--- jsr166/src/jsr166x/Deque.java 2004/09/05 21:28:19 1.1 +++ jsr166/src/jsr166x/Deque.java 2015/01/18 20:17:33 1.13 @@ -1,260 +1,443 @@ /* - * Written by Doug Lea with assistance from members of JCP JSR-166 - * Expert Group and released to the public domain, as explained at - * http://creativecommons.org/licenses/publicdomain + * 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; -import java.util.*; +package jsr166x; // XXX This belongs in java.util!!! XXX + +import java.util.*; // XXX This import goes away XXX /** - * A linear collection in which elements may be inserted and removed - * from both the beginning and end. A Deque (short for - * "double ended queue") provides uniformly named methods to - * get, peek, poll, remove, - * offer, and add the first and - * last element of the collection (for example, methods - * addFirst, pollLast). Unlike interface {@link - * List} the Deque interface does not define support for indexed - * operations or sublists. - * - *

A view of a subset of Deque operations can be obtained using - * method {@link #asFIFO} to support only Last-In-First-Out (LIFO) - * stack behavior, as well as method {@link #asFIFO} to support only - * First-in-First-Out (FIFO) queue behavior. More commonly, a Deque - * is used when various mixtures of LIFO and FIFO operations are - * required. - * - *

Deques additionally provide a few methods to manipulate elements - * embedded within a deque, proceding from either direction: - * removeFirstOccurrence, replaceFirstOccurrence - * removeLastOccurrence, and replaceLastOccurrence. - * They also support {@link Collection} operations including - * contains, iterator, and so on. - * - *

The {@link #offerFirst} and {@link #offerLast} methods insert an - * element if possible, otherwise returning false. They - * differ from {@link java.util.Collection#add Collection.add}, as - * well as {@link #addFirst} and {@link #addLast} methods, which can - * fail to add an element only by throwing an unchecked exception. - * The offer methods are designed for use when failure is a - * normal, rather than exceptional occurrence, for example, in - * fixed-capacity (or "bounded") deques. - * - *

Deque implementations generally do not allow insertion - * of null elements. Even in implementations that permit it, - * null should not be inserted into a Deque, as - * null is also used as a special return value by the poll - * methods to indicate that the deque contains no elements. - * - *

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

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 Collection { - +public interface Deque extends Queue { /** - * Inserts the specified element to the front this deque, if - * possible. When using deques that may impose insertion - * restrictions (for example capacity bounds), method - * offerFirst is generally preferable to method - * addFirst which can fail to insert a non-duplicate - * element only by throwing an exception. + * 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 {@code addFirst}, which + * can fail to insert an element only by throwing an exception. * - * @param o the element to insert. - * @return true if it was possible to add the element to - * this deque, else false + * @param e the element to insert + * @return {@code true} if it was possible to insert the element, + * else {@code false} + * @throws NullPointerException if {@code e} is null and this + * deque does not permit null elements */ - boolean offerFirst(E o); + boolean offerFirst(E e); /** - * Inserts the specified element to the end this deque, if - * possible. When using deques that may impose insertion - * restrictions (for example capacity bounds), method - * offerFirst is generally preferable to method - * addLast which can fail to insert a non-duplicate - * element only by throwing an exception. + * 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 {@code addLast} which + * can fail to insert an element only by throwing an exception. * - * @param o the element to insert. - * @return true if it was possible to add the element to - * this deque, else false + * @param e the element to insert + * @return {@code true} if it was possible to insert the element, + * else {@code false} + * @throws NullPointerException if {@code e} is null and this + * deque does not permit null elements */ - boolean offerLast(E o); + boolean offerLast(E e); /** - * Inserts the specified element to the front this deque, if - * this deque permits insertion of the given element. This - * method has the same semantics as {@link Collection#add}. + * Inserts the specified element to the front of this deque unless it + * would violate capacity restrictions. * - * @param o the element to insert. - * @return true if this deque changed as a result of this - * call, else false + * @param e the element to insert + * @throws IllegalStateException if it was not possible to insert + * the element due to capacity restrictions + * @throws NullPointerException if {@code e} is null and this + * deque does not permit null elements */ - boolean addFirst(E o); + void addFirst(E e); /** - * Inserts the specified element to the end this deque, if - * this deque permits insertion of the given element. This - * method has the same semantics as {@link Collection#add}. + * Inserts the specified element to the end of this deque unless it would + * violate capacity restrictions. * - * @param o the element to insert. - * @return true if this deque changed as a result of this - * call, else false + * @param e the element to insert + * @throws IllegalStateException if it was not possible to insert + * the element due to capacity restrictions + * @throws NullPointerException if {@code e} is null and this + * deque does not permit null elements */ - boolean addLast(E o); + void addLast(E e); /** * Retrieves and removes the first element of this deque, or - * null if this deque is empty. + * {@code null} if this deque is empty. * - * @return the first element of this deque, or null if - * this deque is empty. + * @return the first element of this deque, or {@code null} if + * this deque is empty */ E pollFirst(); /** * Retrieves and removes the last element of this deque, or - * null if this deque is empty. + * {@code null} if this deque is empty. * - * @return the last element of this deque, or null if - * this deque is empty. + * @return the last element of this deque, or {@code null} if + * this deque is empty */ E pollLast(); /** - * Retrieves and removes the first element of this deque. This method - * differs from the pollFirst method in that it throws an + * Removes and returns the first element of this deque. This method + * differs from the {@code 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. + * @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 in that it throws an + * differs from the {@code 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. + * @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. + * returning {@code null} if this deque is empty. * - * @return the first element of this deque, or null if - * this deque is empty. + * @return the first element of this deque, or {@code 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. + * returning {@code null} if this deque is empty. * - * @return the last element of this deque, or null if this deque - * is empty. + * @return the last element of this deque, or {@code 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 + * deque. This method differs from the {@code 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. + * @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 + * deque. This method differs from the {@code 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. + * @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 + * unchanged. More formally, removes the first element {@code e} + * such that {@code (o==null ? e==null : o.equals(e))} (if * such an element exists). * - * @param o 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 + * @param e element to be removed from this deque, if present + * @return {@code true} if the deque contained the specified element + * @throws NullPointerException if the specified element is {@code null} */ - boolean removeFirstOccurrence(E o); + 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 + * unchanged. More formally, removes the last element {@code e} + * such that {@code (o==null ? e==null : o.equals(e))} (if * such an element exists). * - * @param o 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 + * @param e element to be removed from this deque, if present + * @return {@code true} if the deque contained the specified element + * @throws NullPointerException if the specified element is {@code null} */ - boolean removeLastOccurrence(E o); + boolean removeLastOccurrence(Object e); + + + // *** Queue methods *** /** - * Replaces the first occurrence of the specified element in this - * deque. If the deque does not contain the element, it is - * unchanged. More formally, replaces the first element e - * such that (o==null ? e==null : o.equals(e)) (if - * such an element exists). + * 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. * - * @param oldElement element to be replaced in this deque, if present. - * @param newElement replacement value - * @return true if an element was replaced + *

This method is equivalent to {@link #offerLast}. + * + * @param e the element to insert + * @return {@code true} if it was possible to insert the element, + * else {@code false} + * @throws NullPointerException if {@code e} is null and this + * deque does not permit null elements */ - public boolean replaceFirstOccurrence(E oldElement, E newElement); + boolean offer(E e); /** - * Replaces the last occurrence of the specified element in this - * deque. If the deque does not contain the element, it is - * unchanged. More formally, replaces the last element e - * such that (o==null ? e==null : o.equals(e)) (if - * such an element exists). + * 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 {@code 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 {@code 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 {@code null} if this deque is empty. In other words, + * retrieves and removes the first element of this deque, or {@code null} + * if this deque is empty. + * + *

This method is equivalent to {@link #pollFirst()}. * - * @param oldElement element to be replaced in this deque, if present. - * @param newElement replacement value - * @return true if an element was replaced + * @return the first element of this deque, or {@code null} if + * this deque is empty */ - public boolean replaceLastOccurrence(E oldElement, E newElement); + E poll(); /** - * Returns a view of this deque as a first-in-first-out queue, - * mapping {@link Queue#offer} to offerLast, {@link - * Queue#poll} to pollFirst, and other operations - * accordingly. - * @return a first-in-first-out view of this deque. + * Retrieves and removes the head of the queue represented by this deque. + * This method differs from the {@code 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 {@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(); + + /** + * Retrieves, but does not remove, the head of the queue represented by + * this deque. This method differs from the {@code 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 {@code e} is null and this + * deque does not permit null elements */ - public Queue asFIFO(); + void push(E e); /** - * Returns a view of this deque as a first-in-Last-out stack, - * mapping {@link Queue#offer} to offerFirst, {@link - * Queue#poll} to pollFirst, and other operations - * accordingly. - * @return a first-in-last-out view of this deque. + * 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 */ - public Queue asLIFO(); + 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 {@code Iterator} over the elements in this deque + */ + Iterator iterator(); }