Sealed classes in Android Kotlin

Introduction

Sealed classes help you group related types together in a safe way. They make your code easier to understand and handle different cases clearly.

When you want to represent a fixed set of types, like states of a screen.

When you need to handle different results from a network call, like success or error.

When you want to make sure all possible cases are checked in a when expression.

When you want to avoid using many unrelated subclasses and keep types organized.

Syntax

Android Kotlin

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

A sealed class can have subclasses defined inside it or in the same file.

All subclasses must be known at compile time, so no new subclasses outside the file.

Examples

This sealed class represents three states of a screen: loading, showing content, and error.

Android Kotlin

sealed class ScreenState {
    object Loading : ScreenState()
    class Content(val text: String) : ScreenState()
    object Error : ScreenState()
}

Here, Response can be either Success or Failure, each holding different data.

Android Kotlin

sealed class Response {
    data class Success(val code: Int) : Response()
    data class Failure(val message: String) : Response()
}

Sample App

This program defines a sealed class LoginResult with three states. The handleLogin function uses a when expression to print messages based on the state.

Android Kotlin

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

fun handleLogin(result: LoginResult) {
    when(result) {
        is LoginResult.Success -> println("Login successful!")
        is LoginResult.Error -> println("Login failed: ${result.message}")
        LoginResult.Loading -> println("Loading...")
    }
}

fun main() {
    handleLogin(LoginResult.Loading)
    handleLogin(LoginResult.Success)
    handleLogin(LoginResult.Error("Wrong password"))
}

Important Notes

Sealed classes improve safety by forcing you to handle all cases.

You can use sealed interfaces in Kotlin 1.5+ for similar behavior.

Sealed classes are great for representing state machines or result types.

Summary

Sealed classes group related types in one place.

They help handle all possible cases safely with when expressions.

Use them to represent fixed sets of states or results.

Practice

1. What is the main purpose of a sealed class in Kotlin?

easy

A. To replace interfaces completely

B. To allow unlimited subclassing from any package

C. To create anonymous classes only

D. To restrict class inheritance to a fixed set of subclasses

Solution

1. Step 1: Understand sealed class inheritance

   A sealed class restricts which classes can inherit from it, limiting subclasses to those defined in the same file.

2. Step 2: Compare with other class types

   Unlike open classes, sealed classes do not allow unlimited subclassing, ensuring all subclasses are known at compile time.

3. Final Answer:

   To restrict class inheritance to a fixed set of subclasses -> Option D

4. Quick Check:

   Sealed class = fixed subclass set [OK]

Hint: Sealed classes limit subclasses to known types only [OK]

Common Mistakes:

• Thinking sealed classes allow any subclass anywhere
• Confusing sealed classes with abstract classes
• Assuming sealed classes create anonymous classes

2. Which of the following is the correct syntax to declare a sealed class named Result in Kotlin?

easy

A. sealed Result class {}

B. sealed class Result {}

C. class sealed Result {}

D. sealed interface Result {}

Solution

1. Step 1: Recall sealed class declaration syntax

   The keyword sealed precedes class followed by the class name and braces.

2. Step 2: Check each option

   sealed class Result {} matches the correct syntax. Options B and C have wrong keyword order. sealed interface Result {} declares a sealed interface, not a sealed class.

3. Final Answer:

   sealed class Result {} -> Option B

4. Quick Check:

   sealed class syntax = "sealed class Name {}" [OK]

Hint: sealed class always starts with 'sealed class' keywords [OK]

Common Mistakes:

• Swapping 'sealed' and 'class' keywords
• Using 'sealed interface' instead of 'sealed class'
• Omitting braces after class name

3. Given the sealed class and when expression below, what will be printed?

sealed class Response {
  object Success : Response()
  object Error : Response()
}

fun handle(response: Response) {
  when(response) {
    is Response.Success -> println("Success")
    is Response.Error -> println("Error")
  }
}

handle(Response.Success)

medium

A. Nothing is printed

B. Error

C. Success

D. Compile-time error

Solution

1. Step 1: Identify the passed object type

   The function is called with Response.Success, which matches the first when branch.

2. Step 2: Check the when branches

   The when expression prints "Success" for Response.Success and "Error" for Response.Error.

3. Final Answer:

   Success -> Option C

4. Quick Check:

   Response.Success triggers "Success" print [OK]

Hint: Match sealed subclasses exactly in when to get correct output [OK]

Common Mistakes:

• Confusing Success and Error outputs
• Forgetting to call the function with correct subclass
• Expecting compile error due to sealed class

4. What is wrong with this sealed class declaration?

sealed class State {}
class Loading : State()
class Success : State()
class Error : State()

medium

A. Subclasses must be declared inside the sealed class file

B. Sealed classes cannot have subclasses

C. Subclasses must be objects, not classes

D. Sealed class must be abstract

Solution

1. Step 1: Recall sealed class subclass rules

   All subclasses of a sealed class must be declared in the same file as the sealed class.

2. Step 2: Analyze the code snippet

   The subclasses Loading, Success, Error are declared separately after the sealed class. This violates the rule because direct subclasses of a sealed class must be declared in the same file as the sealed class.

3. Final Answer:

   Subclasses must be declared inside the sealed class file -> Option A

4. Quick Check:

   Sealed subclasses must be in same file [OK]

Hint: Keep all sealed subclasses in the same file [OK]

Common Mistakes:

• Declaring subclasses in different files
• Thinking sealed classes cannot have subclasses
• Confusing sealed classes with abstract classes

5. You want to represent a network request state with a sealed class NetworkState having Loading, Success with data, and Error with message. Which is the best way to define Success and Error subclasses?

hard

A. Use data classes with properties inside the sealed class

B. Use object declarations for all subclasses

C. Use abstract classes for Success and Error

D. Use interfaces for Success and Error

Solution

1. Step 1: Understand subclass data needs

   Success needs to hold data, and Error needs to hold a message, so they require properties.

2. Step 2: Choose subclass type

   Data classes are ideal for subclasses with properties because they provide equals, hashCode, and toString automatically.

3. Final Answer:

   Use data classes with properties inside the sealed class -> Option A

4. Quick Check:

   Data classes hold data in sealed subclasses [OK]

Hint: Use data classes for sealed subclasses with data [OK]

Common Mistakes:

• Using object for subclasses needing data
• Using abstract classes unnecessarily
• Using interfaces instead of sealed subclasses