Sealed classes for restricted hierarchies in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a sealed class in Kotlin?

A sealed class is a special kind of class that restricts which other classes can inherit from it. It helps to define a closed set of subclasses, making the hierarchy controlled and known at compile time.

intermediate

Why use sealed classes instead of open classes?

Sealed classes limit subclassing to a fixed set of types, which helps the compiler check when all cases are handled, especially in when expressions. Open classes allow unlimited subclassing, which can lead to less safe code.

intermediate

How do sealed classes improve when expressions?

When you use a sealed class in a <code>when</code> expression, the compiler knows all possible subclasses. This means you don't need an <code>else</code> branch if you cover all subclasses, making your code safer and clearer.

beginner

Can sealed classes have subclasses in other packages?

No. All direct subclasses of a sealed class must be declared in the same package or the same file as the sealed class. This restriction keeps the hierarchy closed and controlled.

beginner

Example: Define a sealed class Shape with subclasses Circle and Rectangle.

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

What is a key feature of sealed classes in Kotlin?

A They restrict subclassing to the same package.

B They allow subclassing from any file.

C They cannot have subclasses.

D They are the same as abstract classes.

Sealed classes restrict direct subclassing to the same package or file, ensuring a closed set of subclasses.

Why does Kotlin allow omitting the else branch in a when expression with sealed classes?

A Because sealed classes have no subclasses.

B Because <code>else</code> is never needed in Kotlin.

C Because sealed classes are final.

D Because the compiler knows all subclasses at compile time.

The compiler knows all subclasses of a sealed class, so it can check if all cases are covered without needing an else.

Which of these is NOT true about sealed classes?

A They can have abstract members.

B They can be instantiated directly.

C Their subclasses must be in the same file.

D They help with exhaustive <code>when</code> checks.

Direct subclasses of sealed classes can be declared in the same package or the same file but do not have to be in the same file (since Kotlin 1.5).

What keyword is used to declare a sealed class in Kotlin?

A sealed

B open

C abstract

D final

The keyword sealed is used to declare a sealed class.

Which of the following is a benefit of using sealed classes?

A Unlimited subclassing across packages.

B Better control over class hierarchies and safer code.

C They automatically generate getters and setters.

D They allow multiple inheritance.

Sealed classes provide better control over class hierarchies and help write safer code by restricting subclassing.

Explain what a sealed class is and why it is useful in Kotlin.

Think about how sealed classes help the compiler know all possible subclasses.

Describe how sealed classes improve safety and readability when using when expressions.

Focus on the relationship between sealed classes and exhaustive checks.