public static Collection subtract(final Collection a, final Collection b) {
        ArrayList list = new ArrayList(a);
        for (Iterator it = b.iterator(); it.hasNext(); ) {
            list.remove(it.next());
        }
        return list;
    }
@Test
    void testSubtract() {
        List<String> subtract = (List<String>) CollectionUtils.subtract(Arrays.asList("a", "b"),
                Arrays.asList("b", "c"));
        assertEquals(1, subtract.size());
        assertEquals("a", subtract.get(0));
    }