Sealed Classes in Kotlin

1. What is a sealed class in Kotlin?

A sealed class is a special kind of class that represents a restricted class hierarchy. All subclasses must be declared within the sealed class itself.

2. What is the main purpose of using sealed classes?

Sealed classes are used to represent a fixed set of subclasses in a class hierarchy. They are often used in conjunction with when expressions to ensure exhaustive handling of all possible subclasses.

3. Can a sealed class have non-private constructors?

Yes, sealed classes can have constructors, but the constructors must be private. Subclasses can have different visibility modifiers (e.g., internal, protected).

4. How does a sealed class differ from an enum class?

While enums represent a fixed set of instances, sealed classes represent a fixed set of types with their own instances.

5. Provide an example of a sealed class and its subclasses.

6. How is a sealed class useful in exhaustive when expressions?

Since all subclasses are known, the compiler can check if all possible cases are handled, making it easier to avoid missing cases.

7. Can you instantiate a sealed class directly?

No, you cannot instantiate a sealed class directly. It must be instantiated through one of its subclasses.

8. How can you use a sealed class to represent state in Android applications?

Sealed classes can be used to represent different states of a view or component in a concise and type-safe manner.

9. Can a sealed class have abstract methods or properties?

Yes, a sealed class can have abstract methods or properties that are implemented by its subclasses.

10. How does a sealed class contribute to code maintainability?

Sealed classes help in maintaining a clear and concise class hierarchy, making it easier to understand and modify the code as it evolves.

11. Explain the role of sealed classes in expression trees or algebraic data types.

Sealed classes are useful for representing hierarchical structures in expression trees, where each subclass represents a different node type.

12. Can sealed classes be used in combination with generics?

Yes, sealed classes can be used with generics to create flexible and type-safe structures.

13. How does pattern matching in Kotlin relate to sealed classes?

Pattern matching in Kotlin, often used with when expressions, is enhanced when working with sealed classes as it ensures exhaustive matching.

14. Can sealed classes have multiple instances of the same subclass?

No, sealed classes have a limited set of subclasses, and each subclass can only have one instance.

15. How are sealed classes related to data classes in Kotlin?

Sealed classes can have data classes as their subclasses, providing a concise way to represent different types of data.

16. What is the significance of the internal modifier in a sealed class?

The internal modifier allows subclasses to be visible within the same module, providing a level of encapsulation.

17. How can you use sealed classes in conjunction with extension functions?

Extension functions can be defined for sealed classes, providing additional functionality for each subclass.

18. What are the limitations or considerations when using sealed classes?

One limitation is that all subclasses must be defined in the same file. Also, the hierarchy should be relatively stable.

19. How do you handle sealed classes when working with persistence or serialization?

When persisting or serializing sealed classes, it's essential to ensure that the class hierarchy remains intact during deserialization.

20. Explain how sealed classes contribute to making code more expressive and readable.

Sealed classes provide a clear and expressive way to represent a finite set of related types, improving code readability and maintainability.