Gson deserialize current object with extended class

Question

I'm new to java (Hum... No... I learned Java at school 10 years ago but never really used it since today).

I have an object class which corresponds to my json and was generated with the website http://www.jsonschema2pojo.org/ (simplified here) :

public class ServerDatasObject {
  private Integer error;
  private Boolean isOffline;
  public Integer getError() {
      return error;
  }
  public Boolean getIsOffline() {
      return isOffline;
  }
}

And another class used to access all object data (simplified too) :

public class ServerDatasHandler extends ServerDatasObject {
  public ServerDatasHandler(String json) {
    Gson gson = new Gson();
    // how to populate current object using : gson.fromJson(json, ServerDatasObject.class);
  }
}

The question is in the code: how to populate current object? I searched and found something about InstanceCreator :

final Foo existing;
InstanceCreator<Foo> creator = new InstanceCreator<Foo>() {
  public Foo createInstance(Type type) { return existing; }
}

Gson gson = new GsonBuilder().registerTypeAdapter(Foo.class, creator).create();
Foo value = gson.fromJson(jsonString, Foo.class);
// value should be same as existing

but I don't understand how to use it and if it is what I need.

The final goal is to

{
  ServerDatasHandler serverDatasHandler = new ServerDatasHandler (json);
  do something with serverDatasHandler.getError()
}

Thanks for your help

Answer 1

You can create a separate static method which creates your handler from json:

public static ServerDatasHandler fromJsonConfig(String json) {
  Gson gson = new Gson();
  ServerDatasHandler handler = gson.fromJson(json, ServerDatasHandler.class);
  return handler;
}

Of course you can also move this method to a factory.

I never used InstanceCreator inside constructor because parsing JSON is almost never a task for a constructor. Preferably it should be hidden inside framework or your own factories. Though, your example with InstanceCreator should also work if you will return ServerDatasHandler.this from the createInstance method.