@Override
 protected void listUsers() {
   if (group != null) {
     users = iam.getGroup(new GetGroupRequest(group.getGroupName())).getUsers();
   } else {
     users = null;
   }
 }
 private void removeUser(String userName) {
   iam.removeUserFromGroup(
       new RemoveUserFromGroupRequest()
           .withGroupName(group.getGroupName())
           .withUserName(userName));
 }