The jsonexample sample has reached the end of it's useful life. It was great code when it has written, but the world has changed. Dart 3 introduced Patterns and records. There is also various JSON parsing tools like json_serializable.

For further information, please see:

• The Dive into Dart's patterns and records codelab for a deep dive into Dart 3's patterns and records with an application to parsing JSON
• The Flutter.dev documentation on JSON and serialization