How to properly initialize items in RecyclerView?

Question

Android

Here is my code for the snippet:

public class Fragment_Username_Search extends Fragment {
private List<User> userList = new ArrayList<>();
    private RecyclerView recyclerView;
    private UserAdapter userAdapter;

    @TargetApi(Build.VERSION_CODES.M)
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

    }

    private void prepareUserData() {
User user = new User(R.drawable.emilypic,"Marry Young","marbear",R.id.item_follow_button);
userList.add(user);
        user = new User(R.drawable.emilypic,"Marry Young","marbear",R.id.item_follow_button);
        userList.add(user);
        user = new User(R.drawable.emilypic,"Sammy Lee","sambam",R.id.item_follow_button);
        userList.add(user);
        user = new User(R.drawable.emilypic,"Lilly Martinson","lilmartini",R.id.item_follow_button);
        userList.add(user);
        userAdapter.notifyDataSetChanged();

    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {

       View view =  inflater.inflate(R.layout.fragment__username__search, container, false);
        RecyclerView.LayoutManager mLayoutManager = new LinearLayoutManager(getActivity());
        recyclerView = (RecyclerView) view.findViewById(R.id.RecyclerView);

        userAdapter = new UserAdapter(userList);
        prepareUserData();
        recyclerView.setLayoutManager(mLayoutManager);
        recyclerView.setItemAnimator(new DefaultItemAnimator());

        recyclerView.setAdapter(userAdapter);
        userAdapter.SetOnItemClickListener(new UserAdapter.OnItemClickListener() {
            @Override
            public void onItemClick(View view, int position) {
                FragmentManager fm = getFragmentManager();
                android.app.DialogFragment dialogFragment = new DialogFragment();


                dialogFragment.show(fm, "Sample Fragment");
            }
        });


        return view;
    }


}

how to properly use the onPrepareData() method? displays only one element on the screen, but it should be 4, I don’t understand what the problem is.
UserAdapter

public class UserAdapter extends RecyclerView.Adapter<UserAdapter.MyViewHolder> {
    private List<User> UserList;
OnItemClickListener mItemClickListener;
    public MyViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
RecyclerView.ViewHolder viewHolder = null;
        LayoutInflater inflater = LayoutInflater.from(parent.getContext());

                    View viewUser = inflater.inflate(R.layout.row_item_layout,parent,false);
                    viewHolder = new MyViewHolder(viewUser,mItemClickListener);


        return (MyViewHolder) viewHolder;
    }

    @Override
    public void onBindViewHolder(MyViewHolder holder, int position) {
        holder.UserTitle.setText(UserList.get(position).getTitle());
        holder.UserDescription.setText(UserList.get(position).getDescription());
        holder.UserIcon.setImageResource(R.drawable.emilypic);
    }


    public UserAdapter(List<User> UserList){
    this.UserList = UserList;
}
    @Override
    public int getItemCount() {
        return UserList.size();
    }

    public class MyViewHolder extends RecyclerView.ViewHolder implements View.OnClickListener {
        public ImageView UserIcon;
        public TextView UserTitle;
        public TextView UserDescription;
        public Button UserFollowButton;
        public MyViewHolder(View itemView, UserAdapter.OnItemClickListener clickListener) {
            super(itemView);
            clickListener = (OnItemClickListener) mItemClickListener;
            UserIcon = (ImageView) itemView.findViewById(R.id.item_icon);
            UserTitle = (TextView) itemView.findViewById(R.id.item_title);
            UserDescription = (TextView) itemView.findViewById(R.id.item_description);
            UserFollowButton = (Button) itemView.findViewById(R.id.item_follow_button);
//itemView.setOnClickListener(this);
            UserFollowButton.setOnClickListener(this);
        }

        @Override
        public void onClick(View v) {
            mItemClickListener.onItemClick(v, getPosition());
        }

    }
    public interface OnItemClickListener{
        public void onItemClick(View view,int position);

    }
    public void SetOnItemClickListener(final OnItemClickListener mItemClickListener) {
        this.mItemClickListener = mItemClickListener;
    }

}

xml layout:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.example.myapplication.Fragment_Username_Search">
<LinearLayout
    android:id="@+id/Search_field"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:orientation="horizontal"
    android:focusable="true"
    android:focusableInTouchMode="true"

    >
    <ImageView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:src="@drawable/ic_search_black_48dp"
        />
    <EditText
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:tag="Search"
        android:textIsSelectable="false"

        />




</LinearLayout>

    <android.support.v7.widget.RecyclerView
       android:layout_below="@id/Search_field"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:id="@+id/RecyclerView"

        >

    </android.support.v7.widget.RecyclerView>
</RelativeLayout>