In Android development, Room is a popular persistence library that provides an abstraction layer over SQLite, making it easier for developers to work with databases. During Android Room Database interviews, candidates are often asked about their understanding of Room, its features, and how it can be effectively utilized in Android applications. In this blog, we'll cover the top 10 Room Database interview questions along with detailed answers and Kotlin code examples to help you prepare for your interview.

Question 1: What is Room Database, and why is it used in Android development?

Answer: Room Database is an Android library that serves as a part of the Android Architecture Components. It provides an SQLite database abstraction layer, making it easier for developers to work with databases in their Android applications. Room offers benefits like compile-time query verification, reduced boilerplate code, and better performance compared to traditional SQLiteOpenHelper.

Question 2: What are the main components of Room Database?

Answer: Room Database consists of three main components:

1. Entity: Represents a table in the database and is defined as a data class annotated with the @Entity annotation. Each property of the data class corresponds to a column in the table.
2. DAO (Data Access Object): Contains methods to define database operations (CRUD) for entities. DAOs are interfaces annotated with @Dao, and methods inside the DAO are annotated with SQL queries using annotations like @Insert, @Update, @Delete, etc.
3. Database: Represents the database itself and is defined as an abstract class annotated with @Database. It includes a list of entities and a version number. Room generates the implementation of this class during compile-time.

Code Example:

@Entity(tableName = "user")
data class User(
    @PrimaryKey val id: Int,
    val name: String,
    val age: Int
)
@Dao
interface UserDao {
    @Insert
    fun insert(user: User)
    @Update
    fun update(user: User)
    @Delete
    fun delete(user: User)
    @Query("SELECT * FROM user")
    fun getAllUsers(): List<User>
}

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

Question 3: Explain the difference between @PrimaryKey, @ColumnInfo, and @ForeignKey annotations.

Answer:

• @PrimaryKey: Marks a field in the entity as the primary key for the table. Each entity must have at least one primary key.
• @ColumnInfo: Specifies the details of a column, such as its name in the table, whether it's indexed, or if it allows null values.
• @ForeignKey: Defines a foreign key relationship between two entities. It ensures referential integrity between the tables.

Question 4: How does Room handle database migrations?

Answer: Room provides database migrations using the Migration class. When you update your database schema by modifying entities or DAOs, you need to create a new Migration object and add it to the RoomDatabase.Builder. Room will automatically execute the necessary migrations to keep the existing data intact.

Code Example:

val migration1to2 = object : Migration(1, 2) {
    override fun migrate(database: SupportSQLiteDatabase) {
        // Migration code from version 1 to 2
    }
}
val appDatabase = Room.databaseBuilder(context, AppDatabase::class.java, "app_database")
    .addMigrations(migration1to2)
    .build()

Question 5: What is the purpose of the @Relation annotation in Room?

Answer: The @Relation annotation is used to define relationships between entities in Room. It allows you to retrieve related entities with a single query, simplifying complex database queries.

Code Example:

@Entity(tableName = "user")
data class User(
    @PrimaryKey val id: Int,
    val name: String
)
@Entity(tableName = "book")
data class Book(
    @PrimaryKey val id: Int,
    val title: String,
    @ColumnInfo(name = "user_id") val userId: Int
)
data class UserWithBooks(
    @Embedded val user: User,
    @Relation(
        parentColumn = "id",
        entityColumn = "user_id"
    )
    val books: List<Book>
)
@Dao
interface UserDao {
    @Transaction
    @Query("SELECT * FROM user")
    fun getUsersWithBooks(): List<UserWithBooks>
}

Question 6: How can you observe database changes using LiveData with Room?

Answer: Room provides the LiveData class from the Android Architecture Components, which can be used to observe changes in the database. By returning a LiveData object from a DAO method, Room automatically updates the returned LiveData whenever there are changes in the database.

Code Example:

@Dao
interface UserDao {
    @Query("SELECT * FROM user")
    fun getAllUsers(): LiveData<List<User>>
}
// Observe the LiveData in an Activity or Fragment
userDao.getAllUsers().observe(this) { users ->
    // Update UI with the latest user data
}

Question 7: How can you execute complex database queries using Room?

Answer: Room supports complex database queries using the @Query annotation in DAO methods. You can write custom SQL queries and pass parameters using placeholders.

Code Example:

@Dao
interface UserDao {
    @Query("SELECT * FROM user WHERE age > :minAge")
    fun getUsersOlderThan(minAge: Int): List<User>
}

Question 8: What is the purpose of the @TypeConverter annotation in Room?

Answer: The @TypeConverter annotation allows you to define custom type converters in Room. It is useful when you need to store complex data types (such as Date, List, or custom objects) in the database as primitive types.

Code Example:

class Converters {
    @TypeConverter
    fun fromTimestamp(value: Long): Date {
        return Date(value)
    }
   
@TypeConverter
    fun dateToTimestamp(date: Date): Long {
        return date.time
    }
}

@Database(entities = [User::class], version = 1)
@TypeConverters(Converters::class)
abstract class AppDatabase : RoomDatabase() {
    // Database definition
}

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