com.google.gson.internal.LinkedTreeMap cannot be cast to my class [duplicate]

Answer 1

I also had problems with GSON complaining about casting LinkedTreeMaps.

The answer provided by Alexis and the comment by Aljoscha explains why the error occurs; "Generics on a type are typically erased at runtime." My issue was that my code worked when I ran it normally, but using ProGuard caused code to be stripped that was vital to casting.

You can follow Alexis's answer and more clearly define the cast and that should fix the problems. You can also add the ProGuard rules given by Google (simply doing this cleared the issue up for me).

##---------------Begin: proguard configuration for Gson  ----------
# Gson uses generic type information stored in a class file when working with fields. Proguard
# removes such information by default, so configure it to keep all of it.
-keepattributes Signature

# For using GSON @Expose annotation
-keepattributes *Annotation*

# Gson specific classes
-keep class sun.misc.Unsafe { *; }
#-keep class com.google.gson.stream.** { *; }

# Application classes that will be serialized/deserialized over Gson
-keep class com.google.gson.examples.android.model.** { *; }

##---------------End: proguard configuration for Gson  ----------

Moral of the Story: Always check to see what ProGuard rules you need.

Answer 2

I had the same problem. I noticed it only occures when you have List as argument.

My solution is to wrap the list in another Object:

class YourObjectList {

    private List<YourObject> items;

    // constructor, getter and setter
}

With that single object, i had no more problems with class cast exception.

Answer 3

Similar to Alexis C's answers. but in Kotlin.
Just pass the class type into function and clarify what generic type is.
Here is simplified example.

inline fun <reified T> parseArray(json: String, typeToken: Type): T {
    val gson = GsonBuilder().create()
    return gson.fromJson<T>(json, typeToken)
}

Here is example call

fun test() {
    val json: String = "......."
    val type = object : TypeToken<List<MyObject>>() {}.type
    val result: List<MyObject> = parseArray<List<MyObject>>(json = json, typeToken = type)
    println(result)
}

Answer 4

To add to the answers already mentioned here, if you have a generic class that handles, say, HTTP calls, it maybe useful to pass Class<T> as part of the constructor.

To give a little more detail, this happens because Java cannot infer the Class<T> during runtime with just T. It needs the actual solid class to make the determination.

So, if you have something like this, like I do:

class HttpEndpoint<T> implements IEndpoint<T>

you can allow the inheriting code to also send the class<T>, since at that point is it clear what T is.

public HttpEndpoint(String baseurl, String route, Class<T> cls) {
    this.baseurl = baseurl;
    this.route = route;
    this.cls = cls;
}

inheriting class:

public class Players extends HttpEndpoint<Player> {

    public Players() {
        super("http://127.0.0.1:8080", "/players",  Player.class);
    }
}

while not entirely a clean solution, it does keep the code packaged up and you don't have to Class<T> between methods.