Estoy lidiando con una situación en la que tengo clases polimórficas que necesito deserializar.

Class Pen{
  String name;
  List<Animal> animals;
}

//Animal can be an interface or parent class: I am flexible

Class Animal{
  AnimalType type;//enum
  int legs;
}

enum AnimalType{
  dog,cat,pig,chicken;
}

Class AnimalDog extends Animal{
  //…
}

Class AnimalCat extends Animal{
  //…
}


Class AnimalPig extends Animal{
  //…
}

entonces creo mi instancia de Gson con

public static Gson instanceUpperCamelCaseWithTypeAdapterFactory() {
    if (null == sGsonUpperCamelCase) {
        final RuntimeTypeAdapterFactory<Animal> typeFactory = RuntimeTypeAdapterFactory
                .of(Animal.class, “type")
                .registerSubtype(AnimalDog.class, “dog”)
                .registerSubtype(AnimalCat.class, “cat”)
                .registerSubtype(AnimalPig.class, “pig”);

        sGsonUpperCamelCase = new GsonBuilder().setFieldNamingPolicy(FieldNamingPolicy.UPPER_CAMEL_CASE)
                .registerTypeAdapterFactory(typeFactory).create();
    }//the naming policy is because server sends me upper case fields whereas Java fields are lowercase.
    return sGsonUpperCamelCase;
}

Para obtener los animales de un json que contiene una lista de animales, hago

List<Animal> animals = gson.fromJson(json, new TypeToken<List<Animal>>() {}.getType());

Soy completamente un novato para Gson. Completamente. Entonces, sin confundirme demasiado, ¿cómo podría resolver este problema?

Traza de error:

com.google.gson.JsonParseException: cannot deserialize class com.company.appname.data.model.Animal because it does not define a field named type
com.company.appname.utils.RuntimeTypeAdapterFactory$1.read(RuntimeTypeAdapterFactory.java:204)
com.google.gson.TypeAdapter$1.read(TypeAdapter.java:199)
com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.read(TypeAdapterRuntimeTypeWrapper.java:40)
com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.read(CollectionTypeAdapterFactory.java:82)
com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.read(CollectionTypeAdapterFactory.java:61)
com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.read(ReflectiveTypeAdapterFactory.java:117)
com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:217)
com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.read(TypeAdapterRuntimeTypeWrapper.java:40)
com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.read(CollectionTypeAdapterFactory.java:82)
com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.read(CollectionTypeAdapterFactory.java:61)
com.google.gson.Gson.fromJson(Gson.java:861)
com.google.gson.Gson.fromJson(Gson.java:826)
com.google.gson.Gson.fromJson(Gson.java:775)

Ejecuto el json a través de un validador en línea, no hay problema con él. Tiene una serie de artículos. Aquí os muestro dos.

{“Animals”:[{  
           “id":9,
           “type”:”dog”,
           “name”:”maximus”
        },
        {  
           “id":10,
           “type”:”cat”,
           “name”:”meowy”,
           “yarns”:5,
           “nice”:true
        }]}