LinkedQueue.java

public class LinkedQueue<T> implements QueueADT<T> {
  private int count;
  private LinearNode<T> front, rear;

  /**
  * Creates an empty queue.
  */
  public LinkedQueue(){
    count = 0;
    front = rear = null;
  }

  /**
  * Adds the specified element to the rear of this queue.
  *
  * @param element  the element to be added to the rear of this queue
  */
  public void enqueue(T element){
    LinearNode<T> node = new LinearNode<T>(element);

    if (isEmpty()){
      front = node;
    }else{
      rear.setNext (node);
    }
    rear = node;
    count++;
  }

  /**
  * Removes the element at the front of this queue and returns a
  * reference to it. Throws an EmptyCollectionException if the
  * queue is empty.
  *
  * @return                           the element at the front of this queue
  * @throws EmptyCollectionException  if an empty collection exception occurs
  */
  public T dequeue() throws EmptyCollectionException{
    if (isEmpty()){
      throw new EmptyCollectionException ("queue");
    }
    T result = front.getElement();
    front = front.getNext();
    count--;

    if (isEmpty()){
      rear = null;
    }
    return result;
  }

  /**
  * Returns a reference to the element at the front of this queue.
  * The element is not removed from the queue.  Throws an
  * EmptyCollectionException if the queue is empty.
  *
  * @return                            a reference to the first element in
  *                                    this queue
  * @throws EmptyCollectionsException  if an empty collection exception occurs
  */
  public T first() throws EmptyCollectionException{
    if (isEmpty()){
      throw new EmptyCollectionException ("queue");
    }
    return front.getElement();
  }

  /**
  * Returns true if this queue is empty and false otherwise.
  *
  * @return  true if this queue is empty and false if otherwise
  */
  public boolean isEmpty(){
    return (count == 0);
  }

  /**
  * Returns the number of elements currently in this queue.
  *
  * @return  the integer representation of the size of this queue
  */
  public int size(){
    return count;
  }

  /**
  * Returns a string representation of this queue.
  *
  * @return  the string representation of this queue
  */
  public String toString(){
    String result = "";
    LinearNode<T> current = front;

    while (current != null){
      result = result + (current.getElement()).toString() + "\n";
      current = current.getNext();
    }

    return result;
  }
}