Sealed classes with when exhaustive check in Kotlin - Step-by-Step Execution

Concept Flow - Sealed classes with when exhaustive check

Define sealed class

↓

Create subclasses

↓

Use when expression

↓

Check all subclasses covered?

No→Compile error

Yes↓

Code compiles and runs safely

Sealed classes restrict subclassing to known types. The when expression must cover all subclasses, ensuring safe exhaustive checks.

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) = when(shape) {
    is Shape.Circle -> Math.PI * shape.radius * shape.radius
    is Shape.Rectangle -> shape.width * shape.height
}

Calculates area for all Shape subclasses using when with exhaustive check.

Execution Table

Step	Action	Input	Condition	Branch Taken	Output
1	Call area with Circle(2.0)	Circle(radius=2.0)	shape is Circle?	Yes	Area = π * 2.0 * 2.0 = 12.566370614359172
2	Call area with Rectangle(3.0,4.0)	Rectangle(width=3.0,height=4.0)	shape is Circle?	No	Check next branch
3	Check shape is Rectangle?	Rectangle(width=3.0,height=4.0)	shape is Rectangle?	Yes	Area = 3.0 * 4.0 = 12.0
4	All subclasses covered?	Circle and Rectangle	Yes	No else needed	Function returns correct area
5	Call area with unknown subclass	Not possible	N/A	N/A	Compile error if missing branch

💡 Execution stops after matching the correct subclass; all subclasses must be covered for compilation.

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
shape	N/A	Circle(radius=2.0)	Rectangle(width=3.0,height=4.0)	N/A	N/A
Output	N/A	12.566370614359172	N/A	12.0	N/A

Key Moments - 2 Insights

Why does the compiler force me to cover all subclasses in the when expression?

Because the sealed class restricts subclasses, the compiler knows all possible types. If you miss one, it warns or errors to prevent runtime mistakes. See execution_table row 5.

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

The compiler will show an error for non-exhaustive when, forcing you to handle the new subclass. This keeps your code safe and up-to-date.

Visual Quiz - 3 Questions

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

A 12.0

B 12.566370614359172

C Error

D 0

💡 Hint

Check execution_table row 1 for output when shape is Circle.

At which step does the when expression check if the shape is a Rectangle?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at execution_table rows 2 and 3 for the branch checks.

If a new subclass Triangle is added but not handled in when, what happens?

A Compile error for non-exhaustive when

B Runtime exception thrown

C Code compiles but returns null

D Triangle is ignored silently

💡 Hint

See key_moments about compiler forcing exhaustive when checks.

Concept Snapshot

sealed class Shape {
  data class Circle(val r: Double) : Shape()
  data class Rectangle(val w: Double, val h: Double) : Shape()
}

fun area(s: Shape) = when(s) {
  is Shape.Circle -> Math.PI * s.r * s.r
  is Shape.Rectangle -> s.w * s.h
} // must cover all subclasses, else compile error

Full Transcript

Sealed classes in Kotlin let you define a fixed set of subclasses. When you use a when expression on a sealed class, Kotlin requires you to cover all subclasses. This makes your code safe because you handle every possible case. For example, if Shape is sealed with Circle and Rectangle subclasses, your when must handle both. If you miss one, the compiler shows an error. This exhaustive check prevents bugs. The execution table shows how the when expression checks the type step-by-step and returns the correct area. If you add a new subclass later, you must update the when expression or the compiler will warn you. This helps keep your code correct and easy to maintain.

Practice

1. What is the main benefit of using sealed classes in Kotlin when combined with a when expression?

easy

A. The when expression becomes exhaustive, ensuring all cases are handled.

B. It allows subclassing from any file in the project.

C. It automatically generates getter and setter methods.

D. It makes the class open for inheritance everywhere.

Solution

1. Step 1: Understand sealed classes restriction

   Sealed classes restrict subclassing to the same file, so all subclasses are known at compile time.

2. Step 2: Effect on when expression

   This allows the when expression to be exhaustive because the compiler knows all possible subclasses.

3. Final Answer:

   The when expression becomes exhaustive, ensuring all cases are handled. -> Option A

4. Quick Check:

   Sealed classes + when = exhaustive check [OK]

Hint: Sealed classes limit subclasses, making when exhaustive [OK]

Common Mistakes:

• Thinking sealed classes allow subclassing anywhere
• Confusing sealed classes with data classes
• Believing sealed classes auto-generate methods

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

easy

A. sealed class Shape {}

B. open sealed class Shape {}

C. sealed interface Shape {}

D. final sealed class Shape {}

Solution

1. Step 1: Recall sealed class declaration syntax

   In Kotlin, sealed classes are declared with the keyword sealed followed by class and the class name.

2. Step 2: Check options for correctness

   sealed class Shape {} uses correct syntax: sealed class Shape {}. open sealed class Shape {} incorrectly adds open, which is not allowed. sealed interface Shape {} uses sealed interface, which is invalid. final sealed class Shape {} uses final sealed class, which is not allowed.

3. Final Answer:

   sealed class Shape {} -> Option A

4. Quick Check:

   sealed class declaration = sealed class Shape {} [OK]

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

Common Mistakes:

• Adding 'open' modifier to sealed class
• Trying to declare sealed interfaces (not allowed)
• Using 'final' modifier with sealed class

3. Given the sealed class hierarchy below, what will be the output of the describe function when called with Shape.Circle(5.0)?

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

fun describe(shape: Shape): String = when(shape) {
    is Shape.Circle -> "Circle with radius ${shape.radius}"
    is Shape.Rectangle -> "Rectangle with width ${shape.width} and height ${shape.height}"
}

medium

A. Rectangle with width 5.0 and height 5.0

B. Compilation error due to missing else branch

C. Shape.Circle

D. Circle with radius 5.0

Solution

1. Step 1: Identify the input and matching when branch

   The input is Shape.Circle(5.0). The when expression matches is Shape.Circle branch.

2. Step 2: Evaluate the returned string

   The branch returns the string "Circle with radius 5.0" using the radius property.

3. Final Answer:

   Circle with radius 5.0 -> Option D

4. Quick Check:

   Input Circle(5.0) returns "Circle with radius 5.0" [OK]

Hint: Match input type in when to get correct output string [OK]

Common Mistakes:

• Confusing Circle with Rectangle output
• Expecting a compilation error due to else missing
• Returning class name instead of formatted string

4. Consider the following code snippet. What is the error and how can it be fixed?

sealed class Result {
    object Success : Result()
    object Failure : Result()
}

fun handle(result: Result) = when(result) {
    is Result.Success -> "Success"
}

medium

A. Runtime error: Result.Failure is not handled and causes crash.

B. Compilation error: when expression is not exhaustive; add branch for Result.Failure.

C. No error; code runs fine and returns null for Result.Failure.

D. Syntax error: sealed classes cannot have object subclasses.

Solution

1. Step 1: Identify the sealed class subclasses

   Result has two subclasses: Success and Failure.

2. Step 2: Check when expression branches

   The when expression only handles Result.Success, missing Result.Failure branch.

3. Step 3: Understand Kotlin's exhaustiveness check

   Since Result is sealed, when must handle all subclasses or have an else branch. Missing Failure causes compilation error.

4. Final Answer:

   Compilation error: when expression is not exhaustive; add branch for Result.Failure. -> Option B

5. Quick Check:

   Sealed class when must cover all subclasses [OK]

Hint: Add all sealed subclasses in when to avoid errors [OK]

Common Mistakes:

• Assuming runtime error instead of compile error
• Ignoring missing branches in when
• Thinking sealed classes can't have object subclasses

5. You have a sealed class Operation with subclasses Add, Subtract, and Multiply. Write a Kotlin function calculate that takes an Operation and two integers, and returns the result of applying the operation. Which of the following implementations correctly uses an exhaustive when expression?

hard

A. fun calculate(op: Operation, a: Int, b: Int): Int = when(op) {
is Operation.Add -> a + b
else -> 0
}

B. fun calculate(op: Operation, a: Int, b: Int): Int = when(op) {
is Operation.Add -> a + b
is Operation.Subtract -> a - b
}

C. fun calculate(op: Operation, a: Int, b: Int): Int = when(op) {
is Operation.Add -> a + b
is Operation.Subtract -> a - b
is Operation.Multiply -> a * b
}

D. fun calculate(op: Operation, a: Int, b: Int): Int = when(op) {
is Operation.Multiply -> a * b
else -> a + b
}

Solution

1. Step 1: Check all subclasses handled in when

   Operation has Add, Subtract, Multiply. fun calculate(op: Operation, a: Int, b: Int): Int = when(op) { is Operation.Add -> a + b is Operation.Subtract -> a - b is Operation.Multiply -> a * b } handles all three explicitly.

2. Step 2: Verify exhaustiveness and correctness

   fun calculate(op: Operation, a: Int, b: Int): Int = when(op) { is Operation.Add -> a + b is Operation.Subtract -> a - b is Operation.Multiply -> a * b } covers all subclasses, so when is exhaustive. The other options miss some subclasses or rely on else, which is less clear.

3. Final Answer:

   fun calculate(op: Operation, a: Int, b: Int): Int = when(op) { is Operation.Add -> a + b is Operation.Subtract -> a - b is Operation.Multiply -> a * b } -> Option C

4. Quick Check:

   All subclasses covered in when = exhaustive [OK]

Hint: Cover all sealed subclasses explicitly for exhaustive when [OK]

Common Mistakes:

• Missing some subclasses in when branches
• Using else instead of explicit branches
• Returning wrong default values