@VisibleForTesting
  public Map<String, String> getOptionsMap() {
    return mMountOptions.stream()
        .collect(ImmutableMap.toImmutableMap(
            Pair::getFirst,
            Pair::getSecond,
            // merge function: use value of the last occurrence if the key occurs multiple times
            (oldValue, newValue) -> newValue
        ));
  }
@Test
  public void parseKeyOnly() throws Exception {
    MountCliOptions mountOptions = new MountCliOptions();
    JCommander jCommander = JCommander.newBuilder()
        .addObject(mountOptions)
        .build();
    jCommander.parse("-o", "key1", "-o", "key2");
    Map<String, String> optionsMap = mountOptions.getOptionsMap();
    assertEquals(Optional.of(""), Optional.ofNullable(optionsMap.get("key1")));
    assertEquals(Optional.of(""), Optional.ofNullable(optionsMap.get("key2")));
  }