deque

offer

public boolean offer(E e)
Inserts the specified element at the end of this deque.

This method is equivalent to offerLast(E).

Specified by:
offer in interface Deque<E>
offer in 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

offerFirst

public boolean offerFirst(E e)
Inserts the specified element at the front of this deque.
Specified by:
offerFirst in 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

offerLast

public boolean offerLast(E e)
Inserts the specified element at the end of this deque.
Specified by:
offerLast in 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

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 Deque<E>
peek in interface Queue<E>
Returns:
the head of the queue represented by this deque, or null if this deque is empty

peekFirst

public E peekFirst()
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<E>
Returns:
the head of this deque, or null if this deque is empty

peekLast

public E peekLast()
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<E>
Returns:
the tail of this deque, or null if this deque 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 Deque<E>
poll in interface Queue<E>
Returns:
the head of the queue represented by this deque, or null if this deque is empty

pollFirst

public E pollFirst()
Retrieves and removes the first element of this deque, or returns null if this deque is empty.
Specified by:
pollFirst in interface Deque<E>
Returns:
the head of this deque, or null if this deque is empty

pollLast

public E pollLast()
Retrieves and removes the last element of this deque, or returns null if this deque is empty.
Specified by:
pollLast in interface Deque<E>
Returns:
the tail of this deque, or null if this deque is empty

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<E>
Returns:
the element at the front of this deque (which is the top of the stack represented by this deque)

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(E).

Specified by:
push in interface Deque<E>
Parameters:
e - the element to push
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 Deque<E>
remove in interface Queue<E>
Returns:
the head of the queue represented by this deque

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(Object).

Specified by:
remove in interface Deque<E>
remove in interface Collection<E>
Overrides:
remove in interface AbstractCollection<E>
Parameters:
o - element to be removed from this deque, if present
Returns:
true if this deque contained the specified element

removeFirst

public E removeFirst()
Specified by:
removeFirst in interface Deque<E>

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:
removeFirstOccurrence in interface Deque<E>
Parameters:
o - element to be removed from this deque, if present
Returns:
true if the deque contained the specified element

removeLast

public E removeLast()
Specified by:
removeLast in interface Deque<E>

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:
removeLastOccurrence in interface Deque<E>
Parameters:
o - element to be removed from this deque, if present
Returns:
true if the deque contained the specified element

size

public int size()
Returns the number of elements in this deque.
Specified by:
size in interface Deque<E>
size in interface Collection<E>
Overrides:
size in interface AbstractCollection<E>
Returns:
the number of elements in this deque

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<E>
Overrides:
toArray in interface AbstractCollection<E>
Returns:
an array containing all of the elements in this deque