AS Room編➂ 改変後（AsyncTaskを使わず、Runnable、Handler、Messageを使用する）

MainActivity.java

package com.example.singleroomrunnable;

import androidx.appcompat.app.AppCompatActivity;
import androidx.room.Dao;

import android.os.AsyncTask;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

import java.sql.Timestamp;
import java.util.ArrayList;
import java.util.List;

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        TextView showData = findViewById(R.id.showData);


        //Handler
        Handler handler = new Handler(){
            public void handleMessage(Message msg){
               String[] recievedArray =(String[])msg.obj;
               StringBuilder sb = new StringBuilder();
                for(int i=0;i<recievedArray.length;i++){
                    sb.append(recievedArray[i]).append("\n");
                }
                showData.setText(sb.toString());
            }
        };



        Button registerBtn = findViewById(R.id.registerBtn);
        AppDatabase db = AppDatabaseSingleton.getInstance(getApplicationContext());
        registerBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                new Thread(new Runnable(){

                    @Override
                    public void run() {
                        // 本質的な処理
                        UserDao userDao = db.userDao();
                        EditText editName = findViewById(R.id.editName);
                        userDao.insert(new User(editName.getText().toString()));
                        StringBuilder sb = new StringBuilder();
                        List<User> atList = userDao.getAll();
                        ArrayList<String> filteredList = new ArrayList<>();
                        for(User user:atList){
                            filteredList.add(user.getName());
                        }
                        String[] userNameArray = filteredList.toArray(new String[filteredList.size()]);

                        //メッセージオブジェクトを利用
                        Message msg = new Message();
                        msg.what = 456;
                        msg.arg1 = 1234;
                        msg.arg2 = 5678;
                        msg.obj = userNameArray;

                        handler.sendMessage(msg);
                    }
                }).start();

            }
        });
    }

}

他は前に同じ。