public class Queue {
    ListItem front;
    ListItem back;
    Queue() {
        front = back = null;
    }
    public void enqueue(Object x) {
        if (isEmpty())
           front = back = new ListItem(x,null);
        else {
            back.next = new ListItem(x,null);
            back = back.next;
        }
    }
    public Object dequeue() {
        if (isEmpty())
           return null;
        else {
           Object first = front.item;
           front = front.next;
           if (front == null)
              back = null;
           return first;
        }
    }
    public Object peek() {
        if (isEmpty())
           return null;
        else
           return front.item;
    }
    public boolean isEmpty() {
        return (front == null);
    }
    public String toString() {
        if (isEmpty())
           return "empty queue";
        else
           return "queue (front to back): " + front;
    }
}