为什么不能recycleview工作？[关闭]

Adapter  

package com.example.notes.adapter;

import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

import androidx.annotation.NonNull;
import androidx.recyclerview.widget.RecyclerView;

import com.example.notes.R;

import java.util.ArrayList;
import java.util.List;

public class MainAdapter extends RecyclerView.Adapter<MainAdapter.MyViewHolder> {
    private Context context;
    private List<String> mainArray;

    public MainAdapter(Context context) {
        this.context = context;
        mainArray = new ArrayList<>();
    }

    @NonNull
    @Override
    public MyViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        View view = LayoutInflater.from(context).inflate(R.layout.item_list_layout, parent,false);
        return new MyViewHolder(view);
    }

    @Override
    public void onBindViewHolder(@NonNull MyViewHolder holder, int position) {
        holder.setData(mainArray.get(position));
    }

    @Override
    public int getItemCount() {
        return mainArray.size();
    }

    static class MyViewHolder extends RecyclerView.ViewHolder {
        private TextView ltitle;

        public MyViewHolder(@NonNull View itemView) {
            super(itemView);
            ltitle = itemView.findViewById(R.id.ltitle);
        }
        public void setData(String title) {
            ltitle.setText(title);
        }
    }

    public void updateAdapter(List<String> newList) {
        mainArray.clear();
        mainArray.addAll(newList);
        notifyDataSetChanged();
    }
}


MainActivity

package com.example.notes;

import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.EditText;

import androidx.appcompat.app.AppCompatActivity;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;


import com.example.notes.adapter.MainAdapter;
import com.example.notes.db.MyDbManager;


public class MainActivity extends AppCompatActivity {
    private MyDbManager myDbManager;
    private EditText title_id, desc_id;
    private RecyclerView rView;
    private MainAdapter mainAdapter;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        init();
    }

    private void init() {
        myDbManager = new MyDbManager(this);
        title_id = findViewById(R.id.title_id);
        desc_id = findViewById(R.id.desc_id);
        rView = findViewById(R.id.rView);
        rView.setLayoutManager(new LinearLayoutManager(this));
        rView.setAdapter(mainAdapter);
        mainAdapter = new MainAdapter(this);
    }

    @Override
    protected void onResume() {
        super.onResume();
        myDbManager.openDb();
        mainAdapter.updateAdapter(myDbManager.getDb());

    }

    public void onClickAdd(View view) {
        Intent add = new Intent(MainActivity.this, EditActivity.class);
        startActivity(add);
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();

        myDbManager.closeDb();
    }

}