Sealed classes in Android 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 classes can inherit from it. It helps represent restricted class hierarchies, making code safer and easier to understand.

intermediate

Why use sealed classes instead of enums?

Sealed classes can hold state and have multiple properties, unlike enums which are limited to fixed constants. Sealed classes allow more complex data and behavior.

beginner

How do you declare a sealed class in Kotlin?

Use the keyword <code>sealed</code> before <code>class</code>. For example: <br><code>sealed class Result</code>

intermediate

Can sealed classes have subclasses outside their file?

No. All subclasses of a sealed class must be declared in the same file. This restriction helps the compiler know all possible subclasses.

intermediate

How does a sealed class improve <code>when</code> expressions?

When you use a sealed class in a <code>when</code> expression, the compiler knows all subclasses and can enforce that all cases are handled, making the code safer.

What keyword do you use to declare a sealed class in Kotlin?

A sealed

B open

C abstract

D final

The keyword sealed declares a sealed class.

Where must subclasses of a sealed class be declared?

A In a different module

B Anywhere in the project

C In a different package

D In the same file as the sealed class

All subclasses must be declared in the same file to keep the hierarchy closed.

Which of these is a benefit of using sealed classes?

A Compiler enforces exhaustive when expressions

B Allows subclasses anywhere

C Can be instantiated directly

D No restrictions on inheritance

Sealed classes help the compiler check that all cases are handled in when expressions.

Can sealed classes hold state or properties?

A No, they are like enums with fixed constants

B Only if marked abstract

C Yes, they can have properties and state

D Only if subclasses are data classes

Sealed classes can have properties and hold state, unlike enums.

Which of these is NOT true about sealed classes?

A They restrict subclassing to the same file

B They allow unlimited subclassing anywhere

C They improve safety in when expressions

D They can be used to represent restricted hierarchies

Sealed classes do NOT allow unlimited subclassing; subclasses must be in the same file.

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 the use of when expressions in Kotlin.

Consider what happens if you forget to handle a subclass in a when expression.