Sealed classes in Android Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

What is the main purpose of sealed classes in Kotlin?

Choose the best explanation for why sealed classes are used in Kotlin.

A To enable multiple inheritance of classes.

B To restrict class inheritance to a fixed set of subclasses known at compile time.

C To allow any class to inherit from them without restrictions.

D To create classes that cannot have any subclasses.

💡 Hint

Think about how sealed classes control which classes can extend them.

Sealed classes restrict inheritance to a known set of subclasses, enabling exhaustive when expressions and safer code.

❓ ui_behavior

intermediate

What will be the output of this Kotlin code using a sealed class?

Given the sealed class and when expression below, what will be printed?

Android Kotlin

sealed class Result
class Success(val data: String) : Result()
class Error(val error: String) : Result()

fun handle(result: Result) = when(result) {
  is Success -> "Data: ${result.data}"
  is Error -> "Error: ${result.error}"
}

println(handle(Success("Hello")))

A Data: Hello

B Runtime exception

C Compilation error due to missing else branch

D Error: Hello

💡 Hint

Check which subclass instance is passed and how when handles sealed classes.

The handle function matches the Success subclass and returns its data string. No else is needed because sealed classes cover all cases.

❓ lifecycle

advanced

What happens if you add a new subclass to a sealed class but forget to update the when expression?

Consider a sealed class with subclasses A and B. You add subclass C but do not update the when expression that handles A and B. What is the result?

A Compilation error due to non-exhaustive when expression.

B The when expression runs but skips subclass C silently.

C Runtime exception when subclass C is passed.

D The code compiles and runs without issues.

💡 Hint

Think about Kotlin's compiler checks for sealed classes and when expressions.

Kotlin requires when expressions on sealed classes to be exhaustive. Adding a new subclass without updating when causes a compile error.

❓ navigation

advanced

How can sealed classes improve navigation state management in Android apps?

Which option best describes how sealed classes help manage navigation states in Android apps?

A By automatically generating navigation graphs without code.

B By allowing dynamic creation of navigation states at runtime.

C By defining a fixed set of navigation states, enabling safe and clear state handling in UI.

D By replacing fragments with sealed classes.

💡 Hint

Think about how sealed classes limit possible states and help with exhaustive checks.

Sealed classes let you define all possible navigation states explicitly, making UI code safer and easier to maintain.

📝 Syntax

expert

What error does this Kotlin code produce?

Examine the sealed class and subclass below. What error occurs when compiling?

Android Kotlin

sealed class Shape
class Circle(val radius: Double) : Shape()
class Square(val side: Double) : Shape()

A Syntax error: sealed class cannot have subclasses.

B No error, code compiles successfully.

C Runtime error due to missing constructor in Square.

D Syntax error: Square class missing parentheses after Shape superclass.

💡 Hint

Check the syntax for subclass declarations in Kotlin.

In Kotlin, when inheriting from a class, parentheses are required even if the superclass has no constructor parameters.