Sealed classes for restricted hierarchies in Kotlin - Step-by-Step Execution

Concept Flow - Sealed classes for restricted hierarchies

Define sealed class

↓

Define subclasses inside or outside

↓

Use when expression to check subclass

↓

Handle all subclasses exhaustively

↓

No other subclasses allowed outside hierarchy

↓

End

Sealed classes restrict subclassing to a known set, enabling exhaustive checks in when expressions.

Execution Sample

Kotlin

sealed class Shape {
    data class Circle(val radius: Double) : Shape()
    data class Rectangle(val width: Double, val height: Double) : Shape()
}

fun area(shape: Shape): Double = when(shape) {
    is Shape.Circle -> 3.14 * shape.radius * shape.radius
    is Shape.Rectangle -> shape.width * shape.height
}

Calculates area for Circle or Rectangle using sealed class Shape and exhaustive when.

Execution Table

Step	Action	Input	Condition	Branch Taken	Output
1	Call area with Circle(radius=2.0)	Circle(radius=2.0)	shape is Circle?	Yes	Area = 3.14 * 2.0 * 2.0 = 12.56
2	Return area for Circle				12.56
3	Call area with Rectangle(width=3.0, height=4.0)	Rectangle(width=3.0, height=4.0)	shape is Circle?	No
4	Check if shape is Rectangle	Rectangle(width=3.0, height=4.0)	shape is Rectangle?	Yes	Area = 3.0 * 4.0 = 12.0
5	Return area for Rectangle				12.0
6	Call area with unknown subclass (not allowed)	UnknownShape()	No matching subclass	Error	Compilation error: sealed class restricts subclasses

💡 Execution stops because all subclasses are handled; no other subclasses allowed outside sealed class.

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
shape	-	Circle(radius=2.0)	Rectangle(width=3.0, height=4.0)	UnknownShape()	-
area	-	12.56	12.0	Error	-

Key Moments - 3 Insights

Why does the when expression not need an else branch?

Because the sealed class restricts subclasses, the when expression covers all cases explicitly (see execution_table rows 1-5).

Can we create a subclass of the sealed class outside its file?

No, sealed classes only allow subclasses defined in the same file, preventing unknown subclasses (see execution_table row 6).

What happens if a new subclass is added but not handled in when?

The compiler forces you to handle it or add an else branch, ensuring exhaustive checks.

Visual Quiz - 3 Questions

Look at the execution_table, what is the area output when shape is Circle(radius=2.0)?

A 3.14

B 12.56

C 4.0

D Error

💡 Hint

Check execution_table row 1 and 2 for Circle input and output.

At which step does the when expression handle the Rectangle subclass?

A Step 1

B Step 3

C Step 4

D Step 6

💡 Hint

Look at execution_table rows 3 and 4 for Rectangle handling.

If a new subclass is added outside the sealed class file, what happens?

A Compiler error due to sealed class restrictions

B It compiles and runs normally

C It runs but when expression skips it

D It requires an else branch

💡 Hint

See execution_table row 6 about unknown subclass and error.

Concept Snapshot

sealed class ClassName {
  // subclasses defined here or same file
}

Use when to check all subclasses without else.
Compiler ensures no other subclasses exist.
Enables safe, exhaustive type checks.

Full Transcript

Sealed classes in Kotlin let you define a restricted set of subclasses. You declare a sealed class and its subclasses in the same file. This restriction helps the compiler know all possible subclasses. When you use a when expression on a sealed class, you can check all subclasses explicitly without needing an else branch. The compiler ensures you handle every subclass, making your code safer and clearer. If you try to create a subclass outside the sealed class file, the compiler will give an error. This way, sealed classes help you create restricted hierarchies with exhaustive checks.

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