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Mastering Kotlin Essentials

Introduction to Sealed Classes and Interfaces

Explore how to use sealed classes and interfaces in Kotlin to build restricted and controlled type hierarchies. This lesson helps you understand their rules, differences, and advantages for safer polymorphism and better code organization in Kotlin applications.

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The Result interface and abstract class

Classes and interfaces in Kotlin are not only used to represent a set of operations or data. We can also use classes and inheritance to represent hierarchies through polymorphism. For instance, let’s say that we send a network request. As a result, we either successfully receive the requested data, or the request fails with some information about what went wrong. These two outcomes can be represented using two classes that implement an interface:

Kotlin 1.5
interface Result
class Success(val data: String) : Result
class Failure(val exception: Throwable) : Result

Alternatively, we could use an abstract class:

Kotlin 1.5
abstract class Result
class Success(val data: String) : Result()
class Failure(val exception: Throwable) : Result()

Restricted hierarchies

With either of these, we know that when a function returns ...