Resolving Flutter json_serializable Error: Understanding Null Types in Dart

Learn how to tackle the Flutter json_serializable error regarding `Null` types and how to adjust your data model accordingly.
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Resolving Flutter json_serializable Error: Understanding Null Types in Dart

If you're developing a Flutter application and using the json_serializable package, you may encounter an error that states: Unhandled Exception: type 'Null' is not a subtype of type 'String' in type cast. This error often arises when your JSON data contains null values that you attempt to cast to non-nullable types in your Dart classes. In this guide, we will break down this issue and present a solution that allows you to maintain your JSON structure without running into type casting errors.

Understanding the Problem

In your specific use case, you have two data entries: one for "Benzin" and another for "Diesel". The "Diesel" entry's leistungData is an empty array:

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

Since the leistungData field is empty, attempting to deserialize this JSON directly into a non-nullable type leads to the aforementioned error message. Dart's strict type system prevents null values from being assigned to non-nullable fields which you declared in your classes.

The Solution

To resolve the error, you can modify your Dart classes to accommodate nullable types. This way, your application can handle cases where fields might be absent or null in the JSON data.

1. Modify Your Class Definitions

You'll want to change your data model definitions to allow for null values by using nullable types. Here’s how to adjust your classes:

Update TreibstoffData Class

Replace this line:

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

with:

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

This allows leistungData to be null instead of forcing it to be a non-nullable list.

Update LeistungData Class

Similarly, for the title in the LeistungData class, change this line:

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

to:

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

Now the title can also be null.

2. Example of Modified Classes

Here is how your modified classes would look:

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

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

3. Why This Works

The adjustments allow your code to correctly handle the scenario where the JSON values could be null or missing entirely. By opting for nullable types as shown, your application becomes more robust and handles edge cases gracefully without crashing.

Conclusion

Handling null values effectively is crucial when working with JSON data in Flutter applications. By adopting nullable types in your data models using Dart’s syntax, you can avoid runtime exceptions and keep your app functioning smoothly.

If you ever encounter such issues, remember to revisit your model definitions to see if adjustments to nullable types might be the answer you need. Happy coding!