Sealed Interfaces in Kotlin

Introduction

Sealed Interfaces 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 interfaces work like sealed classes for interface hierarchies. 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 interfaces work like sealed classes for interface hierarchies.
• They restrict implementations to known types.
• Supported in modern Kotlin versions.

Syntax

sealed interface UiState

Sealed Interfaces in Kotlin Example Program in Kotlin

sealed interface UiState
data class Loading(val message: String) : UiState
data class Ready(val count: Int) : UiState

fun main(args: Array<String>) {
    val state: UiState = Ready(3)
    when (state) {
        is Loading -> println(state.message)
        is Ready -> println("Count ${state.count}")
    }
}

Sample Output

Count 3

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 state: UiState = Ready(3) assigns or updates a value used later in the program.

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

4. The println("Count ${state.count}") statement writes a line to the console — this produces part of the sample output below.

5. Sealed interfaces work like sealed classes for interface hierarchies.

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 interfaces work like sealed classes for interface hierarchies.
• 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 interfaces work like sealed classes for interface hierarchies.
• They restrict implementations to known types.
• Supported in modern Kotlin versions.
• 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.