Interface with Generic Method Iterating Over Enum Values in Kotlin

Learn how to generically extract the implementation of the ratings() function into the RatingValues interface in Kotlin, allowing for cleaner and more maintainable code in your enum classes.
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Understanding the Problem: Generics and Enums in Kotlin

In Kotlin, working with interfaces and enums can sometimes lead to repetitive code, especially when you have multiple enums implementing the same interfaces. If you've ever found yourself writing the same function in the companion objects of various enums, you're not alone. This issue commonly arises while attempting to create a common behavior within enums that implement specific interfaces.

The problem revolves around the need to generically extract the implementation of the ratings() function into a RatingValues interface. This means we want to avoid duplicating the same implementation across companion objects for different enums, leading to cleaner and more maintainable code.

Interfaces in Kotlin

In our case, we have the following two interfaces:

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The Enum Example

We have a sample enum named MarkFoo which implements the RatingValue interface:

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As we see here, every enum needs to define its own implementation of the ratings() method, which can become repetitive and cluttered as you define more enums.

The Solution: Generic Extension Functions

To solve this problem, we need to generify the RatingValues interface to access Enum<T> methods and the RatingValue interface. However, since reified generics are not allowed for interfaces, we'll use extension functions to provide the desired behavior.

Step 1: Create a Generic RatingValues Interface

First, we modify the RatingValues interface to accept a type parameter T. This allows us to specify constraints on the type T:

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Step 2: Add Extension Functions

Next, we add extension functions for ratings() and cumulativeRating() that utilize the generic type T:

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Step 3: Implementation in Enums

Now, we can implement this modified interface in our enum classes without needing to duplicate the ratings() implementation:

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This implementation allows the ratings() method to be automatically generated using the provided extension function!

Limitations and Considerations

It's essential to note that if you plan to use this interface with non-enum types, you may find it necessary to add alternative methods (for example, call the method something like ratingsForEnums()). This ensures that interfaces remain usable without overriding behaviors that only apply to enum classes.

Conclusion

By generifying the RatingValues interface and utilizing extension functions, we have effectively reduced redundancy in our Kotlin code for enums. This method leads to a cleaner codebase, making it easier to maintain and understand.

Kotlin provides powerful tools for abstraction, and learning how to use generics effectively will make you a better developer. Try applying this approach when facing duplicative code in your own projects!