dequeue()

/* Removes an item from the front of the queue, and returns its data */
public Data dequeue() {
    if (isEmpty()) {                                                // 1
        throw new NoSuchElementException("Queue is empty!");        // 1
    }

    Data data = head.data;                                          // 2
    head = head.next;                                               // 3
    size--;                                                         // 4

    if (isEmpty()) {                                                // 5
        tail = null;                                                // 5
    }

    return data;                                                    // 6
}