### Question:
CollectionUtils { public static <T> Collection<T> subtract(Set<T> a, Set<T> b) { return org.apache.commons.collections4.CollectionUtils.subtract(a, b); } private  CollectionUtils(); static Collection<T> subtract(Set<T> a, Set<T> b); static boolean isNotEmpty(Collection coll); static boolean isEmpty(Collection coll); static Map<String, String> stringToMap(String str, String separator); static Map<String, String> stringToMap(String str, String separator, String keyPrefix); static boolean equalLists(Collection<?> a, Collection<?> b); static boolean isEqualCollection(final Collection<?> a, final Collection<?> b); static Map<O, Integer> getCardinalityMap(final Iterable<? extends O> coll); static List<Map<String, Object>> getListByExclusion(List<T> originList, Set<String> exclusionSet);  }### Answer:
@Test public void subtract() { Set<Integer> a = new HashSet<Integer>(); a.add(1); a.add(2); a.add(3); Set<Integer> b = new HashSet<Integer>(); b.add(0); b.add(2); b.add(4); Assert.assertArrayEquals(new Integer[]{1,3},CollectionUtils.subtract(a,b).toArray()); }