How would you create a database?

I am using the Room Persistence Library.

1. Adding dependencies:

   // build.gradle (app level)
   dependencies {
       def room_version = "2.5.2" // or the latest version
   
       implementation("androidx.room:room-runtime:$room_version")
       annotationProcessor("androidx.room:room-compiler:$room_version") // For Kotlin use kapt("...")
   
       // Optional: Kotlin Extensions and Coroutines support for Room
       implementation("androidx.room:room-ktx:$room_version")
   }
   

2. Defining Entities (tables):

   Create a class annotated with @Entity. Each field will be a column, and @PrimaryKey will be the primary key.

   // User.java
   import androidx.room.Entity;
   import androidx.room.PrimaryKey;
   
   @Entity(tableName = "users")
   public class User {
       @PrimaryKey(autoGenerate = true)
       public int id;
   
       public String firstName;
       public String lastName;
   }
   

3. Defining DAO (Data Access Object):

   Create an interface annotated with @Dao, with methods for data access (insert, update, delete, query).

   // UserDao.java
   import androidx.room.Dao;
   import androidx.room.Delete;
   import androidx.room.Insert;
   import androidx.room.OnConflictStrategy;
   import androidx.room.Query;
   import androidx.room.Update;
   
   import java.util.List;
   
   @Dao
   public interface UserDao {
       @Query("SELECT * FROM users")
       List<User> getAll();
   
       @Query("SELECT * FROM users WHERE id IN (:userIds)")
       List<User> loadAllByIds(int[] userIds);
   
       @Query("SELECT * FROM users WHERE first_name LIKE :first AND " +
              "last_name LIKE :last LIMIT 1")
       User findByName(String first, String last);
   
       @Insert(onConflict = OnConflictStrategy.IGNORE)
       void insertAll(User... users);
   
       @Delete
       void delete(User user);
   
       @Update
       void updateUsers(User... users);
   }
   

4. Defining the database:

   Create an abstract class extending RoomDatabase, annotated with @Database. Include the list of entities and DAO.

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

5. Initializing the database:

   Obtain an instance of the database, usually in the Application class or Activity (preferably in Application for singleton pattern).

   // In Activity or Application class
   import android.content.Context;
   import androidx.room.Room;
   
   public class DatabaseInitializer {
       private static AppDatabase db;
   
       public static synchronized AppDatabase getDatabase(Context context) {
           if (db == null) {
               db = Room.databaseBuilder(context.getApplicationContext(),
                              AppDatabase.class, "database-name")
                       // .allowMainThreadQueries() // Not recommended for production
                       // .fallbackToDestructiveMigration() // Be cautious with migrations
                       .build();
           }
           return db;
       }
   }
   

6. Performing data operations:

   After obtaining the DAO instance, perform database operations, preferably on a background thread (using AsyncTask, Executors, Coroutines, RxJava).

   // In Activity or elsewhere
   new Thread(() -> {
      AppDatabase db = DatabaseInitializer.getDatabase(getApplicationContext());
      UserDao userDao = db.userDao();
   
      // Insert
      User user1 = new User();
      user1.firstName = "John";
      user1.lastName = "Doe";
      userDao.insertAll(user1);
   
      // Read
      List<User> allUsers = userDao.getAll();
      // ... process data
   }).start();
   

This approach provides an abstraction over SQLite, simplifies database management, and offers compile-time query checking.