@Test
public void testPeek() throws Exception {

    stack.push("a");
    assertEquals(stack.peek(), "a");
    stack.push("b");
    assertEquals(stack.peek(), "b");
    stack.push("c");
    assertEquals(stack.peek(), "c");
    assertEquals(stack.peek(), "c");
    stack.pop();
    assertEquals(stack.peek(), "b");
    stack.pop();
    assertEquals(stack.peek(), "a");
}