-java.lang.ClassCastException: com.google.gson.internal.LinkedTreeMap cannot be cast to

[Anuragcdas-ai]

Gson version

<'com.google.code.gson:gson:2.13.2' -->

Java / Android version

Used tools

• Maven; version:
• Gradle; version:
• ProGuard (attach the configuration file please); version:
• ...

Description

<!-java.lang.ClassCastException: com.google.gson.internal.LinkedTreeMap cannot be cast to- -->

Expected behavior

Actual behavior

Reproduction steps

1. ...
2. ...

Exception stack trace

[eamonnmcmanus]

Hi, I think we need a bit more information. Do you have a stack trace? What does the code in your app that is nearest to the top of the stack trace look like?

[vermautg]

Hi, if i may, having a similar problem (windows 11, java 17):
using: gson.fromJson(jsonStr, TreeMap.class);
The resulting object is indeed a java.util.TreeMap, but deeper layers in this object still contain LinkedTreeMap and since com.google.gson.internal is not exported in gson's module-info this is giving a problem.

In com.google.gson.internal.bind.ObjectTypeAdapter.tryBeginNesting LinkedTreeMap is still hard coded in stead of using the given type (in my case TreeMap)

Regards.

[Marcono1234]

@vermautg, when you do fromJson(jsonStr, TreeMap.class) you are using the raw class TreeMap. You should specify type arguments so that Gson knowns as which type it should deserialize the nested data, for example fromJson(jsonStr, new TypeToken<TreeMap<String, MyType>>() {}).
Otherwise Gson cannot know which types you expect and for the nested values it deserializes JSON objects ({ ... }) as Map without making any guarantees about the specific map implementation (currently that just happens to be the internal LinkedTreeMap).

Alternatively in case your data has a complex structure, it might be better to model it as custom class and then tell Gson to deserialize as that class instead of as TreeMap.

[vermautg]

@marcono, it got a bit more complex since the newer java versions than what I was used to. I understand that, for this to work, a TypeAdapter has to be created for TreeMap and it has to be registered. I will try that later. It would have been easier if LinkedTreeMap had been moved to an exported package.
For now, I did a cast for every LinkedTreeMap to AbstractMap.
Thank you for your help.
Regards.