Sealed Class with when in Kotlin

Introduction

Sealed Class with when is a fundamental concept every Kotlin developer should understand. Sealed classes restrict which subclasses can exist, making when expressions exhaustive and safer for representing finite state machines or result types.

When expression with sealed classes can be exhaustive. In this tutorial you will learn the syntax, walk through a complete example program, study the sample output, and review best practices so you can apply the concept confidently in your own projects.

Definition

• when expression with sealed classes can be exhaustive.
• Compiler knows all possible subclasses.
• else branch is often not required.

Syntax

when (result) { is Success -> ... }

Sealed Class with when in Kotlin Example Program in Kotlin

sealed class Shape
class Circle(val r: Int) : Shape()
class Square(val side: Int) : Shape()

fun describe(shape: Shape) = when (shape) {
    is Circle -> "Circle radius ${shape.r}"
    is Square -> "Square side ${shape.side}"
}

fun main(args: Array<String>) {
    println(describe(Circle(5)))
}

Sample Output

Circle radius 5

When to use

Use sealed classes for closed hierarchies — UI states, network results, or AST node types.

How it works

1. The program starts with a main function — the entry point that runs when you execute the file.

2. fun describe(shape: Shape) = when (shape) { assigns or updates a value used later in the program.

3. The println(describe(Circle(5))) statement writes a line to the console — this produces part of the sample output below.

4. When expression with sealed classes can be exhaustive.

5. Run the program in IntelliJ IDEA, Android Studio, or with the Kotlin command-line compiler (kotlinc / kotlin). Compare your console output with the sample output shown below.

Best Practices

Common Mistakes

Key Points

• when expression with sealed classes can be exhaustive.
• Compiler knows all possible subclasses.
• else branch is often not required.
• Test the example locally and verify the output matches the sample.
• Experiment by changing input values to see how behaviour changes.

Notes

• Semicolons at the end of statements are optional in Kotlin.