FIFO - Implementation Of Immutable Queue

The following codes shows an implementation of an enqueue function of a FIFO immutable queue, and the output result shows the String "c". I don't understand why since I expected it should be null.

The head and the tail of an ImmutableQueue Object are two separate queue, and each time I call the enqueue function, it just return a new object with a new tail, however, the head is not modified except the first two times I call the function.

Therefore, I expected head.next.next should be a null element, but the result is not like that.

public class ImmutableQueue<E> {
public Queue<E> head;
public Queue<E> tail;
public ImmutableQueue(){}
private ImmutableQueue(Queue<E> hd, Queue<E> tl){
head=hd;
tail=tl;

[Code] ......