enqueue()

/* Add a new item to the back of the queue */
public void enqueue(Item item) {
    if (q.length == length) {       // 1
        resize(2 * q.length);       // 1
    }                               // 1

    q[tail] = item;                 // 2
    tail = (tail + 1) % q.length;   // 3
    length++;                       // 4
}