public void testNull() throws Exception {
    doTestNull(CharMatcher.any());
    doTestNull(CharMatcher.none());
    doTestNull(is('a'));
    doTestNull(isNot('a'));
    doTestNull(anyOf(""));
    doTestNull(anyOf("x"));
    doTestNull(anyOf("xy"));
    doTestNull(anyOf("CharMatcher"));
    doTestNull(noneOf("CharMatcher"));
    doTestNull(inRange('n', 'q'));
    doTestNull(forPredicate(Predicates.equalTo('c')));
  }