data-structures/java/Queue/LQueue.java

/** Linked queue implementation */
class Lqueue<E> implements Queue<E> {
    private Link<E> front;  // Pointer to front queue node
    private Link<E> rear;   // Pointer to rear queue node
    int size;   // Number of elements in queue
    
     /** Constructors */
    public LQueue() { init(); }	
    public LQueue(int size) { init(); } // Ignore size

    /** Initialize queue */
    private void init() {
        front = rear = new Link<E>(null);
        size = 0;
    }

    /** Reinitialize queue */
    private void clear() { init(); };

    /** Put element on rear */
    public void enqueue(E it) {
        rear.setNext(new Link<E>(it, null));
        rear = rear.next();
        ++size;
    }

    /** Remove and return element from front */
    public E dequeue() {
        assert size != 0 : "Queue is empty";
        E it = front.next().element();  // Store dequeued value
        front.setNext(front.next().next()); // Advance front
        if (front.next() == null) rear = front; // Last object
        --size;
        return it;  //Return object
    }

    /** @return Front element */
    public E frontValue() {
        assert size != 0 : "Queue is empty";
        return front.next().element();
    }

    /** @return Queue size */
    public int length() { return size; }
}