How to Override a Generic Method in Dart?

Описание: Learn how to successfully override generic methods in Dart with practical examples and tips to enhance your coding skills.
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This video is based on the question https://stackoverflow.com/q/65170597/ asked by the user 'mrj' ( https://stackoverflow.com/u/4610033/ ) and on the answer https://stackoverflow.com/a/65179602/ provided by the user 'lrn' ( https://stackoverflow.com/u/2156621/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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How to Override a Generic Method in Dart?

Dart is a versatile programming language that supports various programming paradigms, including object-oriented programming. One common challenge developers face when working with generics is overriding methods correctly. In this guide, we’ll explore how to effectively override a generic method in Dart, using a concrete example to illustrate the process.

The Problem

Imagine you have a base class designed to manage chat functionalities. This base class generically handles messages and users, potentially looking something like this:

[[See Video to Reveal this Text or Code Snippet]]

Here, IChat is an abstract class that defines a structure for managing messages. The method getUsers is designed to return a list of users of a specified type. However, when creating a subclass (Chat), you may encounter an issue related to type compatibility.

Given an extended model like this:

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You might find that returning List<ChatUser> from getUsers<T>() leads to an error because it does not fit the expected return type List<T>.

The Error

The compiler is throwing an error because it expects a list of the generic type T, but instead, it finds a list of ChatUser. This mismatch prompts the need to adjust the method’s structure to resolve it efficiently.

The Solution

Rethink the Class Design

First, it’s essential to assess what you want your classes to accomplish. It appears you always want the subclass to return a specific type of user, which suggests having the type as a property of the class rather than as a parameter in the method invocation.

Revised Code Structure

Instead of maintaining the generic method signature, we can redefine the getUsers method to not require a specified type parameter, allowing the class's structure to dictate the type. Here’s how you can implement this:

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Code Explanation

Change the Generic Method:

We removed the type parameter from getUsers in the IChat class. This allows us to specify that the method will return a general list of users as defined by IUser.

Override in Subclass:

In the Chat class, we directly return List<ChatUser>, thus aligning with the method in the base class without type conflicts.

Stylistic Improvements:

To conform to Dart syntax and coding styles, consider using final for properties and simplify constructors as shown in the original answer.

Final Example Code

Here’s a complete example after making the mentioned adjustments:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

By rethinking how we structure our classes and their methods, we can effectively resolve challenges related to generics in Dart. Instead of letting the method’s type drive its return value, we realigned our class design to define its purpose more clearly and compatibly.

Adopting these practices not only helps resolve specific coding errors but also promotes better software architecture within your Dart applications. Happy coding!