This is my adapter:

    /**
     * recyclerviewUserList display adapter
     */
    public class UserAdapter extends RecyclerView.Adapter<UserAdapter.UserViewHolder> {
        final private List<ObjectUser> mUserList;
        class UserViewHolder extends RecyclerView.ViewHolder {
            final TextView tvUsername;
            final TextView tvCreated;
            final TextView tvGrade;
            final TextView tvDepartment;
            UserViewHolder(View userView) {
                super(userView);
                tvUsername = userView.findViewById(R.id.tvUsername);
                tvCreated = userView.findViewById(R.id.tvCreated);
                tvGrade = userView.findViewById(R.id.tvGrade);
                tvDepartment = userView.findViewById(R.id.tvDepartment);
            }
        }
        UserAdapter(ArrayList<ObjectUser> userlist) {
            mUserList = userlist;
        }
        @NonNull
        @Override
        public UserViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
            View v = LayoutInflater.from(parent.getContext()).inflate(R.layout.fragment_user_list_item, parent,false);
            return new UserViewHolder(v);
        }
        @Override
        public void onBindViewHolder(@NonNull final UserViewHolder holder, int position) {
            final ObjectUser currentUser = mUserList.get(position);
            holder.tvUsername.setText(currentUser.getUsername());
            holder.tvCreated.setText(currentUser.getCreated());
            holder.tvGrade.setText(currentUser.getGradeName());
            holder.tvDepartment.setText(currentUser.getDepartmentname());
            //getGradeName(currentUser.getGrade(), holder.tvGrade :: setText);
            //getDepartmentName(currentUser.getDepartment(), holder.tvDepartment :: setText);
        }
        @Override
        public int getItemCount() {
            return mUserList.size();
        }
    }
}