Decoding JSON in Flutter: Fixing InternalLinkedHashMap String, dynamic Errors

Learn how to easily decode JSON responses in Flutter without facing type errors like 'InternalLinkedHashMap String, dynamic ' is not a subtype of type 'List String '.
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Decoding JSON in Flutter: Fixing InternalLinkedHashMap<String, dynamic> Errors

When working with APIs in Flutter, you may encounter several challenges, one of which is decoding JSON data. A common issue arises when the type returned from the API doesn't match what you've set up in your Dart models. A specific example of this is the error message: 'InternalLinkedHashMap<String, dynamic>' is not a subtype of type 'List<String>'.

In this guide, we will explore what this error means and how to resolve it effectively by restructuring our data model to align with the JSON structure.

Understanding the Problem

The JSON Structure

Here's a sample JSON response that we might want to decode:

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

This JSON contains a message object, which includes various breeds that are listed as arrays of strings. However, the model you've set up appears to expect a flat list of strings directly, which doesn’t align with the JSON structure.

The Current Dart Model

Here’s how you defined your JsonResponse Dart model:

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

The issue arises because you are trying to directly extract a list of strings from a nested object, leading to the inconsistency and ultimately the error message.

A Step-by-Step Solution

To resolve this type mismatch, we need to restructure our JsonResponse model to accurately represent the JSON data format.

Step 1: Adjusting the Dart Model

Instead of trying to map the nested message object directly to a List<String>, we need to change the type in our model to a Map<String, dynamic>. Here’s how we can do it:

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

Step 2: Accessing the Data

Once the model is adjusted, you can access the specific breed lists like so:

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

Summary of Changes

Change the type of message in JsonResponse to Map<String, dynamic>.

Adjust the factory constructor to correctly parse the JSON to this new structure.

Access breed lists using the appropriate keys from the message map.

Conclusion

By ensuring that your Dart model aligns with the underlying JSON structure, you can effectively avoid type errors like 'InternalLinkedHashMap<String, dynamic>' is not a subtype of type 'List<String>'. Properly decoding JSON responses is crucial for successful API interactions in Flutter applications.

By following the adjustments outlined in this post, you can streamline JSON decoding in your Flutter projects and improve your app's stability and performance.