БД на Android

package com.example.myapplication;

import androidx.annotation.NonNull;

import java.io.Serializable;

public class User implements Serializable {

   private long id;

   private String login;

   private int age;

   public User(long id, String login, int age) {

       this.id = id;

       this.login = login;

       this.age = age;

   }

   public long getId() {

       return id;

   }

   public void setId(long id) {

       this.id = id;

   }

   public String getLogin() {

       return login;

   }

   public void setLogin(String login) {

       this.login = login;

   }

   public int getAge() {

       return age;

   }

   public void setAge(int age) {

       this.age = age;

   }

   @NonNull

   @Override

   public String toString() {

       return id + " " + login + " " + age;

   }

}

package com.example.myapplication;

import android.content.ContentValues;

import android.content.Context;

import android.database.Cursor;

import android.database.sqlite.SQLiteDatabase;

import android.database.sqlite.SQLiteOpenHelper;

import java.util.ArrayList;

public class DBUsers {

   public static final String DATABASE_NAME = "mydb.db";

   public static final int DATABASE_VERSION = 1;

   public static final String USER_TABLE_NAME = "users";

   public static final String USER_COLUMN_ID = "id";

   public static final String USER_COLUMN_LOGIN = "login";

   public static final String USER_COLUMN_AGE = "age";

   private SQLiteDatabase database;

   public DBUsers(Context context) {

       OpenHelper openHelper = new OpenHelper(context);

       database = openHelper.getWritableDatabase();

   }

   public ArrayList<User> selectAll(){

       Cursor cursor = database.query(

               USER_TABLE_NAME,

               null, null, null, null, null, null

       );

       ArrayList<User> users = new ArrayList<>();

       while (cursor.moveToNext()) {

           long id = cursor.getLong(cursor.getColumnIndexOrThrow(USER_COLUMN_ID));

           String login = cursor.getString(cursor.getColumnIndexOrThrow(USER_COLUMN_LOGIN));

           int age = cursor.getInt(cursor.getColumnIndexOrThrow(USER_COLUMN_AGE));

           users.add(new User(id, login, age));

       }

       return users;

   }

   public long insert(User user){

       ContentValues contentValues = new ContentValues();

       contentValues.put(USER_COLUMN_LOGIN, user.getLogin());

       contentValues.put(USER_COLUMN_AGE, user.getAge());

       return database.insert(USER_TABLE_NAME, null, contentValues);

   }

   private static class OpenHelper extends SQLiteOpenHelper{

       OpenHelper(Context context){

           super(context, DATABASE_NAME, null, DATABASE_VERSION);

       }

       @Override

       public void onCreate(SQLiteDatabase sqLiteDatabase) {

           String createSQLQuery = "CREATE TABLE " + USER_TABLE_NAME + " (" +

                   USER_COLUMN_ID + " INTEGER PRIMARY KEY AUTOINCREMENT, " +

                   USER_COLUMN_LOGIN + " TEXT, " +

                   USER_COLUMN_AGE + " INTEGER" +

                   ");";

           sqLiteDatabase.execSQL(createSQLQuery);

       }

       @Override

       public void onUpgrade(SQLiteDatabase sqLiteDatabase, int i, int i1) {

           sqLiteDatabase.execSQL("DROP TABLE IF EXISTS " + USER_TABLE_NAME);

           onCreate(sqLiteDatabase);

       }

   }

}

package com.example.myapplication;

import androidx.appcompat.app.AppCompatActivity;

import android.content.Intent;

import android.graphics.Bitmap;

import android.graphics.BitmapFactory;

import android.os.Bundle;

import android.util.Log;

import android.view.View;

import android.widget.AdapterView;

import android.widget.ArrayAdapter;

import android.widget.Button;

import android.widget.ListView;

import android.widget.Toast;

import java.util.ArrayList;

public class MainActivity extends AppCompatActivity {

   DBUsers dbUsers;

   @Override

   protected void onCreate(Bundle savedInstanceState) {

       super.onCreate(savedInstanceState);

       setContentView(R.layout.activity_main);

       dbUsers = new DBUsers(this);

       ArrayList<String> stringUsers = new ArrayList<>();

       for (User user: dbUsers.selectAll()){

           stringUsers.add(user.toString());

       }

       ListView listView = findViewById(R.id.list);

       ArrayAdapter<String> arrayAdapter = new ArrayAdapter<>(

               this,

               android.R.layout.simple_list_item_1,

               stringUsers

       );

       listView.setAdapter(arrayAdapter);

       Button addButton = findViewById(R.id.add_button);

       addButton.setOnClickListener(new View.OnClickListener() {

           @Override

           public void onClick(View view) {

               Intent intent = new Intent(

                       getApplicationContext(),

                       AddUserActivity.class

               );

               startActivity(intent);

           }

       });

   }

}

<?xml version="1.0" encoding="utf-8"?>

<LinearLayout

   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"

   android:orientation="vertical"

   tools:context=".MainActivity">

   <ListView

       android:id="@+id/list"

       android:layout_width="match_parent"

       android:layout_height="wrap_content"

       android:layout_weight="10"

       />

   <Button

       android:text="Добавить"

       android:id="@+id/add_button"

       android:layout_width="match_parent"

       android:layout_height="wrap_content"

       android:layout_weight="1"

       />

</LinearLayout>