Java tutorial
/* * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. * * This code is free software; you can redistribute it and/or modify it * under the terms of the GNU General Public License version 2 only, as * published by the Free Software Foundation. Oracle designates this * particular file as subject to the "Classpath" exception as provided * by Oracle in the LICENSE file that accompanied this code. * * This code is distributed in the hope that it will be useful, but WITHOUT * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License * version 2 for more details (a copy is included in the LICENSE file that * accompanied this code). * * You should have received a copy of the GNU General Public License version * 2 along with this work; if not, write to the Free Software Foundation, * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. * * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.com if you need additional information or have any * questions. */ /* * This file is available under and governed by the GNU General Public * License version 2 only, as published by the Free Software Foundation. * However, the following notice accompanied the original version of this * file: * * 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 java.util; /** * A linear collection that supports element insertion and removal at * both ends. The name <i>deque</i> 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. * * <p>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. * * <p>The twelve methods described above are summarized in the * following table: * * <table class="striped"> * <caption>Summary of Deque methods</caption> * <thead> * <tr> * <td rowspan="2"></td> * <th scope="col" colspan="2"> First Element (Head)</th> * <th scope="col" colspan="2"> Last Element (Tail)</th> * </tr> * <tr> * <th scope="col" style="font-weight:normal; font-style:italic">Throws exception</th> * <th scope="col" style="font-weight:normal; font-style:italic">Special value</th> * <th scope="col" style="font-weight:normal; font-style:italic">Throws exception</th> * <th scope="col" style="font-weight:normal; font-style:italic">Special value</th> * </tr> * </thead> * <tbody> * <tr> * <th scope="row">Insert</th> * <td>{@link #addFirst(Object) addFirst(e)}</td> * <td>{@link #offerFirst(Object) offerFirst(e)}</td> * <td>{@link #addLast(Object) addLast(e)}</td> * <td>{@link #offerLast(Object) offerLast(e)}</td> * </tr> * <tr> * <th scope="row">Remove</th> * <td>{@link #removeFirst() removeFirst()}</td> * <td>{@link #pollFirst() pollFirst()}</td> * <td>{@link #removeLast() removeLast()}</td> * <td>{@link #pollLast() pollLast()}</td> * </tr> * <tr> * <th scope="row">Examine</th> * <td>{@link #getFirst() getFirst()}</td> * <td>{@link #peekFirst() peekFirst()}</td> * <td>{@link #getLast() getLast()}</td> * <td>{@link #peekLast() peekLast()}</td> * </tr> * </tbody> * </table> * * <p>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: * * <table class="striped"> * <caption>Comparison of Queue and Deque methods</caption> * <thead> * <tr> * <th scope="col"> {@code Queue} Method</th> * <th scope="col"> Equivalent {@code Deque} Method</th> * </tr> * </thead> * <tbody> * <tr> * <th scope="row">{@link #add(Object) add(e)}</th> * <td>{@link #addLast(Object) addLast(e)}</td> * </tr> * <tr> * <th scope="row">{@link #offer(Object) offer(e)}</th> * <td>{@link #offerLast(Object) offerLast(e)}</td> * </tr> * <tr> * <th scope="row">{@link #remove() remove()}</th> * <td>{@link #removeFirst() removeFirst()}</td> * </tr> * <tr> * <th scope="row">{@link #poll() poll()}</th> * <td>{@link #pollFirst() pollFirst()}</td> * </tr> * <tr> * <th scope="row">{@link #element() element()}</th> * <td>{@link #getFirst() getFirst()}</td> * </tr> * <tr> * <th scope="row">{@link #peek() peek()}</th> * <td>{@link #peekFirst() peekFirst()}</td> * </tr> * </tbody> * </table> * * <p>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 equivalent to {@code Deque} * methods as indicated in the table below: * * <table class="striped"> * <caption>Comparison of Stack and Deque methods</caption> * <thead> * <tr> * <th scope="col"> Stack Method</th> * <th scope="col"> Equivalent {@code Deque} Method</th> * </tr> * </thead> * <tbody> * <tr> * <th scope="row">{@link #push(Object) push(e)}</th> * <td>{@link #addFirst(Object) addFirst(e)}</td> * </tr> * <tr> * <th scope="row">{@link #pop() pop()}</th> * <td>{@link #removeFirst() removeFirst()}</td> * </tr> * <tr> * <th scope="row">{@link #peek() peek()}</th> * <td>{@link #getFirst() getFirst()}</td> * </tr> * </tbody> * </table> * * <p>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. * * <p>This interface provides two methods to remove interior * elements, {@link #removeFirstOccurrence removeFirstOccurrence} and * {@link #removeLastOccurrence removeLastOccurrence}. * * <p>Unlike the {@link List} interface, this interface does not * provide support for indexed access to elements. * * <p>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 <i>not</i> 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 indicate that the deque is empty. * * <p>{@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}. * * <p>This interface is a member of the * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework"> * Java Collections Framework</a>. * * @author Doug Lea * @author Josh Bloch * @since 1.6 * @param <E> the type of elements held in this deque */ public interface Deque<E> extends Queue<E> { /** * 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}. * * <p>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 * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>) * @throws NullPointerException if the specified element is null and this * deque does not permit null elements * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>) */ 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 * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>) * @throws NullPointerException if the specified element is null and this * deque does not permit null elements * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>) */ 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}. * * <p>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. * * <p>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. * * <p>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. * * <p>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. * * <p>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. * * <p>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(); /** * Adds all of the elements in the specified collection at the end * of this deque, as if by calling {@link #addLast} on each one, * in the order that they are returned by the collection's iterator. * * <p>When using a capacity-restricted deque, it is generally preferable * to call {@link #offer(Object) offer} separately on each element. * * <p>An exception encountered while trying to add an element may result * in only some of the elements having been successfully added when * the associated exception is thrown. * * @param c the elements to be inserted into this deque * @return {@code true} if this deque changed as a result of the call * @throws IllegalStateException if not all the elements can be added at * this time due to insertion restrictions * @throws ClassCastException if the class of an element of the specified * collection prevents it from being added to this deque * @throws NullPointerException if the specified collection contains a * null element and this deque does not permit null elements, * or if the specified collection is null * @throws IllegalArgumentException if some property of an element of the * specified collection prevents it from being added to this deque */ boolean addAll(Collection<? extends E> c); // *** 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. * * <p>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. * * <p>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). * * <p>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 * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>) * @throws NullPointerException if the specified element is null and this * deque does not permit null elements * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>) */ 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 * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>) * @throws NullPointerException if the specified element is null and this * deque does not permit null elements * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>) */ 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<E> 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<E> descendingIterator(); }