Deque接口是“double ended queue”的缩写(通常读作“deck”)，即双端队列，支持在队列的两端插入和删除元素，继承Queue接口。大多数的实现对元素的数量没有限制，但这个接口既支持有容量限制的deque，也支持没有固定大小限制的。
Deque接口定义了在两端访问元素的方法，主要包括insert、remove和examine。和Queue定义一样，所有这些方法存在两种形式：一种如果操作失败则抛出异常，另一种则返回一个特殊值(null或false)。后者主要是为有容量限制的队列实现的。
Deque的12种方法总结如下：
| 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() |
当deque用作queue时，则表现出FIFO的行为，元素从队尾加入，在队首移除，从Queue接口继承来的方法和Deque中的一些方法等同。如下表：
| {@code Queue} Method | Equivalent {@code Deque} Method |
| {@link java.util.Queue#add add(e)} | {@link #addLast addLast(e)} |
| {@link java.util.Queue#offer offer(e)} | {@link #offerLast offerLast(e)} |
| {@link java.util.Queue#remove remove()} | {@link #removeFirst removeFirst()} |
| {@link java.util.Queue#poll poll()} | {@link #pollFirst pollFirst()} |
| {@link java.util.Queue#element element()} | {@link #getFirst getFirst()} |
| {@link java.util.Queue#peek peek()} | {@link #peek peekFirst()} |
Deque也可以被用作LIFO的栈，此时的接口应该严格参照Stack类的实现。当deque被用作栈时，元素在deque的head端push/pop。栈的方法等价于Deque中的一些方法，如下表：
| Stack Method | Equivalent {@code Deque} Method |
| push(e) | addFirst(e) |
| pop() | removeFirst() |
| peek() | peekFirst() |
注意：peek方法在Deque用作队列和栈时是没问题的，这两种情况都是在deque的head端获得。
Deque接口提供了两种方法来移除内部的元素，removeFirstOccurrence 和 removeLastOccurrence。
Deque与List不同，该接口不支持下标访问元素。Deque的实现并不严格要求禁止插入元素null，但强烈鼓励不插入null。任何Deque的实现都强烈鼓励不要插入null，因为null是多种方法作为一种特殊返回值来表示deque为空。
和Queue一样，Deque的实现通常不定义基于元素的equals和hashCode方法，而是直接继承自Object类。
Deque接口源码：
public interface Deque<E> extends Queue<E> {
/**
* Inserts the specified element at the front of this deque if it is
* possible to do so immediately without violating capacity restrictions,
* throwing an {@code IllegalStateException} if no space is currently
* available. When using a capacity-restricted deque, it is generally
* preferable to use method {@link #offerFirst}.
*
* @param e the element to add
* @throws IllegalStateException if the element cannot be added at this
* time due to capacity restrictions
* @throws ClassCastException if the class of the specified element
* prevents it from being added to this deque
* @throws NullPointerException if the specified element is null and this
* deque does not permit null elements
* @throws IllegalArgumentException if some property of the specified
* element prevents it from being added to this deque
*/
void addFirst(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 an {@code IllegalStateException} if no space is currently
* available. When using a capacity-restricted deque, it is generally
* preferable to use method {@link #offerLast}.
*
* <p>This method is equivalent to {@link #add}.
*
* @param e the element to add
* @throws IllegalStateException if the element cannot be added at this
* time due to capacity restrictions
* @throws ClassCastException if the class of the specified element
* prevents it from being added to this deque
* @throws NullPointerException if the specified element is null and this
* deque does not permit null elements
* @throws IllegalArgumentException if some property of the specified
* element prevents it from being added to this deque
*/
void addLast(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 the {@link #addFirst} method,
* which can fail to insert an element only by throwing an exception.
*
* @param e the element to add
* @return {@code true} if the element was added to this deque, else
* {@code false}
* @throws ClassCastException if the class of the specified element
* prevents it from being added to this deque
* @throws NullPointerException if the specified element is null and this
* deque does not permit null elements
* @throws IllegalArgumentException if some property of the specified
* element prevents it from being added to this deque
*/
boolean offerFirst(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 the {@link #addLast} method,
* which can fail to insert an element only by throwing an exception.
*
* @param e the element to add
* @return {@code true} if the element was added to this deque, else
* {@code false}
* @throws ClassCastException if the class of the specified element
* prevents it from being added to this deque
* @throws NullPointerException if the specified element is null and this
* deque does not permit null elements
* @throws IllegalArgumentException if some property of the specified
* element prevents it from being added to this deque
*/
boolean offerLast(E e);
/**
* 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.
*
* @return the head of this deque
* @throws NoSuchElementException if this deque is empty
*/
E removeFirst();
/**
* 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.
*
* @return the tail of this deque
* @throws NoSuchElementException if this deque is empty
*/
E removeLast();
/**
* Retrieves and removes the first element of this deque,
* or returns {@code null} if this deque is empty.
*
* @return the head of this deque, or {@code null} if this deque is empty
*/
E pollFirst();
/**
* Retrieves and removes the last element of this deque,
* or returns {@code null} if this deque is empty.
*
* @return the tail of this deque, or {@code null} if this deque is empty
*/
E pollLast();
/**
* 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.
*
* @return the head of this deque
* @throws NoSuchElementException if this deque is empty
*/
E getFirst();
/**
* 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.
*
* @return the tail of this deque
* @throws NoSuchElementException if this deque is empty
*/
E getLast();
/**
* Retrieves, but does not remove, the first element of this deque,
* or returns {@code null} if this deque is empty.
*
* @return the head of this deque, or {@code null} if this deque is empty
*/
E peekFirst();
/**
* Retrieves, but does not remove, the last element of this deque,
* or returns {@code null} if this deque is empty.
*
* @return the tail of this deque, or {@code null} if this deque is empty
*/
E peekLast();
/**
* 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 {@code e} such that
* <tt>(o==null ? e==null : o.equals(e))</tt>
* (if such an element exists).
* Returns {@code true} if this deque contained the specified element
* (or equivalently, if this deque changed as a result of the call).
*
* @param o element to be removed from this deque, if present
* @return {@code true} if an element was removed as a result of this call
* @throws ClassCastException if the class of the specified element
* is incompatible with this deque
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if the specified element is null and this
* deque does not permit null elements
* (<a href="Collection.html#optional-restrictions">optional</a>)
*/
boolean removeFirstOccurrence(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 {@code e} such that
* <tt>(o==null ? e==null : o.equals(e))</tt>
* (if such an element exists).
* Returns {@code true} if this deque contained the specified element
* (or equivalently, if this deque changed as a result of the call).
*
* @param o element to be removed from this deque, if present
* @return {@code true} if an element was removed as a result of this call
* @throws ClassCastException if the class of the specified element
* is incompatible with this deque
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if the specified element is null and this
* deque does not permit null elements
* (<a href="Collection.html#optional-restrictions">optional</a>)
*/
boolean removeLastOccurrence(Object o);
// *** Queue methods ***
/**
* 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
* {@code true} upon success and throwing an
* {@code IllegalStateException} if no space is currently available.
* When using a capacity-restricted deque, it is generally preferable to
* use {@link #offer(Object) offer}.
*
* <p>This method is equivalent to {@link #addLast}.
*
* @param e the element to add
* @return {@code true} (as specified by {@link Collection#add})
* @throws IllegalStateException if the element cannot be added at this
* time due to capacity restrictions
* @throws ClassCastException if the class of the specified element
* prevents it from being added to this deque
* @throws NullPointerException if the specified element is null and this
* deque does not permit null elements
* @throws IllegalArgumentException if some property of the specified
* element prevents it from being added to this deque
*/
boolean 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, returning
* {@code true} upon success and {@code 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.
*
* <p>This method is equivalent to {@link #offerLast}.
*
* @param e the element to add
* @return {@code true} if the element was added to this deque, else
* {@code false}
* @throws ClassCastException if the class of the specified element
* prevents it from being added to this deque
* @throws NullPointerException if the specified element is null and this
* deque does not permit null elements
* @throws IllegalArgumentException if some property of the specified
* element prevents it from being added to this deque
*/
boolean offer(E e);
/**
* 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.
*
* <p>This method is equivalent to {@link #removeFirst()}.
*
* @return the head of the queue represented by this deque
* @throws NoSuchElementException if this deque is empty
*/
E remove();
/**
* Retrieves and removes the head of the queue represented by this deque
* (in other words, the first element of this deque), or returns
* {@code null} if this deque is empty.
*
* <p>This method is equivalent to {@link #pollFirst()}.
*
* @return the first element of this deque, or {@code null} if
* this deque is empty
*/
E poll();
/**
* 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.
*
* <p>This method is equivalent to {@link #getFirst()}.
*
* @return the head of the queue represented by this deque
* @throws NoSuchElementException if this deque is empty
*/
E element();
/**
* 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 {@code null} if this deque is empty.
*
* <p>This method is equivalent to {@link #peekFirst()}.
*
* @return the head of the queue represented by this deque, or
* {@code null} if this deque is empty
*/
E peek();
// *** Stack methods ***
/**
* 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 an
* {@code IllegalStateException} if no space is currently available.
*
* <p>This method is equivalent to {@link #addFirst}.
*
* @param e the element to push
* @throws IllegalStateException if the element cannot be added at this
* time due to capacity restrictions
* @throws ClassCastException if the class of the specified element
* prevents it from being added to this deque
* @throws NullPointerException if the specified element is null and this
* deque does not permit null elements
* @throws IllegalArgumentException if some property of the specified
* element prevents it from being added to this deque
*/
void push(E e);
/**
* Pops an element from the stack represented by this deque. In other
* words, removes and returns the first element of this deque.
*
* <p>This method is equivalent to {@link #removeFirst()}.
*
* @return 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
*/
E pop();
// *** Collection methods ***
/**
* 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 {@code e} such that
* <tt>(o==null ? e==null : o.equals(e))</tt>
* (if such an element exists).
* Returns {@code true} if this deque contained the specified element
* (or equivalently, if this deque changed as a result of the call).
*
* <p>This method is equivalent to {@link #removeFirstOccurrence(Object)}.
*
* @param o element to be removed from this deque, if present
* @return {@code true} if an element was removed as a result of this call
* @throws ClassCastException if the class of the specified element
* is incompatible with this deque
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if the specified element is null and this
* deque does not permit null elements
* (<a href="Collection.html#optional-restrictions">optional</a>)
*/
boolean remove(Object o);
/**
* Returns {@code true} if this deque contains the specified element.
* More formally, returns {@code true} if and only if this deque contains
* at least one element {@code e} such that
* <tt>(o==null ? e==null : o.equals(e))</tt>.
*
* @param o element whose presence in this deque is to be tested
* @return {@code true} if this deque contains the specified element
* @throws ClassCastException if the type of the specified element
* is incompatible with this deque
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if the specified element is null and this
* deque does not permit null elements
* (<a href="Collection.html#optional-restrictions">optional</a>)
*/
boolean contains(Object o);
/**
* Returns the number of elements in this deque.
*
* @return the number of elements in this deque
*/
public int size();
/**
* 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).
*
* @return an iterator over the elements in this deque in proper sequence
*/
Iterator<E> iterator();
/**
* 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).
*
* @return an iterator over the elements in this deque in reverse
* sequence
*/
Iterator<E> descendingIterator();
}