Sealed classes for restricted hierarchies in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a sealed class named Shape.

Kotlin

sealed class [1]

Drag options to blanks, or click blank then click option'

AShape

BShapes

Cshape

DshapeClass

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase or plural forms for the class name.

The sealed class must be declared with the exact name Shape to match the requirement.

2fill in blank

medium

Complete the code to declare a subclass Circle of the sealed class Shape.

Kotlin

class Circle(val radius: Double) : [1]()

Drag options to blanks, or click blank then click option'

AshapeClass

Bshape

CShape

DShapes

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect class names or lowercase names for inheritance.

The subclass must inherit from the sealed class Shape using the exact class name.

3fill in blank

hard

Fix the error in the when expression to handle all subclasses of Shape.

Kotlin

fun area(shape: Shape): Double = when(shape) {
    is Circle -> Math.PI * shape.radius * shape.radius
    is Square -> shape.side * shape.side
    [1] -> throw IllegalArgumentException("Unknown shape")
}

Drag options to blanks, or click blank then click option'

Adefault

Belse

Cotherwise

Dcatch

Attempts:

3 left

💡 Hint

Common Mistakes

Using invalid keywords like default or catch instead of else.

The when expression requires an else branch to handle all other cases not explicitly matched.

4fill in blank

hard

Fill both blanks to declare a sealed class Shape with two subclasses Circle and Square.

Kotlin

sealed class [1] {
    class Circle(val radius: Double) : [2]()
    class Square(val side: Double) : Shape()
}

Drag options to blanks, or click blank then click option'

AShape

Bshape

CShapes

DshapeClass

Attempts:

3 left

💡 Hint

Common Mistakes

Using different or lowercase names for the sealed class or superclass.

Both the sealed class and the superclass in the subclasses must be named Shape exactly.

5fill in blank

hard

Fill all three blanks to create a sealed class Shape with subclasses Circle and Square, and a function to calculate area.

Kotlin

sealed class [1] {
    class Circle(val radius: Double) : [2]()
    class Square(val side: Double) : Shape()
}

fun area(shape: [3]): Double = when(shape) {
    is Circle -> Math.PI * shape.radius * shape.radius
    is Square -> shape.side * shape.side
}

Drag options to blanks, or click blank then click option'

AShape

Bshape

DShapes

Attempts:

3 left

💡 Hint

Common Mistakes

Using inconsistent or incorrect class names.

The sealed class and its subclasses must use the exact name Shape. The function parameter type must also be Shape.

Practice

1. What is the main purpose of using sealed classes in Kotlin?

easy

A. To create classes that cannot be instantiated

B. To allow any class in any file to inherit from it

C. To restrict which classes can inherit from it, all in the same file

D. To automatically generate data classes

Solution

1. Step 1: Understand sealed class inheritance rules

   Sealed classes restrict subclassing to the same file, limiting which classes can extend them.

2. Step 2: Compare options with sealed class purpose

   Only To restrict which classes can inherit from it, all in the same file correctly describes this restriction; others describe unrelated features.

3. Final Answer:

   To restrict which classes can inherit from it, all in the same file -> Option C

4. Quick Check:

   Sealed classes restrict inheritance = A [OK]

Hint: Sealed classes limit subclasses to one file only [OK]

Common Mistakes:

• Thinking sealed classes prevent instantiation
• Confusing sealed classes with abstract classes
• Believing sealed classes allow open inheritance

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

easy

A. abstract sealed class Shape

B. open sealed class Shape {}

C. sealed interface Shape {}

D. sealed class Shape {}

Solution

1. Step 1: Recall sealed class declaration syntax

   Sealed classes are declared with the keyword sealed class followed by the name and body.

2. Step 2: Check each option's correctness

   sealed class Shape {} matches the correct syntax. open sealed class Shape {} wrongly combines open and sealed. sealed interface Shape {} uses sealed interface which is invalid. abstract sealed class Shape {} wrongly combines abstract and sealed.

3. Final Answer:

   sealed class Shape {} -> Option D

4. Quick Check:

   Correct sealed class syntax = D [OK]

Hint: Use only 'sealed class ClassName {}' to declare sealed classes [OK]

Common Mistakes:

• Adding 'open' or 'abstract' keywords with sealed
• Using 'sealed interface' instead of 'sealed class'
• Forgetting curly braces after class name

3. Consider the following Kotlin code using sealed classes:

sealed class Result {
    data class Success(val data: String) : Result()
    object Failure : Result()
}

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

println(handle(Result.Success("OK")))

What is the output when this code runs?

medium

A. "Data: OK"

B. "Failed"

C. Compilation error due to missing else branch

D. Runtime exception

Solution

1. Step 1: Understand sealed class and when expression

   The sealed class Result has two subclasses: Success and Failure. The when expression covers both cases without else.

2. Step 2: Trace the function call with Success("OK")

   The argument is Result.Success("OK"), so the first branch matches, returning "Data: OK".

3. Final Answer:

   "Data: OK" -> Option A

4. Quick Check:

   Sealed class when covers all cases = "Data: OK" [OK]

Hint: Sealed classes allow exhaustive when without else [OK]

Common Mistakes:

• Thinking else branch is required for sealed classes
• Confusing Success and Failure outputs
• Expecting runtime errors from sealed classes

4. The following Kotlin code tries to compile but fails. What is the error?

sealed class Animal
class Dog : Animal()
class Cat : Animal()

Why does this code cause a compilation error?

medium

A. Sealed classes cannot have subclasses

B. Subclasses Dog and Cat must be declared in the same file as Animal

C. Dog and Cat must be declared as sealed classes too

D. Animal must be declared as an abstract class instead

Solution

1. Step 1: Recall sealed class subclass rules

   Sealed classes restrict subclasses to be declared in the same file.

2. Step 2: Identify the error cause

   Dog and Cat are subclasses but declared outside the sealed class file, causing compilation error.

3. Final Answer:

   Subclasses Dog and Cat must be declared in the same file as Animal -> Option B

4. Quick Check:

   Sealed subclasses must be in same file = B [OK]

Hint: All sealed subclasses must be in the same file [OK]

Common Mistakes:

• Thinking sealed classes forbid any subclassing
• Believing subclasses must be sealed too
• Confusing sealed with abstract class rules

5. You want to model a fixed set of user roles in Kotlin using sealed classes: Admin, Guest, and Member. Which approach correctly uses sealed classes to restrict roles and safely handle all cases?

hard

A. sealed class UserRole {
object Admin : UserRole()
object Guest : UserRole()
object Member : UserRole()
}

fun describe(role: UserRole) = when(role) {
is UserRole.Admin -> "Admin user"
is UserRole.Guest -> "Guest user"
is UserRole.Member -> "Member user"
}

B. open class UserRole
class Admin : UserRole()
class Guest : UserRole()
class Member : UserRole()

fun describe(role: UserRole) = when(role) {
is Admin -> "Admin user"
is Guest -> "Guest user"
is Member -> "Member user"
else -> "Unknown"
}

C. sealed interface UserRole {
object Admin : UserRole
object Guest : UserRole
object Member : UserRole
}

fun describe(role: UserRole) = when(role) {
UserRole.Admin -> "Admin user"
UserRole.Guest -> "Guest user"
UserRole.Member -> "Member user"
}

D. abstract class UserRole
object Admin : UserRole()
object Guest : UserRole()
object Member : UserRole()

fun describe(role: UserRole) = when(role) {
is Admin -> "Admin user"
is Guest -> "Guest user"
is Member -> "Member user"
}

Solution

1. Step 1: Use sealed class to restrict subclasses

   sealed class UserRole { object Admin : UserRole() object Guest : UserRole() object Member : UserRole() } fun describe(role: UserRole) = when(role) { is UserRole.Admin -> "Admin user" is UserRole.Guest -> "Guest user" is UserRole.Member -> "Member user" } declares a sealed class with all subclasses as objects inside the same file, restricting roles properly.

2. Step 2: Check safe handling with exhaustive when

   The when expression covers all subclasses without else, ensuring safe handling.

3. Final Answer:

   sealed class UserRole { object Admin : UserRole() object Guest : UserRole() object Member : UserRole() } fun describe(role: UserRole) = when(role) { is UserRole.Admin -> "Admin user" is UserRole.Guest -> "Guest user" is UserRole.Member -> "Member user" } -> Option A

4. Quick Check:

   Sealed class + exhaustive when = safe fixed roles [OK]

Hint: Use sealed class with all subclasses inside one file [OK]

Common Mistakes:

• Using open or abstract classes without sealing
• Missing exhaustive when branches
• Using sealed interface incorrectly