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Queue.java Queue.java Below is the syntax highlighted version of Queue.java from §4.3 Stacks and Queues. /****************************************************************************** * Compilation: javac Queue.java * Execution: java Queue < input.txt * Data files: https://introcs.cs.princeton.edu/java/43stack/tobe.txt * * A generic queue, implemented using a linked list. * * % java Queue < tobe.txt * to be or not to be (2 left on queue) * ******************************************************************************/ import java.util.Iterator; import java.util.NoSuchElementException; /** * The {@code Queue} class represents a first-in-first-out (FIFO) queue of * generic items. It supports the usual enqueue and dequeue * operations, along with methods for peeking at the top item, testing if the * queue is empty, getting the number of items in the queue, and iterating * over the items in FIFO order. *

* This implementation uses a singly-linked list with a nested class for * linked-list nodes. The enqueue, dequeue, peek, * size, and is-empty operations all take constant time * in the worst case. *

* For additional documentation, see * Section 4.3 of * Computer Science: An Interdisciplinary Approach * by Robert Sedgewick and Kevin Wayne. * * @param the generic type of an item in this queue * @author Robert Sedgewick * @author Kevin Wayne */ public class Queue implements Iterable { 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; // the item in the node private Node next; // reference to next item } /** * Initializes an empty queue. */ public Queue() { first = null; last = null; n = 0; } /** * Returns true if this queue is empty. * * @return {@code true} if this queue is empty; {@code 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 number of items in this queue. * * @return the number of items in this queue */ public int length() { 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; } /** * Add the item to the queue. * * @param item the item to enqueue */ 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++; } /** * 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 iterator() { return new ListIterator(); } // an iterator, doesn't implement remove() since it's optional private class ListIterator implements Iterator { private Node current = first; // node containing current item public boolean hasNext() { return current != null; } public void remove() { throw new UnsupportedOperationException(); } public Item next() { if (!hasNext()) throw new NoSuchElementException(); Item item = current.item; current = current.next; return item; } } /** * Unit tests the {@code Queue} data type. * * @param args the command-line arguments */ public static void main(String[] args) { Queue queue = new Queue(); 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 © 2000–2017, Robert Sedgewick and Kevin Wayne. Last updated: Wed Jul 31 15:08:53 EDT 2019.