Solving the Problem of null Values in JSON Decoding with Dart

Learn how to fix the issue of null values when decoding JSON in Dart. We'll explore a case study involving a custom model and keys in JSON data.
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Understanding the Issue: Getting Null When Decoding Doubles in Dart

In the world of Dart programming, especially when working with Flutter and JSON data, you may encounter an issue where a double value decodes as null. This problem can be particularly frustrating, especially when you know that the JSON string has the correct value. Let's take a closer look at a real-world example to understand the issue and how to resolve it effectively.

The Scenario

Imagine you have a custom object called MaslulModel designed to work seamlessly with JSON encoding and decoding. Here’s a brief look at what your class is doing:

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After creating an instance of this model, you convert it to JSON using the toMap method and save it as shared preferences. Later on when you need to retrieve the data, you decode the JSON back into a MaslulModel object.

However, you notice that while other values are decoded correctly, the interest field—designed to hold a double—returns as null. As per your JSON string, interest is present and has a value of 5.0:

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

Elucidating the Root Cause

The culprit behind the null value lies in a simple yet common mistake: a typographical error in the key name. When defining the fromJson factory constructor, you have:

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

In your code, notice the use of jsonData['interest;']. The semicolon at the end of interest; is the reason why the decoding function fails to retrieve the correct value. Instead, it looks for a key that doesn't exist, resulting in a null value.

The Solution: Correcting the Key Name

To fix this issue, simply refresh the key name in your fromJson method by removing the semicolon:

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

By making this change, your fromJson function will now correctly map the interest key from the JSON data to the respective property of the MaslulModel class, allowing the double value to be retained.

Conclusion

When working with JSON in Dart, minor encoding mistakes can lead to significant issues such as receiving null instead of the expected value. Always check that your key names match exactly with those present in your JSON structure. In our case, correcting a single typo resolved the issue and enabled the accurate decoding of the double value.

By addressing such pitfalls in your code, you not only enhance the robustness of your application but also streamline the development process, making it a lot easier for future updates or debugging.

Remember, precision is key in programming! Whether you’re managing JSON data in Dart or another language, a small oversight can lead to frustrating bugs. Happy coding!