Can someone review my implementation of a Queue?

Question

So here is my Queue interface:

public interface Queue<E> {
    /**
     * Add an item to the rear of the queue.
     *
     * @param item
     *            Item to be added.
     */
    public void enqueue(E item);

    /**
     * Remove and return the item at the front of the queue.
     *
     * @return The item that was removed.
     */
    public E dequeue();

    /**
     * @return The item at the front of the queue.
     */
    public E frontValue();

    /**
     * @return How many elements are in the queue.
     */
    public int length();

    /**
     * Remove all the items in the queue.
     */
    public void clear();
}

And here is my actual implementation:

public class LinkedQueue<E> implements Queue<E> {
    private SinglyLinkedListNode<E> front;
    private SinglyLinkedListNode<E> rear;
    private int size;

    /**
     * Create a new LinkedQueue object.
     */
    public LinkedQueue() {
    this.init();
    }

    private void init() {
    // only one object is constructed
    this.front = this.rear = new SinglyLinkedListNode<E>(null, null);
    this.size = 0;
    }

    @Override
    public void enqueue(E value) {
    this.rear.setNextNode(new SinglyLinkedListNode<E>(value, null));
    this.rear = this.rear.getNextNode();
    this.size++;
    }

    @Override
    public E dequeue() {
    if (this.size == 0) {
        throw new IllegalStateException("In method dequeue of class "
            + "LinkedQueue the linked queue is empty can cannot"
            + " be dequeued");
    }

    E item = this.front.getNextNode().getValue();
    this.front.setNextNode(this.front.getNextNode().getNextNode());

    if (this.front.getNextNode() == null) {
        this.rear = this.front;
    }
    this.size--;
    return item;
    }

    @Override
    public E frontValue() {
    if (this.size == 0) {
        throw new IllegalStateException("In method frontValue of class "
            + "LinkedQueue the linked queue is empty");
    }
    return this.front.getNextNode().getValue();
    }

    @Override
    public int length() {
    return this.size;
    }

    @Override
    public void clear() {
    this.init();
    }

    /**
     * Creates a easy to read String representation of the linked queue's
     * contents.
     *
     * Example: < 1 2 3 4 5 6 >
     */
    public String toString() {
    StringBuilder linkedQueueAsString = new StringBuilder();
    linkedQueueAsString.append("< ");
    System.out.println("front.getNextNode: " + this.front.getNextNode());
    for (SinglyLinkedListNode<E> node = this.front.getNextNode(); node != null; node = node.getNextNode()) {
        linkedQueueAsString.append(node.getValue());
        System.out.println(node.getValue() + ",");
        linkedQueueAsString.append(" ");
    }
    linkedQueueAsString.append(">");
    return linkedQueueAsString.toString();
    }
}

Answer 1

You should not name your interface Queue, since java.util.Queue already exits.

Also it would be helpful if you post the source of SingleLinkedListNode.

The source looks fine to me, you could however add two methods like (append/en-queue, remove/de-queue):

SingleLinkedListNode<E> append(SingleLinkedListNode<E> newNode) that will replace

 this.rear.setNextNode(new SingleLinkedListNode<E>(value, null));
 this.rear = this.rear.getNextNode();

and returns the next node.