Entity, DAO, Database classes in Android Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

What is the role of an Entity class in Android Room?

In Android Room, what does an Entity class represent?

A A table in the database with columns as class fields

B A class that manages database connections

C A class that handles database queries

D A UI component for displaying data

💡 Hint

Think about how data is stored in a database.

An Entity class defines a table in the database. Each field in the class corresponds to a column in that table.

❓ ui_behavior

intermediate

What happens when you call a DAO query method on the main thread?

Consider a DAO method annotated with @Query that fetches data. What happens if you call this method directly on the main thread?

A The query runs asynchronously automatically

B The query runs smoothly without any issues

C The app shows a warning but continues

D The app crashes with a runtime exception

💡 Hint

Think about Android's rule for long operations on the main thread.

Room does not allow database queries on the main thread to avoid freezing the UI. It throws a runtime exception if you try.

❓ lifecycle

advanced

When is the Room database instance created in a typical Android app?

In an Android app using Room, when is the database instance usually created?

A Only when the user opens a specific screen

B Every time a DAO method is called

C When the app starts, typically in a singleton or Application class

D When the app goes to background

💡 Hint

Think about how to avoid creating multiple database instances.

Creating the database once at app start and reusing it avoids overhead and ensures data consistency.

📝 Syntax

advanced

What is the correct syntax to define a DAO method that inserts a User entity?

Choose the correct Kotlin DAO method syntax to insert a User entity into the database.

Android Kotlin

interface UserDao {
    // Insert method here
}

A @Insert fun insertUser(user: User): Int

B @Insert suspend fun insertUser(user: User)

C fun insertUser(@Insert user: User)

D @Insert suspend fun insertUser(): User

💡 Hint

The @Insert annotation goes before the function, and suspend is used for coroutines.

The correct syntax uses @Insert annotation before a suspend function that takes the entity as a parameter.

🔧 Debug

expert

Why does this Room database build fail with "Cannot find symbol" error?

Given this code snippet, why does the build fail with a "Cannot find symbol" error for the DAO interface? @Entity data class Product(val id: Int, val name: String) @Database(entities = [Product::class], version = 1) abstract class AppDatabase : RoomDatabase() { abstract fun productDao(): ProductDao } // ProductDao interface is missing in the project.

A Because the ProductDao interface is not defined anywhere

B Because the Product entity is missing the @PrimaryKey annotation

C Because the database version is not incremented

D Because RoomDatabase class is not imported

💡 Hint

Check if all referenced classes exist in the project.

The build fails because the AppDatabase references ProductDao, but ProductDao interface is not defined anywhere.