package DataStructures;

import java.util.NoSuchElementException;

/**
 * A linked list implementation of a queue
 * @param <E> the type of elements held in this queue
 * @author Peter Williams
 */

public class QueueList<E> implements Queue<E> {

    private ListNode<E> front; // read from here
    private ListNode<E> back;  // write to here

    /**
     * Creates an empty queue suitable for holding items of type E
     */
    public QueueList() {
        front = null;
    }
  
    public boolean isEmpty() {
        return front == null;
    }
  
    // join the back
    public void enqueue(E item) {
        if (front == null) {
            front = back = new ListNode<E>(item, null);
        } else {
            back = back.next = new ListNode<E>(item, null);
        }
    }

    // leave the front
    public E dequeue() {
        if (front == null) {
            throw new NoSuchElementException();
        } else {
            E item = front.data;
            front = front.next;
            return item; 
        }
    }
}