jsr166x.Deque Maven / Gradle / Ivy
/*
* Written by Doug Lea and Josh Bloch with assistance from members of
* JCP JSR-166 Expert Group and released to the public domain, as explained
* at http://creativecommons.org/publicdomain/zero/1.0/
*/
package jsr166x; // XXX This belongs in java.util!!! XXX
import java.util.*; // XXX This import goes away XXX
/**
* 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:
*
*
*
* 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 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.
*
* @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 to the front this deque unless it would
* violate capacity restrictions. When using a capacity-restricted deque,
* this method is generally preferable to method addFirst, which
* can fail to insert an element only by throwing an exception.
*
* @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 offerFirst(E e);
/**
* Inserts the specified element to the end of this deque unless it would
* violate capacity restrictions. When using a capacity-restricted deque,
* this method is generally preferable to method addLast which
* can fail to insert an element only by throwing an exception.
*
* @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 offerLast(E e);
/**
* Inserts the specified element to the front of this deque unless it
* would violate capacity restrictions.
*
* @param e the element to insert
* @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 addFirst(E e);
/**
* Inserts the specified element to the end of this deque unless it would
* violate capacity restrictions.
*
* @param e the element to insert
* @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 addLast(E e);
/**
* Retrieves and removes the first element of this deque, or
* null if this deque is empty.
*
* @return the first element of this deque, or null if
* this deque is empty
*/
E pollFirst();
/**
* Retrieves and removes the last element of this deque, or
* null if this deque is empty.
*
* @return the last element of this deque, or null if
* this deque is empty
*/
E pollLast();
/**
* Removes and returns the first element of this deque. This method
* differs from the pollFirst method only in that it throws an
* exception if this deque is empty.
*
* @return the first element 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 the pollLast method only in that it throws an
* exception if this deque is empty.
*
* @return the last element of this deque
* @throws NoSuchElementException if this deque is empty
*/
E removeLast();
/**
* Retrieves, but does not remove, the first element of this deque,
* returning null if this deque is empty.
*
* @return the first element of this deque, or null if
* this deque is empty
*/
E peekFirst();
/**
* Retrieves, but does not remove, the last element of this deque,
* returning null if this deque is empty.
*
* @return the last element of this deque, or null if this deque
* is empty
*/
E peekLast();
/**
* Retrieves, but does not remove, the first element of this
* deque. This method differs from the peek method only
* in that it throws an exception if this deque is empty.
*
* @return the first element 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 the peek method only
* in that it throws an exception if this deque is empty.
*
* @return the last element of this deque
* @throws NoSuchElementException if this deque is empty
*/
E getLast();
/**
* Removes the first occurrence of the specified element in 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).
*
* @param e element to be removed from this deque, if present
* @return true if the deque contained the specified element
* @throws NullPointerException if the specified element is null
*/
boolean removeFirstOccurrence(Object e);
/**
* Removes the last occurrence of the specified element in 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).
*
* @param e element to be removed from this deque, if present
* @return true if the deque contained the specified element
* @throws NullPointerException if the specified element is null
*/
boolean removeLastOccurrence(Object e);
// *** Queue methods ***
/**
* Inserts the specified element into the queue represented by this deque
* unless it would violate capacity restrictions. In other words, inserts
* the specified element to the end of this deque. 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 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 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 iterator();
}