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: * *
| * | 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: * *
| {@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: * *
| 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}.
*
* @author Doug Lea
* @author Josh Bloch
* @since 1.6
* @param 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
* {@code Objects.equals(o, 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
* {@code Objects.equals(o, 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
* {@code Objects.equals(o, 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 {@code Objects.equals(o, 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 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 contains(Object o);
/**
* Returns the number of elements in this deque.
*
* @return the number of elements in this deque
*/
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