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.

Practice

1. What is the main purpose of a sealed class in Kotlin?

easy

A. To replace interfaces completely

B. To allow unlimited subclassing from any package

C. To create anonymous classes only

D. To restrict class inheritance to a fixed set of subclasses

Solution

1. Step 1: Understand sealed class inheritance

   A sealed class restricts which classes can inherit from it, limiting subclasses to those defined in the same file.

2. Step 2: Compare with other class types

   Unlike open classes, sealed classes do not allow unlimited subclassing, ensuring all subclasses are known at compile time.

3. Final Answer:

   To restrict class inheritance to a fixed set of subclasses -> Option D

4. Quick Check:

   Sealed class = fixed subclass set [OK]

Hint: Sealed classes limit subclasses to known types only [OK]

Common Mistakes:

• Thinking sealed classes allow any subclass anywhere
• Confusing sealed classes with abstract classes
• Assuming sealed classes create anonymous classes

2. Which of the following is the correct syntax to declare a sealed class named Result in Kotlin?

easy

A. sealed Result class {}

B. sealed class Result {}

C. class sealed Result {}

D. sealed interface Result {}

Solution

1. Step 1: Recall sealed class declaration syntax

   The keyword sealed precedes class followed by the class name and braces.

2. Step 2: Check each option

   sealed class Result {} matches the correct syntax. Options B and C have wrong keyword order. sealed interface Result {} declares a sealed interface, not a sealed class.

3. Final Answer:

   sealed class Result {} -> Option B

4. Quick Check:

   sealed class syntax = "sealed class Name {}" [OK]

Hint: sealed class always starts with 'sealed class' keywords [OK]

Common Mistakes:

• Swapping 'sealed' and 'class' keywords
• Using 'sealed interface' instead of 'sealed class'
• Omitting braces after class name

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

sealed class Response {
  object Success : Response()
  object Error : Response()
}

fun handle(response: Response) {
  when(response) {
    is Response.Success -> println("Success")
    is Response.Error -> println("Error")
  }
}

handle(Response.Success)

medium

A. Nothing is printed

B. Error

C. Success

D. Compile-time error

Solution

1. Step 1: Identify the passed object type

   The function is called with Response.Success, which matches the first when branch.

2. Step 2: Check the when branches

   The when expression prints "Success" for Response.Success and "Error" for Response.Error.

3. Final Answer:

   Success -> Option C

4. Quick Check:

   Response.Success triggers "Success" print [OK]

Hint: Match sealed subclasses exactly in when to get correct output [OK]

Common Mistakes:

• Confusing Success and Error outputs
• Forgetting to call the function with correct subclass
• Expecting compile error due to sealed class

4. What is wrong with this sealed class declaration?

sealed class State {}
class Loading : State()
class Success : State()
class Error : State()

medium

A. Subclasses must be declared inside the sealed class file

B. Sealed classes cannot have subclasses

C. Subclasses must be objects, not classes

D. Sealed class must be abstract

Solution

1. Step 1: Recall sealed class subclass rules

   All subclasses of a sealed class must be declared in the same file as the sealed class.

2. Step 2: Analyze the code snippet

   The subclasses Loading, Success, Error are declared separately after the sealed class. This violates the rule because direct subclasses of a sealed class must be declared in the same file as the sealed class.

3. Final Answer:

   Subclasses must be declared inside the sealed class file -> Option A

4. Quick Check:

   Sealed subclasses must be in same file [OK]

Hint: Keep all sealed subclasses in the same file [OK]

Common Mistakes:

• Declaring subclasses in different files
• Thinking sealed classes cannot have subclasses
• Confusing sealed classes with abstract classes

5. You want to represent a network request state with a sealed class NetworkState having Loading, Success with data, and Error with message. Which is the best way to define Success and Error subclasses?

hard

A. Use data classes with properties inside the sealed class

B. Use object declarations for all subclasses

C. Use abstract classes for Success and Error

D. Use interfaces for Success and Error

Solution

1. Step 1: Understand subclass data needs

   Success needs to hold data, and Error needs to hold a message, so they require properties.

2. Step 2: Choose subclass type

   Data classes are ideal for subclasses with properties because they provide equals, hashCode, and toString automatically.

3. Final Answer:

   Use data classes with properties inside the sealed class -> Option A

4. Quick Check:

   Data classes hold data in sealed subclasses [OK]

Hint: Use data classes for sealed subclasses with data [OK]

Common Mistakes:

• Using object for subclasses needing data
• Using abstract classes unnecessarily
• Using interfaces instead of sealed subclasses