Скачать или смотреть How to Properly Extend Classes in Dart to Access Object Properties

Learn how to correctly extend classes in Dart and effectively access object properties, ensuring proper implementation of your toString method.
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This video is based on the question https://stackoverflow.com/q/71526254/ asked by the user 'Dave' ( https://stackoverflow.com/u/13381244/ ) and on the answer https://stackoverflow.com/a/71526322/ provided by the user 'Anti' ( https://stackoverflow.com/u/6541968/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding Class Extension in Dart

When working with Dart and object-oriented programming, you might encounter situations where you need to extend a class. This allows you to create a new class that inherits properties and methods from a parent class. However, there can be challenges in accessing those inherited properties correctly, particularly in methods like toString() which are intended to provide a string representation of an object.

In this post, we’ll break down the issue faced in accessing object properties within the toString method while extending a class in Dart, and how you can resolve this elegantly.

The Problem

Consider the following scenario: you have already defined a Car class and you want to create a CarPrice class that extends it. In your implementation, you try to access the properties of your object in the toString() method, but instead, you end up with null null being printed.

This happens due to the improper assignment of the class properties in the constructors of your classes.

Let's take a closer look at the initial code structure that causes this problem:

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

The Solution

The key to resolving this issue is to ensure that you use the this keyword when assigning values to the class properties in your constructors. The this keyword differentiates between the constructor parameters and the class properties, ensuring proper assignment.

Here's how you can modify your code to achieve the desired effect:

Revised Class Definitions

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

Explanation of Key Changes

Use of this: By adding this. before the property names in the constructor, you're ensuring that the parameters passed to the constructor are assigned to the correct class properties.

Output After Fix: After these adjustments, running the main function will now print test 1000, reflecting your expectations of the CarPrice object.

Conclusion

Understanding the nuances of extending classes in Dart, and how to properly reference properties within constructors, is crucial for effective programming. With just a small tweak by using the this keyword, you can access and display the corresponding properties of your objects seamlessly.

If you found this guide helpful, feel free to reach out with more questions about Dart programming or object-oriented principles. Happy coding!