Queue_Linked_List.java

/******************************************************************************
 *
 *  A generic queue, implemented using a singly-linked list.
 *  Reference:  http://algs4.cs.princeton.edu/13stacks/Queue_Linked_List.java.html
 *
 *
 ******************************************************************************/

import java.util.Iterator;
import java.util.NoSuchElementException;

/**
 *  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>
 */
public class Queue_Linked_List<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 Queue_Linked_List() {
        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 ListIterator();
    }

    // an iterator, doesn't implement remove() since it's optional
    private class ListIterator implements Iterator<Item> {
        private Node current = first;

        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;
        }
    }

    public static void main(String[] args) {
        Queue_Linked_List<Integer> queue = new Queue_Linked_List<Integer>();
	queue.enqueue(1);
        queue.enqueue(2);
        queue.enqueue(3);
	System.out.println("(" + queue.size() + " left on queue)");
	System.out.println("Dequeued 1 item off the Queue");
        queue.dequeue();
        System.out.println("(" + queue.size() + " left on queue)");
	System.out.println(queue.toString());
    }
}

/* Output
   (3 left on queue)
   Dequeued 1 item off the Queue
   (2 left on queue)
   2 3
*/