StackedQueueTest.java

import org.junit.Assert;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.junit.runners.JUnit4;

import java.util.NoSuchElementException;
import java.util.Stack;

/**
 * Решение задачи собеседования: сделать очередь из двух стеков
 *
 */
@RunWith(JUnit4.class)
public class StackedQueueTest {

    @Test
    public void test() {
        final MyStackedQueue<Integer> queue = new MyStackedQueue<>();

        queue.push(1);
        Assert.assertEquals(1, (int) queue.pop());

        queue.push(2);
        queue.push(3);

        Assert.assertEquals(2, (int) queue.pop());
        Assert.assertEquals(3, (int) queue.pop());
    }

    class MyStackedQueue<E>  {

        private Stack<E> outStack = new Stack<>();
        private Stack<E> inStack = new Stack<>();

        public void push(final E element) {
            inStack.push(element);
        }

        public E pop() {
            if (outStack.empty()
                    && !inStack.empty()) {
                while (!inStack.empty()) {
                    outStack.push(inStack.pop());
                }
            }

            if (outStack.empty()) {
                throw new NoSuchElementException();
            } else {
                return outStack.pop();
            }
        }
    }
}