Sealed Class Basics in Kotlin

Introduction

Sealed Class Basics 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.

Sealed classes restrict class hierarchies to known subclasses. 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

• Sealed classes restrict class hierarchies to known subclasses.
• All subclasses must be declared in the same file.
• Useful for representing restricted states.

Syntax

sealed class Result

Sealed Class Basics in Kotlin Example Program in Kotlin

sealed class Result
class Success(val data: String) : Result()
class Error(val message: String) : Result()

fun main(args: Array<String>) {
    val result: Result = Success("Done")
    when (result) {
        is Success -> println(result.data)
        is Error -> println(result.message)
    }
}

Sample Output

Done

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. val result: Result = Success("Done") assigns or updates a value used later in the program.

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

4. The println(result.message) statement writes a line to the console — this produces part of the sample output below.

5. Sealed classes restrict class hierarchies to known subclasses.

6. 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

• Understand the core idea: sealed classes restrict class hierarchies to known subclasses.
• Prefer readable names and small functions so examples map directly to real projects.
• Run and modify the example — change values and observe how the output changes.

Common Mistakes

• Skipping the example and only reading the definition — hands-on practice cements the concept.
• Copying syntax without understanding nullable vs non-nullable types or scope rules.
• Ignoring compiler warnings that often point to safer alternatives.

Key Points

• Sealed classes restrict class hierarchies to known subclasses.
• All subclasses must be declared in the same file.
• Useful for representing restricted states.
• 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.