AS Room編①解説～EditText欄にテキストを記入し、その下のボタンを押すと、データベースに登録され、その一覧が下記に表示される～

すべて同じディレクトリに入れてます。

MainActivity.java

package com.example.roomappli;

import androidx.appcompat.app.AppCompatActivity;

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

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

public class MainActivity extends AppCompatActivity {



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


        TextView index = findViewById(R.id.index);
        Button button = findViewById(R.id.button);
        AppDatabase db = AppDatabaseSingleton.getInstance(getApplicationContext());
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                new DataStoreAsyncTask(db,index).execute();
            }
        });
    }

    public class DataStoreAsyncTask extends AsyncTask<Void,Void,Integer> {
        private AppDatabase db;
        private TextView textView;
        StringBuilder sb;

        public DataStoreAsyncTask(AppDatabase db,TextView textView) {
           this.db= db;
           this.textView =textView;
        }

        @Override
        protected Integer doInBackground(Void... voids) {
            UserDao userDao = db.userDao();
            EditText editText = findViewById(R.id.editText);
            userDao.insert(new User(editText.getText().toString()));
            sb = new StringBuilder();
            List<User> atList = userDao.getAll();
            for(User user: atList){
                sb.append(user.getAccessTime()).append("\n");
            }
            return 0;
        }

        protected  void onPostExecute(Integer code){
            textView.setText(sb.toString());

        }
    }
}

User.java

package com.example.roomappli;


import androidx.annotation.NonNull;
import androidx.room.ColumnInfo;
import androidx.room.Entity;
import androidx.room.PrimaryKey;

@Entity(tableName ="users")
public class User {
    @PrimaryKey(autoGenerate = true)
    @NonNull
    @ColumnInfo(name = "user_id")
    private int user_id;

    @ColumnInfo(name = "accessTime")
    private String accessTime;

    public User(String accessTime) {
        this.accessTime = accessTime;
    }

    public int getUser_id() {
        return user_id;
    }

    public void setUser_id(int user_id) {
        this.user_id = user_id;
    }

    public String getAccessTime() {
        return accessTime;
    }

    public void setAccessTime(String accessTime) {
        this.accessTime = accessTime;
    }
}

UserDao(注：これはインターフェースです）

package com.example.roomappli;

import androidx.room.Dao;
import androidx.room.Delete;
import androidx.room.Insert;
import androidx.room.Query;

import java.util.List;

@Dao
public interface UserDao {
    @Insert
    void insertAll(User... users);

    @Insert
    void insert(User user);

    @Delete
    void delete(User user);

    @Query("SELECT * FROM users")
    List<User> getAll();
}

AppDatabase.java( RoomDatabaseを継承した抽象クラス）

package com.example.roomappli;

import androidx.room.Database;
import androidx.room.RoomDatabase;

@Database(entities = {User.class},version = 1)
public abstract class AppDatabase extends RoomDatabase {
    public abstract UserDao userDao();
}

AppDatabaseSingleton.java

package com.example.roomappli;

import android.content.Context;

import androidx.room.Room;

public class AppDatabaseSingleton {
    private static AppDatabase instance = null;

    static AppDatabase getInstance(Context context){
        instance = Room.databaseBuilder(context,AppDatabase.class,"dtabase-name").build();
        return instance;
    }
}