Скачать или смотреть How to Declare a Variable of Type Class for Inherited Classes in Kotlin

Learn how to properly declare a variable of type class for inherited classes in Kotlin, avoiding type mismatch errors.
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This video is based on the question https://stackoverflow.com/q/67427177/ asked by the user 'KingFish' ( https://stackoverflow.com/u/145129/ ) and on the answer https://stackoverflow.com/a/67427192/ provided by the user 'Louis Wasserman' ( https://stackoverflow.com/u/869736/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding Class Declarations in Kotlin: A Guide for Inherited Classes

When working with Kotlin, particularly with classes that extend from a base class, developers often encounter situations where they need to declare a variable that can hold classes of a specific type. This article addresses a common challenge faced by Kotlin developers regarding variable declarations for inherited classes, specifically when using AppCompatActivity as the base class for activities.

The Problem Explained

Let's consider two classes, Activity1 and Activity2, both of which inherit from AppCompatActivity. If you attempt to declare a variable called toOpen that can hold an instance of the class type for these activities, you might run into a type mismatch error. The initial approach may look like this:

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Attempting to Assign Classes

Suppose we try to assign Activity1 as follows:

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In this case, an error occurs because toOpen is explicitly expecting a Class<AppCompatActivity>, while Activity1 is not recognized as a valid type for that declaration.

The Solution

The key to solving this issue lies in correctly declaring the type of the variable toOpen. Instead of declaring it as Class<AppCompatActivity>, you should declare it as Class<in AppCompatActivity>. This modification leverages Kotlin's generics system, allowing for a more flexible assignment.

Correct Declaration

Here’s how to correctly declare the variable:

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What Does Class<in AppCompatActivity> Mean?

The declaration Class<in AppCompatActivity> indicates that toOpen can accept any class that is AppCompatActivity or any of its subclasses. This is a notable aspect of Kotlin's type system which allows for covariance using the in keyword. It ensures that the variable is flexible enough to accommodate derived classes without throwing type mismatch errors.

Assigning the Classes

With the correct declaration, you can now assign either of the activities to toOpen without encountering issues:

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This change resolves the type mismatch and allows you to work seamlessly with inherited classes in your Kotlin applications.

Conclusion

Understanding how to declare variables that handle classes in Kotlin, particularly for inherited structures, is crucial for any developer working with this programming language. By using Class<in AppCompatActivity>, you can avoid common pitfalls related to type mismatches and ensure that your code remains clean and functional.

With these insights, you are now better equipped to handle class declarations in Kotlin. Happy coding!