ArticleRoomDatabase.java

// Define database class which takes {@code Article} as table schema.
@Database(entities = Article.class, version = 1, exportSchema = false)
abstract class ArticleRoomDatabase extends RoomDatabase {

    // APIs for accessing table data.
    public abstract ArticleDao articleDao();

    private static volatile ArticleRoomDatabase SINGLETON;

    private static final int NUMBER_OF_THREADS = 4;
    static final ExecutorService databaseWriteExecutor =
            Executors.newFixedThreadPool(NUMBER_OF_THREADS);

    // Create database instance for Repository to access.
    static ArticleRoomDatabase getDatabase(final Context context) {
        if (SINGLETON == null){
            synchronized (ArticleRoomDatabase.class){
                if (SINGLETON != null) {
                    return SINGLETON;
                }

                SINGLETON = Room.databaseBuilder(context.getApplicationContext(),
                        ArticleRoomDatabase.class,
                        "articles").build();
            }
        }

        return SINGLETON;
    }
}