Скачать или смотреть Resolving the _InternalLinkedHashMap String, dynamic Error in Flutter When Fetching JSON Data

Learn how to fix the error: `type '_InternalLinkedHashMap String, dynamic ' is not a subtype of type 'FutureOr List dynamic '` in your Flutter application when handling JSON data.
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This video is based on the question https://stackoverflow.com/q/64090984/ asked by the user 'Hagga Nagga' ( https://stackoverflow.com/u/12658617/ ) and on the answer https://stackoverflow.com/a/64091038/ provided by the user 'Claudio Castro' ( https://stackoverflow.com/u/693319/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding the Problem

When developing applications in Flutter that make HTTP requests to fetch data, it’s common to use JSON format. However, issues may arise during the decoding process, particularly with data type mismatches. One common error encountered is:

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This error typically indicates that the data being returned from the server is in a format that doesn’t match what the Flutter application is expecting. Let’s dive deeper into this problem and how to effectively solve it.

What Causes the Error?

The error occurs when you attempt to decode the JSON response and return it as a List, but the actual response is a Map. Here’s a breakdown of the issue based on your provided example:

Successful Response Format: If your server returns {"Number":5}, it returns a JSON object that corresponds to a Map in Flutter.

Error Triggering Format: If you try to decode a single JSON object but declare your function to return a List, Flutter raises a type error.

Additionally, another error formatting concern is shown when improper data structure is received, such as string{"Number":5}, which results in a FormatException.

The Solution

To resolve the above errors, we need to adjust your method to correctly handle the data type being returned from your server.

Step-by-Step Fix

Change the Return Type:
You need to alter the return type of your senddata function to reflect that you're receiving a Map, not a List.

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

Full Code Implementation:
Below is the corrected version of the initState and senddata method in your Flutter code:

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

Further Processing:
After fetching the data, you can access the elements in the returned Map using their respective keys:

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

Conclusion

By changing the return type of your senddata function to a Map<String, dynamic>, you can correctly handle the JSON response without encountering type errors. Remember, understanding the data structure you expect to receive from the server is key to avoiding these issues.

If you continue to experience problems, double-check the response structure from your server to ensure it aligns with your data handling logic.

Thank you for reading, and happy coding!