Скачать или смотреть How to Fix Gson Child Object Attributes Returning Null in Kotlin Data Classes

Discover how to resolve the issue of child object attributes returning null in Gson when mapping JSON to Kotlin data classes. Understand the necessary structure adjustments for seamless data mapping.
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This video is based on the question https://stackoverflow.com/q/63792594/ asked by the user 'UIB' ( https://stackoverflow.com/u/8248268/ ) and on the answer https://stackoverflow.com/a/63793258/ provided by the user 'Mafor' ( https://stackoverflow.com/u/9090751/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding the Gson Mapping Challenge in Kotlin

Working with JSON and data classes in Kotlin can be straightforward, yet sometimes, it presents challenges. A common issue arises when trying to map nested JSON objects to Kotlin classes using Gson, particularly when attributes return null.

This guide addresses the problem illustrated by a user's attempt to parse a specific JSON structure. The provided code attempts to map JSON data representing time zones, but the attributes of child objects end up returning null, which can be quite frustrating.

The JSON Structure

The JSON in question looks like this:

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

It contains a list of TimeZones, each of which includes a MapZone object mapping different attributes.

The Original Kotlin Data Classes

The Kotlin data classes initially set up to handle this JSON structure are as follows:

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

When this code is executed, the values of the MapZone attributes consistently return null. So, what went wrong?

Solution: Adjusting the Structure

The key to resolving this issue lies in the structure of the data classes. You are missing an additional layer that maps the nested JSON structure correctly. Here’s how you can adjust your data classes:

Updated Kotlin Data Classes

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

Breakdown of the Changes

Added a New Class (TimeZone): The TimeZone class encapsulates the MapZone object. This additional layer directly corresponds to the structure of the JSON you are trying to parse.

Correct Mapping: By separating MapZone into its own class within TimeZone, Gson can now correctly navigate the JSON hierarchy. The TimeZones class now holds a list of TimeZone objects, each of which contains a MapZone object.

Conclusion

By adjusting your data class structure, you can effectively resolve the issue of child object attributes returning null when using Gson for JSON mapping in Kotlin. Always ensure that your Kotlin classes accurately reflect the JSON hierarchy to facilitate smooth data parsing.

Now, with the revised structure in place, you should be able to run your code successfully without encountering null values for the attributes in MapZone. Happy coding!