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 {@code null} or {@code false}, depending on * the operation). The latter form of the insert operation is * designed specifically for use with capacity-restricted * {@code Deque} implementations; in most implementations, insert * operations cannot fail. * *

The twelve methods described above are summarized in the * following table: * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *

Summary of Deque methods

	First Element (Head)		Last Element (Tail)
	Throws exception	Special value	Throws exception	Special value
Insert	{@link Deque#addFirst addFirst(e)}	{@link Deque#offerFirst offerFirst(e)}	{@link Deque#addLast addLast(e)}	{@link Deque#offerLast offerLast(e)}
Remove	{@link Deque#removeFirst removeFirst()}	{@link Deque#pollFirst pollFirst()}	{@link Deque#removeLast removeLast()}	{@link Deque#pollLast pollLast()}
Examine	{@link Deque#getFirst getFirst()}	{@link Deque#peekFirst peekFirst()}	{@link Deque#getLast getLast()}	{@link Deque#peekLast peekLast()}

* *

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 {@code Queue} interface are precisely equivalent to * {@code Deque} methods as indicated in the following table: * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *

Comparison of Queue and Deque methods

{@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()}

* *

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 * {@code Deque} methods as indicated in the table below: * * * * * * * * * * * * * * * * * * * *

Comparison of Stack and Deque methods

Stack Method	Equivalent {@code Deque} Method
{@link #push push(e)}	{@link #addFirst addFirst(e)}
{@link #pop pop()}	{@link #removeFirst removeFirst()}
{@link #peek peek()}	{@link #peekFirst peekFirst()}

* *

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 {@code Deque} implementations are not strictly required * to prohibit the insertion of null elements, they are strongly * encouraged to do so. Users of any {@code Deque} implementations * that do allow null elements are strongly encouraged not to * take advantage of the ability to insert nulls. This is so because * {@code null} is used as a special return value by various methods * to indicated that the deque is empty. * *

{@code Deque} implementations generally do not define * element-based versions of the {@code equals} and {@code hashCode} * methods, but instead inherit the identity-based versions from class * {@code Object}. * *

This interface is a member of the Java Collections * Framework. * * @author Doug Lea * @author Josh Bloch * @since 1.6 * @param the type of elements held in this collection */ public interface Deque extends Queue { /** * 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}. * *

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 * (o==null ? e==null : o.equals(e)) * (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 * (optional) * @throws NullPointerException if the specified element is null and this * deque does not permit null elements * (optional) */ 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 * (o==null ? e==null : o.equals(e)) * (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 * (optional) * @throws NullPointerException if the specified element is null and this * deque does not permit null elements * (optional) */ 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}. * *

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. * *

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. * *

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. * *

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. * *

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. * *

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. * *

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. * *

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 * (o==null ? e==null : o.equals(e)) * (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). * *

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 * (optional) * @throws NullPointerException if the specified element is null and this * deque does not permit null elements * (optional) */ 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 * (o==null ? e==null : o.equals(e)). * * @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 * (optional) * @throws NullPointerException if the specified element is null and this * deque does not permit null elements * (optional) */ 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 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 descendingIterator(); }