@Test
public void testFrequency() {

    CheckedIterator<String, RuntimeException> iterator;
    iterator = create("one", "two");
    assertEquals(0, iterator.frequency("three"));
    iterator = create("one", "two");
    assertEquals(1, iterator.frequency("one"));
    iterator = create("one", "two", "one");
    assertEquals(2, iterator.frequency("one"));
}