@Deprecated
    public DomainNameMapping<V> add(String hostname, V output) {
        map.put(normalizeHostname(checkNotNull(hostname, "hostname")), checkNotNull(output, "output"));
        return this;
    }
@Test
    public void testNullValuesAreForbidden() {

        assertThrows(NullPointerException.class, new Executable() {
            @Override
            public void execute() {
                new DomainNameMappingBuilder<String>("NotFound").add("Some key", null);
            }
        });
    }