Sealed classes for restricted hierarchies in Kotlin - Step-by-Step Execution

Concept Flow - Sealed classes for restricted hierarchies

Define sealed class

↓

Define subclasses inside or outside

↓

Use when expression to check subclass

↓

Handle all subclasses exhaustively

↓

No other subclasses allowed outside hierarchy

↓

End

Sealed classes restrict subclassing to a known set, enabling exhaustive checks in when expressions.

Execution Sample

Kotlin

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

fun area(shape: Shape): Double = when(shape) {
    is Shape.Circle -> 3.14 * shape.radius * shape.radius
    is Shape.Rectangle -> shape.width * shape.height
}

Calculates area for Circle or Rectangle using sealed class Shape and exhaustive when.

Execution Table

Step	Action	Input	Condition	Branch Taken	Output
1	Call area with Circle(radius=2.0)	Circle(radius=2.0)	shape is Circle?	Yes	Area = 3.14 * 2.0 * 2.0 = 12.56
2	Return area for Circle				12.56
3	Call area with Rectangle(width=3.0, height=4.0)	Rectangle(width=3.0, height=4.0)	shape is Circle?	No
4	Check if shape is Rectangle	Rectangle(width=3.0, height=4.0)	shape is Rectangle?	Yes	Area = 3.0 * 4.0 = 12.0
5	Return area for Rectangle				12.0
6	Call area with unknown subclass (not allowed)	UnknownShape()	No matching subclass	Error	Compilation error: sealed class restricts subclasses

💡 Execution stops because all subclasses are handled; no other subclasses allowed outside sealed class.

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
shape	-	Circle(radius=2.0)	Rectangle(width=3.0, height=4.0)	UnknownShape()	-
area	-	12.56	12.0	Error	-

Key Moments - 3 Insights

Why does the when expression not need an else branch?

Because the sealed class restricts subclasses, the when expression covers all cases explicitly (see execution_table rows 1-5).

Can we create a subclass of the sealed class outside its file?

No, sealed classes only allow subclasses defined in the same file, preventing unknown subclasses (see execution_table row 6).

What happens if a new subclass is added but not handled in when?

The compiler forces you to handle it or add an else branch, ensuring exhaustive checks.

Visual Quiz - 3 Questions

Look at the execution_table, what is the area output when shape is Circle(radius=2.0)?

A 3.14

B 12.56

C 4.0

D Error

💡 Hint

Check execution_table row 1 and 2 for Circle input and output.

At which step does the when expression handle the Rectangle subclass?

A Step 1

B Step 3

C Step 4

D Step 6

💡 Hint

Look at execution_table rows 3 and 4 for Rectangle handling.

If a new subclass is added outside the sealed class file, what happens?

A Compiler error due to sealed class restrictions

B It compiles and runs normally

C It runs but when expression skips it

D It requires an else branch

💡 Hint

See execution_table row 6 about unknown subclass and error.

Concept Snapshot

sealed class ClassName {
  // subclasses defined here or same file
}

Use when to check all subclasses without else.
Compiler ensures no other subclasses exist.
Enables safe, exhaustive type checks.

Full Transcript

Sealed classes in Kotlin let you define a restricted set of subclasses. You declare a sealed class and its subclasses in the same file. This restriction helps the compiler know all possible subclasses. When you use a when expression on a sealed class, you can check all subclasses explicitly without needing an else branch. The compiler ensures you handle every subclass, making your code safer and clearer. If you try to create a subclass outside the sealed class file, the compiler will give an error. This way, sealed classes help you create restricted hierarchies with exhaustive checks.