public class LinkedQueue implements Queue {

    private class Node {
	private Object value;
	private Node next;

	private Node(Object value, Node next) {
	    this.value = value;
	    this.next = next;
	}
    }

    private Node front, rear;
    private int size, maxSize;

    public LinkedQueue() {
	front = rear = null;
	size = maxSize = 0;
    }

    public boolean isEmpty() {
	return front == null;
    }

    public boolean isFull() {
	return false;
    }

    public void enqueue(Object obj) {
	if (obj==null)
	    throw new IllegalArgumentException();

	Node newNode = new Node(obj, null);
	if (rear == null) {
	    rear = newNode;
	    front = rear;
	} else {
	    rear.next = newNode;
	    rear = newNode;
	}
	incrSize();
    }

    public Object dequeue() {
	if (isEmpty())
	    throw new IllegalStateException("empty");

	Object obj = front.value;
	front = front.next;
	if (front == null)
	    rear = null;
	decrSize();
	return obj;
    }

    public int size() {
	int n = 0;
	Node p=front;
	while (p!=null) {
	    p = p.next;
	    n++;
	}
	return n;
    }

    private void incrSize() {
	size++;
	if (size > maxSize)
	    maxSize = size;
    }

    private void decrSize() {
	size--;
    }

    public int maxSize() {
	return maxSize;
    }

    public String toString() {
	String out = "front -> [";
	if (! isEmpty()) {
	    Node p = front;
	    out = out + p.value;
	    p = p.next;
	    while (p != null) {
		out = out + ", " + p.value;
		p = p.next;
	    }
	}
	out = out + "] <- rear";
	return out;
    }

    public static void main(String[] args) {
	Queue q = new LinkedQueue();
	for (int i=0; i< 10; i++) {
	    q.enqueue("elem-" + i);
	    System.out.println(q);
	}
	while (! q.isEmpty())
	    System.out.println(q.dequeue());
	for (int i=0; i< 10; i++) {
	    q.enqueue("elem-" + i);
	    System.out.println(q);
	}
	while (! q.isEmpty())
	    System.out.println(q.dequeue());
    }
}