Queue类表示泛型项的先进先出（FIFO）队列。
/****************************************************************************** * Copyright 2002-2015, Robert Sedgewick and Kevin Wayne. * * This file is part of algs4.jar, which accompanies the textbook * * Algorithms, 4th edition by Robert Sedgewick and Kevin Wayne, * Addison-Wesley Professional, 2011, ISBN 0-321-57351-X. * http://algs4.cs.princeton.edu * * * algs4.jar is free software: you can redistribute it and/or modify * it under the terms of the GNU General Public License as published by * the Free Software Foundation, either version 3 of the License, or * (at your option) any later version. * * algs4.jar 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 for more details. * * You should have received a copy of the GNU General Public License * along with algs4.jar. If not, see http://www.gnu.org/licenses. ******************************************************************************/ /* 来自 n o w j a v a . c o m - 时 代 Java*/ /****************************************************************************** * Compilation: javac Queue.java * Execution: java Queue < input.txt * Dependencies: StdIn.java StdOut.java * Data files: http://algs4.cs.princeton.edu/13stacks/tobe.txt * * A generic queue, implemented using a linked list. * * % java Queue < tobe.txt * to be or not to be (2 left on queue) * ******************************************************************************/ package edu.pucit.cs.algo; import java.util.Iterator; import java.util.NoSuchElementException; /** * The <tt>Queue</tt> class represents a first-in-first-out (FIFO) * queue of generic items. * It supports the usual <em>enqueue</em> and <em>dequeue</em> * operations, along with methods for peeking at the first item, * testing if the queue is empty, and iterating through * the items in FIFO order. * <p> * This implementation uses a singly-linked list with a static nested class for * linked-list nodes. See {@link LinkedQueue} for the version from the * textbook that uses a non-static nested class. * The <em>enqueue</em>, <em>dequeue</em>, <em>peek</em>, <em>size</em>, and <em>is-empty</em> * operations all take constant time in the worst case. * <p> * For additional documentation, see <a href="http://algs4.cs.princeton.edu/13stacks">Section 1.3</a> of * <i>Algorithms, 4th Edition</i> by Robert Sedgewick and Kevin Wayne. * * @author Robert Sedgewick * @author Kevin Wayne * * @param <Item> the generic type of an item in this bag */ public class Queue<Item> implements Iterable<Item> { private Node<Item> first; // beginning of queue private Node<Item> last; // end of queue private int N; // number of elements on queue // helper linked list class private static class Node<Item> { private Item item; private Node<Item> next; /* *来 自 N o w J a v a . c o m */ } /** * Initializes an empty queue. */ public Queue() { first = null; last = null; N = 0; } /** * Returns true if this queue is empty. * * @return <tt>true</tt> if this queue is empty; <tt>false</tt> otherwise */ public boolean isEmpty() { return first == null; } /** * Returns the number of items in this queue. * * @return the number of items in this queue */ public int size() { return N; } /** * Returns the item least recently added to this queue. * * @return the item least recently added to this queue * @throws NoSuchElementException if this queue is empty */ public Item peek() { if (isEmpty()) throw new NoSuchElementException("Queue underflow"); return first.item; } /** * Adds the item to this queue. * * @param item the item to add */ public void enqueue(Item item) { Node<Item> oldlast = last; last = new Node<Item>(); last.item = item; last.next = null; if (isEmpty()) first = last; else oldlast.next = last; N++; } /** * Removes and returns the item on this queue that was least recently added. * * @return the item on this queue that was least recently added * @throws NoSuchElementException if this queue is empty */ public Item dequeue() { if (isEmpty()) throw new NoSuchElementException("Queue underflow"); Item item = first.item; first = first.next; N--; if (isEmpty()) last = null; // to avoid loitering return item; } /** * Returns a string representation of this queue. * * @return the sequence of items in FIFO order, separated by spaces */ public String toString() { StringBuilder s = new StringBuilder(); for (Item item : this) s.append(item + " "); return s.toString(); } /** * Returns an iterator that iterates over the items in this queue in FIFO order. * * @return an iterator that iterates over the items in this queue in FIFO order */ public Iterator<Item> iterator() { return new ListIterator<Item>(first); } // an iterator, doesn't implement remove() since it's optional private class ListIterator<Item> implements Iterator<Item> { private Node<Item> current; public ListIterator(Node<Item> first) { current = first; } public boolean hasNext() { return current != null;