public static class CustomTypeAdapterFactory implements TypeAdapterFactory {
    @SuppressWarnings("unchecked")
    @Override
    public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
       if (!UserProfile.class.isAssignableFrom(type.getRawType())) {
         return null; // this class only serializes 'UserProfile' and its subtypes
       }
       final TypeAdapter<JsonElement> elementAdapter = gson.getAdapter(JsonElement.class);
       final TypeAdapter<UserProfile> thisAdapter
                        = gson.getDelegateAdapter(this, TypeToken.get(UserProfile.class));

       return (TypeAdapter<T>) new TypeAdapter<UserProfile>() {
           @Override
           public void write(JsonWriter out, UserProfile value) throws IOException {
             JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
             obj.remove("additionalProperties");
             // serialize additonal properties
             if (value.getAdditionalProperties() != null) {
               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
                 if (entry.getValue() instanceof String)
                   obj.addProperty(entry.getKey(), (String) entry.getValue());
                 else if (entry.getValue() instanceof Number)
                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
                 else if (entry.getValue() instanceof Boolean)
                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
                 else if (entry.getValue() instanceof Character)
                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
                 else {
                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
                 }
               }
             }
             elementAdapter.write(out, obj);
           }

           @Override
           public UserProfile read(JsonReader in) throws IOException {
             JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
             validateJsonObject(jsonObj);
             // store additional fields in the deserialized instance
             UserProfile instance = thisAdapter.fromJsonTree(jsonObj);
             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
               if (!openapiFields.contains(entry.getKey())) {
                 if (entry.getValue().isJsonPrimitive()) { // primitive type
                   if (entry.getValue().getAsJsonPrimitive().isString())
                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
                   else
                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
                 } else { // non-primitive type
                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
                 }
               }
             }
             return instance;
           }

       }.nullSafe();
    }
  }