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.