Скачать или смотреть Why Your Parsing JSON Code Returns Null: Understanding the Issue and the Fix

Discover why parsing a JSON list of maps returns null in Flutter/Dart. Here’s how to identify and resolve the issue with your model classes!
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This video is based on the question https://stackoverflow.com/q/63187854/ asked by the user 'Talha Javed Khan' ( https://stackoverflow.com/u/12450079/ ) and on the answer https://stackoverflow.com/a/63187989/ provided by the user 'julemand101' ( https://stackoverflow.com/u/1953515/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding Null Values When Parsing JSON in Flutter/Dart

Parsing JSON data in a Flutter or Dart application can sometimes lead to confusing issues, such as unexpectedly receiving null values. If you've encountered this problem, you're not alone. In this guide, we’ll explore a common scenario where parsing a JSON list of maps returns null values and how to fix it effectively.

The Problem

While parsing JSON responses, developers often find that the data they expect ends up being null. In this case, the API returns a list of customer addresses, and even though the printed API response and the decoded response looks correct, the modeled objects contain null values. Here’s a look at the provided code:

API Call Code Snippet

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

What Works

Printing individual fields from the decoded map:

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

These print correctly and return the expected values.

What Doesn’t Work

When accessing properties through the model classes:

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

These return null, which indicates there’s an issue in the model class structure.

The Solution: Fixing the Constructor

Upon inspecting the provided code, the root of the issue lies in the constructor of the CustomerAddress model class. Here’s why the problem occurs:

Identifying the Constructor Issue

The constructor you’ve defined does not properly initialize the properties of the CustomerAddress class. As a result, the values are being discarded. The correct constructor should include the this keyword to ensure the variables are correctly assigned when an object is instantiated.

Correct Constructor Code

You need to change the constructor of CustomerAddress to:

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

Why This Change Matters

Adding the this keyword before the properties in the constructor allows Dart to assign the passed values to the respective properties of the class. Without it, your constructor merely accepts the values but does nothing with them, leading to the problematic null values.

Conclusion

Encountering null values while parsing JSON can be a frustrating experience when working with Flutter and Dart, but understanding how Dart's constructors work helps solve the issue quickly. By fixing the constructor of the CustomerAddress model, you’ll be able to successfully parse the JSON response and access your data as intended.

Implement this fix, and you should start seeing the expected values in your object properties. Remember, correctly defining your model classes is crucial for effective JSON handling in your applications!