Sample of Room using Kotlin.

Room.kt

class App: Application() {
    companion object {
        lateinit var database: MyDatabase
    }
    override fun onCreate() {
        super.onCreate()
        App.database =  Room.databaseBuilder(this, MyDatabase::class.java, "my-database").build()

    }
}

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        AsyncTask.execute {
            val operation = Operation(0, 4.5f, 4.5f, "+", 9.0f)
            App.database.operationDao().insert(operation)

            val operations = App.database.operationDao().getAllOperations()

            val arrayList = ArrayList<String>()
            operations.forEach { operation ->
                arrayList.add(operation.expression())
            }
            println(arrayList)
        }
    }
}

@Database(entities = arrayOf(Operation::class), version = 0)
abstract class MyDatabase : RoomDatabase() {
    abstract fun operationDao(): OperationDao
}

@Dao
interface OperationDao {
    @Query("SELECT * FROM operation")
    fun getAllOperations(): List<Operation>

    @Insert
    fun insert(operation: Operation)
}

@Entity
data class Operation(
        @PrimaryKey(autoGenerate = true)
        val uid: Long,
        val firstNumber: Float = 0.0f,
        val secondNumber: Float = 0.0f,
        val operator: String = "",
        val result: Float = 0.0f
)

fun Operation.expression(): String = "$firstNumber $operator $secondNumber = $result"