@Test
void testUnion() {

    Set<Integer> a = newHashSet(2, 3, 4);
    Set<Integer> b = newHashSet(3, 4, 5, 6);
    Set<Integer> result = CollectionUtils.union(a, b);
    assertEquals(newHashSet(2, 3, 4, 5, 6), result);
    result = CollectionUtils.union(a, newHashSet());
    assertEquals(a, result);
}