54. Sealed classes in Kotlin: Creating restricted class hierarchies

March 14, 2025

🔒 Sealed Classes in Kotlin: Creating Restricted Class Hierarchies

Welcome, Kotlin developers! Today we'll dive deep into one of the most powerful and interesting features of Kotlin - sealed classes. These special classes provide a robust mechanism for creating restricted and predictable class hierarchies, enabling more type-safe and elegant code designs.

📘 What Are Sealed Classes?

Sealed classes in Kotlin are used to represent restricted class hierarchies. They allow you to define a limited set of subclasses that can inherit from a specific sealed class. This feature provides compile-time safety and helps prevent unexpected class extensions.

🔍 Key Characteristics of Sealed Classes

All direct subclasses must be defined in the same file
Constructors are private by default
They are implicitly abstract
Excellent for use with when expressions

💡 Basic Sealed Class Example

sealed class Result {
    data class Success(val data: String) : Result()
    data class Error(val message: String) : Result()
    object Loading : Result()
}

fun handleResult(result: Result) = when(result) {
    is Result.Success -> println("Data loaded: ${result.data}")
    is Result.Error -> println("Error occurred: ${result.message}")
    Result.Loading -> println("Loading in progress")
}

🏗️ Advanced Sealed Class Patterns

Sealed classes can be used in various sophisticated scenarios, such as representing complex state machines, creating type-safe domain models, and implementing functional programming patterns.

🚀 Performance and Compiler Benefits

The Kotlin compiler provides special optimizations for sealed classes, ensuring exhaustive checks and enabling more efficient pattern matching compared to traditional inheritance.

🎯 Practical Exercises

1. Create a sealed class representing geometric shapes with subclasses for Circle, Rectangle, and Triangle 2. Implement a payment processing system using sealed classes 3. Design a network request state management class 4. Create a sealed class for representing game character types 5. Build a configuration management system with sealed classes

⚠️ Common Pitfalls and Best Practices

Always define all subclasses in the same file
Use data classes for immutable state representations
Leverage sealed classes with when expressions
Consider using interfaces for more complex scenarios

Pro Tip: Always ensure comprehensive coverage in when expressions with sealed classes to handle all possible subtype scenarios.

🔗 Code Comparison: Sealed vs Regular Inheritance

// Sealed Class Approach
sealed class PaymentMethod {
    data class CreditCard(val number: String) : PaymentMethod()
    data class PayPal(val email: String) : PaymentMethod()
    object Cash : PaymentMethod()
}

// Traditional Inheritance
abstract class PaymentMethodTraditional {
    class CreditCard(val number: String) : PaymentMethodTraditional()
    class PayPal(val email: String) : PaymentMethodTraditional()
    class Cash : PaymentMethodTraditional()
}

🎉 Conclusion

Sealed classes represent a powerful abstraction in Kotlin, offering type-safe, expressive, and compile-time checked class hierarchies. By understanding and leveraging sealed classes, developers can create more robust and maintainable code structures.

#Kotlin #ProgrammingPatterns #SealedClasses #AndroidDev

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