How to serialize a class with an interface?

Here is a generic solution that works for all cases where only interface is known statically.

1. Create serialiser/deserialiser:

   final class InterfaceAdapter<T> implements JsonSerializer<T>, JsonDeserializer<T> {
       public JsonElement serialize(T object, Type interfaceType, JsonSerializationContext context) {
           final JsonObject wrapper = new JsonObject();
           wrapper.addProperty("type", object.getClass().getName());
           wrapper.add("data", context.serialize(object));
           return wrapper;
       }
   
       public T deserialize(JsonElement elem, Type interfaceType, JsonDeserializationContext context) throws JsonParseException {
           final JsonObject wrapper = (JsonObject) elem;
           final JsonElement typeName = get(wrapper, "type");
           final JsonElement data = get(wrapper, "data");
           final Type actualType = typeForName(typeName); 
           return context.deserialize(data, actualType);
       }
   
       private Type typeForName(final JsonElement typeElem) {
           try {
               return Class.forName(typeElem.getAsString());
           } catch (ClassNotFoundException e) {
               throw new JsonParseException(e);
           }
       }
   
       private JsonElement get(final JsonObject wrapper, String memberName) {
           final JsonElement elem = wrapper.get(memberName);
           if (elem == null) throw new JsonParseException("no '" + memberName + "' member found in what was expected to be an interface wrapper");
           return elem;
       }
   }
   

2. make Gson use it for the interface type of your choice:

   Gson gson = new GsonBuilder().registerTypeAdapter(Animal.class, new InterfaceAdapter<Animal>())
                                .create();