Why Sealed classes with when exhaustive check in Kotlin? - Purpose & Use Cases

The Big Idea

What if your code could warn you every time you forget to handle a new case?

The Scenario

Imagine you have a list of different types of animals, and you want to write code that handles each type separately. You try to check each animal type manually with many if-else statements.

The Problem

This manual approach is slow and error-prone because you might forget to handle a new animal type added later. Your code becomes messy and hard to maintain, leading to bugs.

The Solution

Sealed classes let you define a fixed set of types in one place. When you use a when expression with sealed classes, Kotlin forces you to handle all possible cases. This makes your code safer and easier to read.

Before vs After

✗ Before

if (animal is Dog) { /* handle dog */ } else if (animal is Cat) { /* handle cat */ } // what if new animal added?

✓ After

when (animal) {
  is Dog -> /* handle dog */
  is Cat -> /* handle cat */
  // compiler ensures all cases handled
}

What It Enables

You can write clear, safe code that automatically checks you cover all possible cases, preventing bugs before they happen.

Real Life Example

In a messaging app, you might have sealed classes for message types like Text, Image, and Video. Using when with sealed classes ensures you handle every message type properly.

Key Takeaways

Manual checks can miss new cases and cause bugs.

Sealed classes define a fixed set of types clearly.

when with sealed classes forces exhaustive checks for safety.

Practice

1. What is the main benefit of using sealed classes in Kotlin when combined with a when expression?

easy

A. The when expression becomes exhaustive, ensuring all cases are handled.

B. It allows subclassing from any file in the project.

C. It automatically generates getter and setter methods.

D. It makes the class open for inheritance everywhere.

Solution

1. Step 1: Understand sealed classes restriction

   Sealed classes restrict subclassing to the same file, so all subclasses are known at compile time.

2. Step 2: Effect on when expression

   This allows the when expression to be exhaustive because the compiler knows all possible subclasses.

3. Final Answer:

   The when expression becomes exhaustive, ensuring all cases are handled. -> Option A

4. Quick Check:

   Sealed classes + when = exhaustive check [OK]

Hint: Sealed classes limit subclasses, making when exhaustive [OK]

Common Mistakes:

• Thinking sealed classes allow subclassing anywhere
• Confusing sealed classes with data classes
• Believing sealed classes auto-generate methods

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

easy

A. sealed class Shape {}

B. open sealed class Shape {}

C. sealed interface Shape {}

D. final sealed class Shape {}

Solution

1. Step 1: Recall sealed class declaration syntax

   In Kotlin, sealed classes are declared with the keyword sealed followed by class and the class name.

2. Step 2: Check options for correctness

   sealed class Shape {} uses correct syntax: sealed class Shape {}. open sealed class Shape {} incorrectly adds open, which is not allowed. sealed interface Shape {} uses sealed interface, which is invalid. final sealed class Shape {} uses final sealed class, which is not allowed.

3. Final Answer:

   sealed class Shape {} -> Option A

4. Quick Check:

   sealed class declaration = sealed class Shape {} [OK]

Hint: Use only 'sealed class ClassName' to declare sealed classes [OK]

Common Mistakes:

• Adding 'open' modifier to sealed class
• Trying to declare sealed interfaces (not allowed)
• Using 'final' modifier with sealed class

3. Given the sealed class hierarchy below, what will be the output of the describe function when called with Shape.Circle(5.0)?

sealed class Shape {
    data class Circle(val radius: Double) : Shape()
    data class Rectangle(val width: Double, val height: Double) : Shape()
}

fun describe(shape: Shape): String = when(shape) {
    is Shape.Circle -> "Circle with radius ${shape.radius}"
    is Shape.Rectangle -> "Rectangle with width ${shape.width} and height ${shape.height}"
}

medium

A. Rectangle with width 5.0 and height 5.0

B. Compilation error due to missing else branch

C. Shape.Circle

D. Circle with radius 5.0

Solution

1. Step 1: Identify the input and matching when branch

   The input is Shape.Circle(5.0). The when expression matches is Shape.Circle branch.

2. Step 2: Evaluate the returned string

   The branch returns the string "Circle with radius 5.0" using the radius property.

3. Final Answer:

   Circle with radius 5.0 -> Option D

4. Quick Check:

   Input Circle(5.0) returns "Circle with radius 5.0" [OK]

Hint: Match input type in when to get correct output string [OK]

Common Mistakes:

• Confusing Circle with Rectangle output
• Expecting a compilation error due to else missing
• Returning class name instead of formatted string

4. Consider the following code snippet. What is the error and how can it be fixed?

sealed class Result {
    object Success : Result()
    object Failure : Result()
}

fun handle(result: Result) = when(result) {
    is Result.Success -> "Success"
}

medium

A. Runtime error: Result.Failure is not handled and causes crash.

B. Compilation error: when expression is not exhaustive; add branch for Result.Failure.

C. No error; code runs fine and returns null for Result.Failure.

D. Syntax error: sealed classes cannot have object subclasses.

Solution

1. Step 1: Identify the sealed class subclasses

   Result has two subclasses: Success and Failure.

2. Step 2: Check when expression branches

   The when expression only handles Result.Success, missing Result.Failure branch.

3. Step 3: Understand Kotlin's exhaustiveness check

   Since Result is sealed, when must handle all subclasses or have an else branch. Missing Failure causes compilation error.

4. Final Answer:

   Compilation error: when expression is not exhaustive; add branch for Result.Failure. -> Option B

5. Quick Check:

   Sealed class when must cover all subclasses [OK]

Hint: Add all sealed subclasses in when to avoid errors [OK]

Common Mistakes:

• Assuming runtime error instead of compile error
• Ignoring missing branches in when
• Thinking sealed classes can't have object subclasses

5. You have a sealed class Operation with subclasses Add, Subtract, and Multiply. Write a Kotlin function calculate that takes an Operation and two integers, and returns the result of applying the operation. Which of the following implementations correctly uses an exhaustive when expression?

hard

A. fun calculate(op: Operation, a: Int, b: Int): Int = when(op) {
is Operation.Add -> a + b
else -> 0
}

B. fun calculate(op: Operation, a: Int, b: Int): Int = when(op) {
is Operation.Add -> a + b
is Operation.Subtract -> a - b
}

C. fun calculate(op: Operation, a: Int, b: Int): Int = when(op) {
is Operation.Add -> a + b
is Operation.Subtract -> a - b
is Operation.Multiply -> a * b
}

D. fun calculate(op: Operation, a: Int, b: Int): Int = when(op) {
is Operation.Multiply -> a * b
else -> a + b
}

Solution

1. Step 1: Check all subclasses handled in when

   Operation has Add, Subtract, Multiply. fun calculate(op: Operation, a: Int, b: Int): Int = when(op) { is Operation.Add -> a + b is Operation.Subtract -> a - b is Operation.Multiply -> a * b } handles all three explicitly.

2. Step 2: Verify exhaustiveness and correctness

   fun calculate(op: Operation, a: Int, b: Int): Int = when(op) { is Operation.Add -> a + b is Operation.Subtract -> a - b is Operation.Multiply -> a * b } covers all subclasses, so when is exhaustive. The other options miss some subclasses or rely on else, which is less clear.

3. Final Answer:

   fun calculate(op: Operation, a: Int, b: Int): Int = when(op) { is Operation.Add -> a + b is Operation.Subtract -> a - b is Operation.Multiply -> a * b } -> Option C

4. Quick Check:

   All subclasses covered in when = exhaustive [OK]

Hint: Cover all sealed subclasses explicitly for exhaustive when [OK]

Common Mistakes:

• Missing some subclasses in when branches
• Using else instead of explicit branches
• Returning wrong default values