Deserialize Map<String, List<?>> using Gson

Question

I serialize data in server:

Gson gson = new Gson();
Map<String, List<?>> resultMap = BackendUpdateManager.getInstance()
    .getUpdates(timeHolder, shopIdInt, buyerIdInt);
gson.toJson(resultMap);

and deserialize:

Gson gson = new Gson();
Map<String, List<?>> resultMap =  gson.fromJson(json,
    new TypeToken<Map<String, List<?>>>() {
    }.getType());

However, when I try use items from the Map:

List<ProductCategory> productCategoryList = (List<ProductCategory>)updateMap.get(key);

for(ProductCategory productCategory : productCategoryList) {

}

I get error:

Caused by: java.lang.ClassCastException: com.google.gson.internal.LinkedTreeMap cannot be cast to com.example.model.entity.ProductCategory

How can I fix this error or otherwise create a Map with List<different classes>?

I've tried creating classes with getters and setters, that contains Lists of different classes instead of Map<String, List<?>> and use it for serialization and deserialization. But I'm looking for a better way.

Answer 1

How is Gson supposed to know that a particular Json string is a ProductCategory? For instance, if the definition of ProductCategory is this:

package com.stackoverflow.vitvetal;  

public class ProductCategory {
    String name;
    String type;
}

And the Json is this:

{
    "name":"bananas",
    "type":"go-go"
}

Where is the link that tells Gson to create an instance of a com.stackoverflow.vitvetal.ProductCategory?

This link doesn't exist, because you didn't tell Gson about it.

So what gson does instead is, it creates a Map<String, String> that looks like

"name" -> "bananas"
"type" -> "go-go"

If you want to do something different, the easiest thing to do - but also the least powerful - is to fully specify the parameterized type when you create your TypeToken; no wildcard <?> allowed.

If you need to do something more powerful, like creating maps with more variety in objects, you need to create a custom deserializer, using a TypeAdapter<T>, that teaches Gson how to handle your particular sort of object.

• Here is a tutorial on how to do write gson TypeAdapters