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 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; /** * This class provides skeletal implementations of some {@link Queue} * operations. The implementations in this class are appropriate when * the base implementation does <em>not</em> allow {@code null} * elements. Methods {@link #add add}, {@link #remove remove}, and * {@link #element element} are based on {@link #offer offer}, {@link * #poll poll}, and {@link #peek peek}, respectively, but throw * exceptions instead of indicating failure via {@code false} or * {@code null} returns. * * <p>A {@code Queue} implementation that extends this class must * minimally define a method {@link Queue#offer} which does not permit * insertion of {@code null} elements, along with methods {@link * Queue#peek}, {@link Queue#poll}, {@link Collection#size}, and * {@link Collection#iterator}. Typically, additional methods will be * overridden as well. If these requirements cannot be met, consider * instead subclassing {@link AbstractCollection}. * * <p>This class is a member of the * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework"> * Java Collections Framework</a>. * * @since 1.5 * @author Doug Lea * @param <E> the type of elements held in this queue */ public abstract class AbstractQueue<E> extends AbstractCollection<E> implements Queue<E> { /** * Constructor for use by subclasses. */ protected AbstractQueue() { } /** * Inserts the specified element into this queue 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. * * <p>This implementation returns {@code true} if {@code offer} succeeds, * else throws an {@code IllegalStateException}. * * @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 queue * @throws NullPointerException if the specified element is null and * this queue does not permit null elements * @throws IllegalArgumentException if some property of this element * prevents it from being added to this queue */ public boolean add(E e) { if (offer(e)) return true; else throw new IllegalStateException("Queue full"); } /** * Retrieves and removes the head of this queue. This method differs * from {@link #poll poll} only in that it throws an exception if this * queue is empty. * * <p>This implementation returns the result of {@code poll} * unless the queue is empty. * * @return the head of this queue * @throws NoSuchElementException if this queue is empty */ public E remove() { E x = poll(); if (x != null) return x; else throw new NoSuchElementException(); } /** * Retrieves, but does not remove, the head of this queue. This method * differs from {@link #peek peek} only in that it throws an exception if * this queue is empty. * * <p>This implementation returns the result of {@code peek} * unless the queue is empty. * * @return the head of this queue * @throws NoSuchElementException if this queue is empty */ public E element() { E x = peek(); if (x != null) return x; else throw new NoSuchElementException(); } /** * Removes all of the elements from this queue. * The queue will be empty after this call returns. * * <p>This implementation repeatedly invokes {@link #poll poll} until it * returns {@code null}. */ public void clear() { while (poll() != null) ; } /** * Adds all of the elements in the specified collection to this * queue. Attempts to addAll of a queue to itself result in * {@code IllegalArgumentException}. Further, the behavior of * this operation is undefined if the specified collection is * modified while the operation is in progress. * * <p>This implementation iterates over the specified collection, * and adds each element returned by the iterator to this * queue, in turn. A runtime exception encountered while * trying to add an element (including, in particular, a * {@code null} element) may result in only some of the elements * having been successfully added when the associated exception is * thrown. * * @param c collection containing elements to be added to this queue * @return {@code true} if this queue changed as a result of the call * @throws ClassCastException if the class of an element of the specified * collection prevents it from being added to this queue * @throws NullPointerException if the specified collection contains a * null element and this queue 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 * queue, or if the specified collection is this queue * @throws IllegalStateException if not all the elements can be added at * this time due to insertion restrictions * @see #add(Object) */ public boolean addAll(Collection<? extends E> c) { if (c == null) throw new NullPointerException(); if (c == this) throw new IllegalArgumentException(); boolean modified = false; for (E e : c) if (add(e)) modified = true; return modified; } }