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 are summarized in the * follwoing table:
* *
| * | First Element (Head) | *Last Element (Tail) | *||
| * | Throws exception | *Returns special value | *Throws exception | *Returns special value | *
| Insert | *{@link #addFirst addFirst(e)} | *{@link #offerFirst offerFirst(e)} | *{@link #addLast addLast(e)} | *{@link #offerLast offerLast(e)} | *
| Remove | *{@link #removeFirst removeFirst()} | *{@link #pollFirst pollFirst()} | *{@link #removeLast removeLast()} | *{@link #pollLast pollLast()} | *
| Examine | *{@link #getFirst getFirst()} | *{@link #peekFirst peekFirst()} | *{@link #getLast getLast()} | *{@link #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 to 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:
* *
| Queue Method | *Equivalent Deque Method | *
| {@link java.util.Queue#offer offer(e)} | *{@link #offerLast offerLast(e)} | *
| {@link java.util.Queue#add add(e)} | *{@link #addLast addLast(e)} | *
| {@link java.util.Queue#poll poll()} | *{@link #pollFirst pollFirst()} | *
| {@link java.util.Queue#remove remove()} | *{@link #removeFirst removeFirst()} | *
| {@link java.util.Queue#peek peek()} | *{@link #peek peekFirst()} | *
| {@link java.util.Queue#element element()} | *{@link #getFirst getFirst()} | *
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 dequeue 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:
* *
| Stack Method | *Equivalent 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 inteface provides two methods to 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.
*
* @author Doug Lea
* @author Josh Bloch
* @since 1.6
* @param This method is equivalent to {@link #offerLast}.
*
* @param e the element to insert
* @return true if it was possible to insert the element,
* else false
* @throws NullPointerException if e is null and this
* deque does not permit null elements
*/
boolean offer(E e);
/**
* Inserts the specified element into the queue represented by this
* deque unless it would violate capacity restrictions. In other words,
* inserts the specified element as the last element of this deque.
*
* This method is equivalent to {@link #addLast}.
*
* @param e the element to insert
* @return true (as per the spec for {@link Collection#add})
* @throws IllegalStateException if it was not possible to insert
* the element due to capacity restrictions
* @throws NullPointerException if e is null and this
* deque does not permit null elements
*/
boolean add(E e);
/**
* Retrieves and removes the head of the queue represented by
* this deque, or null if this deque is empty. In other words,
* retrieves and removes the first element of this deque, or null
* if this deque is empty.
*
* This method is equivalent to {@link #pollFirst()}.
*
* @return the first element of this deque, or null if
* this deque is empty
*/
E poll();
/**
* Retrieves and removes the head of the queue represented by this deque.
* This method differs from the poll method 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, but does not remove, the head of the queue represented by
* this deque, returning null if this deque is empty.
*
* This method is equivalent to {@link #peekFirst()}
*
* @return the head of the queue represented by this deque, or
* null if this deque is empty
*/
E peek();
/**
* Retrieves, but does not remove, the head of the queue represented by
* this deque. This method differs from the peek method 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();
// *** Stack methods ***
/**
* Pushes an element onto the stack represented by this deque. In other
* words, inserts the element to the front this deque unless it would
* violate capacity restrictions.
*
* This method is equivalent to {@link #addFirst}.
*
* @throws IllegalStateException if it was not possible to insert
* the element due to capacity restrictions
* @throws NullPointerException if e is null and this
* deque does not permit null elements
*/
void push(E e);
/**
* Pops an element from the stack represented by this deque. In other
* words, removes and returns the 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 Method ***
/**
* Returns an iterator over the elements in this deque. The elements
* will be ordered from first (head) to last (tail).
*
* @return an Iterator over the elements in this deque
*/
Iterator