CareerCup_3.5 - Queue implemented by two stacks

StackQueue.java

package Chapter3;

public class StackQueue<T> {
    private Stack<T> fir;
    private Stack<T> sec;

    public StackQueue() {
	fir = new Stack<>();
	sec = new Stack<>();
    }

    public void enqueue(T value) {
	fir.push(value);
    }

    public T dequeue() {
	if (sec.isEmpty()) {
	    while (!fir.isEmpty()) {
		sec.push(fir.pop());
	    }
	}
	if (sec.isEmpty())
	    return null;
	return sec.pop();
    }

    public boolean isEmpty() {
	return fir.isEmpty() && sec.isEmpty();
    }

    public static void main(String[] args) {
	StackQueue<Integer> queue = new StackQueue<>();
	for (int i = 1; i <= 10; i++)
	    queue.enqueue(i);

	for (int i = 1; i <= 5; i++)
	    System.out.println("Dequeue: " + queue.dequeue());

	for (int i = 11; i <= 15; i++)
	    queue.enqueue(i);

	while (!queue.isEmpty())
	    System.out.println("Dequeue: " + queue.dequeue());
	//test dequeue empty stack
	System.out.println("Dequeue: " + queue.dequeue());
    }
}