Below is the syntax highlighted version of LinkedQueue.java.
/****************************************************************************** * Compilation: javac LinkedQueue.java * Execution: java LinkedQueue < input.txt * Dependencies: StdIn.java StdOut.java * Data files: https://algs4.cs.princeton.edu/13stacks/tobe.txt * * A generic queue, implemented using a singly linked list. * * % java Queue < tobe.txt * to be or not to be (2 left on queue) * ******************************************************************************/ package edu.princeton.cs.algs4; import java.util.Iterator; import java.util.NoSuchElementException; /** * The {@code LinkedQueue} 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 non-static nested class * for linked-list nodes. See {@link Queue} for a version that uses a 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="https://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 */ public class LinkedQueue<Item> implements Iterable<Item> { private int n; // number of elements on queue private Node first; // beginning of queue private Node last; // end of queue // helper linked list class private class Node { private Item item; private Node next; } /** * Initializes an empty queue. */ public LinkedQueue() { first = null; last = null; n = 0; assert check(); } /** * Is this queue empty? * @return true if this queue is empty; false 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 java.util.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 oldlast = last; last = new Node(); last.item = item; last.next = null; if (isEmpty()) first = last; else oldlast.next = last; n++; assert check(); } /** * 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 java.util.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 assert check(); 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(); } // check internal invariants private boolean check() { if (n < 0) { return false; } else if (n == 0) { if (first != null) return false; if (last != null) return false; } else if (n == 1) { if (first == null || last == null) return false; if (first != last) return false; if (first.next != null) return false; } else { if (first == null || last == null) return false; if (first == last) return false; if (first.next == null) return false; if (last.next != null) return false; // check internal consistency of instance variable n int numberOfNodes = 0; for (Node x = first; x != null && numberOfNodes <= n; x = x.next) { numberOfNodes++; } if (numberOfNodes != n) return false; // check internal consistency of instance variable last Node lastNode = first; while (lastNode.next != null) { lastNode = lastNode.next; } if (last != lastNode) return false; } return true; } /** * 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 LinkedIterator(); } // a linked-list iterator private class LinkedIterator implements Iterator<Item> { private Node current = first; public boolean hasNext() { return current != null; } public Item next() { if (!hasNext()) throw new NoSuchElementException(); Item item = current.item; current = current.next; return item; } } /** * Unit tests the {@code LinkedQueue} data type. * * @param args the command-line arguments */ public static void main(String[] args) { LinkedQueue<String> queue = new LinkedQueue<String>(); while (!StdIn.isEmpty()) { String item = StdIn.readString(); if (!item.equals("-")) queue.enqueue(item); else if (!queue.isEmpty()) StdOut.print(queue.dequeue() + " "); } StdOut.println("(" + queue.size() + " left on queue)"); } } /****************************************************************************** * Copyright 2002-2022, 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. ******************************************************************************/