java.util
interface Deque

All Superinterfaces:

Collection, Iterable, Queue

All Known Subinterfaces:

BlockingDeque

All Known Implementing Classes:

ArrayDeque, LinkedList

public interface Deque
extends Queue

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". Most 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:

This interface extends the {@link 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:

Deques can also be used as LIFO (Last-In-First-Out) stacks. This interface should be used in preference to the legacy {@link Stack} class. When a deque is used as a stack, elements are pushed and popped from the beginning of the deque. Stack methods are precisely equivalent to Deque methods as indicated in the table below:

Note that the {@link #peek 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, {@link #removeFirstOccurrence removeFirstOccurrence} and {@link #removeLastOccurrence removeLastOccurrence}.

Unlike the {@link 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 indicated 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.

add

public boolean add(Object 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, returning true upon success and throwing an IllegalStateException if no space is currently available. When using a capacity-restricted deque, it is generally preferable to use {@link #offer(Object) offer}.

This method is equivalent to {@link #addLast}.

Parameters:

e - the element to add

Returns:

true (as specified by {@link Collection#add})

addFirst

public void addFirst(Object e)

Inserts the specified element at the front of this deque if it is possible to do so immediately without violating capacity restrictions. When using a capacity-restricted deque, it is generally preferable to use method {@link #offerFirst}.

Parameters:

e - the element to add

addLast

public void addLast(Object e)

Inserts the specified element at the end of this deque if it is possible to do so immediately without violating capacity restrictions. When using a capacity-restricted deque, it is generally preferable to use method {@link #offerLast}.

This method is equivalent to {@link #add}.

Parameters:

e - the element to add

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==null ? e==null : o.equals(e)).

Parameters:

o - element whose presence in this deque is to be tested

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).

Returns:

an iterator over the elements in this deque in reverse sequence

element

public Object 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 from {@link #peek peek} only in that it throws an exception if this deque is empty.

This method is equivalent to {@link #getFirst()}.

Returns:

the head of the queue represented by this deque

getFirst

public Object getFirst()

Retrieves, but does not remove, the first element of this deque. This method differs from {@link #peekFirst peekFirst} only in that it throws an exception if this deque is empty.

Returns:

the head of this deque

getLast

public Object getLast()

Retrieves, but does not remove, the last element of this deque. This method differs from {@link #peekLast peekLast} only in that it throws an exception if this deque is empty.

Returns:

the tail of this deque

iterator

public Iterator 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).

Returns:

an iterator over the elements in this deque in proper sequence

offer

public boolean offer(Object 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, returning true upon success and false if no space is currently available. When using a capacity-restricted deque, this method is generally preferable to the {@link #add} method, which can fail to insert an element only by throwing an exception.

This method is equivalent to {@link #offerLast}.

Parameters:

e - the element to add

Returns:

true if the element was added to this deque, else false

offerFirst

public boolean offerFirst(Object 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 the {@link #addFirst} method, which can fail to insert an element only by throwing an exception.

Parameters:

e - the element to add

Returns:

true if the element was added to this deque, else false

offerLast

public boolean offerLast(Object 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 the {@link #addLast} method, which can fail to insert an element only by throwing an exception.

Parameters:

e - the element to add

Returns:

true if the element was added to this deque, else false

peek

public Object 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 returns null if this deque is empty.

This method is equivalent to {@link #peekFirst()}.

Returns:

the head of the queue represented by this deque, or null if this deque is empty

peekFirst

public Object peekFirst()

Retrieves, but does not remove, the first element of this deque, or returns null if this deque is empty.

Returns:

the head of this deque, or null if this deque is empty

peekLast

public Object peekLast()

Retrieves, but does not remove, the last element of this deque, or returns null if this deque is empty.

Returns:

the tail of this deque, or null if this deque is empty

poll

public Object 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 {@link #pollFirst()}.

Returns:

the first element of this deque, or null if this deque is empty

pollFirst

public Object pollFirst()

Retrieves and removes the first element of this deque, or returns null if this deque is empty.

Returns:

the head of this deque, or null if this deque is empty

pollLast

public Object pollLast()

Retrieves and removes the last element of this deque, or returns null if this deque is empty.

Returns:

the tail of this deque, or null if this deque is empty

pop

public Object 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 {@link #removeFirst()}.

Returns:

the element at the front of this deque (which is the top of the stack represented by this deque)

push

public void push(Object 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, returning true upon success and throwing an IllegalStateException if no space is currently available.

This method is equivalent to {@link #addFirst}.

Parameters:

e - the element to push

remove

public Object 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 from {@link #poll poll} only in that it throws an exception if this deque is empty.

This method is equivalent to {@link #removeFirst()}.

Returns:

the head of the queue represented by this deque

remove

public boolean 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 element e such that (o==null ? e==null : 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 {@link #removeFirstOccurrence}.

Parameters:

o - element to be removed from this deque, if present

Returns:

true if an element was removed as a result of this call

removeFirst

public Object removeFirst()

Retrieves and removes the first element of this deque. This method differs from {@link #pollFirst pollFirst} only in that it throws an exception if this deque is empty.

Returns:

the head of this deque

removeFirstOccurrence

public boolean 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 element e such that (o==null ? e==null : 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).

Parameters:

o - element to be removed from this deque, if present

Returns:

true if an element was removed as a result of this call

removeLast

public Object removeLast()

Retrieves and removes the last element of this deque. This method differs from {@link #pollLast pollLast} only in that it throws an exception if this deque is empty.

Returns:

the tail of this deque

removeLastOccurrence

public boolean 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 element e such that (o==null ? e==null : 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).

Parameters:

o - element to be removed from this deque, if present

Returns:

true if an element was removed as a result of this call

size

public int size()

Returns the number of elements in this deque.

Returns:

the number of elements in this deque