Entities, DAOs & Compile-Time-Verified Queries

An Abstraction Over SQLite, Not a Replacement

Room is an Android Jetpack library over SQLite — still the same on-disk storage engine, but with a developer-facing API, annotations, and generated code replacing raw SQL string manipulation and manual Cursor parsing. Its compile-time query validation catches a typo'd column name as a build error rather than a runtime crash.

Entities, DAOs & the Database Class

@Entity
data class User(
    @PrimaryKey(autoGenerate = true) val id: Long = 0,
    val name: String
)

@Dao
interface UserDao {
    @Query("SELECT * FROM User WHERE id = :id")
    fun getById(id: Long): Flow<User>

    @Insert(onConflict = OnConflictStrategy.REPLACE)
    suspend fun insert(user: User)
}

@Database(entities = [User::class], version = 1)
abstract class AppDatabase : RoomDatabase() {
    abstract fun userDao(): UserDao
}

An @Entity class defines a table (instance = row, property = column). A @Dao interface declares database operations as methods; Room generates the implementation. @Query methods hold raw SQL validated against the schema at compile time. The @Database class is the central hub, listing entities and exposing DAO accessors. onConflict (e.g. REPLACE) controls how a uniqueness violation on insert/update is resolved, rather than the operation failing unpredictably.

Threading & Coroutines

Room throws by default if a query runs on the main thread — blocking I/O there risks visible jank or an ANR crash. A suspend DAO function lets calling code await the result via coroutines' structured concurrency, with Room ensuring the actual query executes off the main thread.