@Test
 public void replaceOptional() {
   assertEquals(Optional.of(1), map.replaceOptional("Key1", 11));
   assertEquals(Optional.of(11), map.replaceOptional("Key1", 12));
   assertEquals(Optional.empty(), map.replaceOptional("NonExistingKey", 999));
   assertEquals(Optional.empty(), map.replaceOptional("NonExistingKey", 1000));
 }