Sealed Classes Kotlin

What You’ll Learn 📚

• Understanding sealed classes and their purpose
• Key terminology explained simply
• Basic to advanced examples
• Common questions and troubleshooting

Introduction to Sealed Classes

In Kotlin, a sealed class is a special kind of class that allows you to represent restricted class hierarchies. What does that mean? 🤔 Essentially, a sealed class lets you define a set of subclasses that are known at compile time. This is super handy when you want to model a restricted set of types, like a specific set of states or actions.

Think of a sealed class like a sealed envelope 📩 – you know exactly what’s inside!

Key Terminology

• Sealed Class: A class that restricts its subclasses to a specific set known at compile time.
• Subclass: A class that inherits from another class.
• Compile Time: The phase in which the source code is translated to executable code.

The Simplest Example

sealed class Result {  class Success(val data: String) : Result()  class Error(val exception: Exception) : Result()}

Progressively Complex Examples

Example 1: Using Sealed Classes in a Function

fun handleResult(result: Result) {  when (result) {    is Result.Success -> println("Success with data: ${result.data}")    is Result.Error -> println("Error with exception: ${result.exception.message}")  }}

Example 2: Extending Sealed Classes

sealed class Operation {  object Add : Operation()  object Subtract : Operation()  object Multiply : Operation()  object Divide : Operation()}

Example 3: Sealed Classes with Data

sealed class NetworkResponse {  data class Success(val data: String) : NetworkResponse()  data class Failure(val error: String) : NetworkResponse()  object Loading : NetworkResponse()}

Common Questions

1. Why use sealed classes? They provide a way to represent a fixed set of types, ensuring type safety and exhaustiveness in when expressions.
2. Can sealed classes have non-sealed subclasses? No, all direct subclasses must be defined within the same file.
3. What’s the difference between sealed and abstract classes? Sealed classes restrict subclassing to a known set, while abstract classes do not.
4. Can sealed classes be instantiated? No, you cannot create an instance of a sealed class directly.
5. How do sealed classes improve code? They make your code more robust by ensuring all cases are handled explicitly.

Troubleshooting Common Issues

If you get a “sealed class cannot be instantiated” error, remember that you can only instantiate its subclasses.

If your when expression isn’t exhaustive, check if you’ve covered all subclasses of your sealed class.

Practice Exercises

• Create a sealed class to represent different types of notifications (e.g., Email, SMS, Push).
• Write a function that takes your notification sealed class and prints a message based on the type.

Great job making it through this tutorial! 🎉 Sealed classes are a powerful feature in Kotlin, and understanding them will make your code more robust and easier to maintain. Keep practicing, and soon you’ll be a sealed class pro!

For more information, check out the official Kotlin documentation.