Question

I have a factory for creating model instances. The factory provides this method:

public <Model> Model createFromJson(String json, final Class<Model> model) { 
    Model modelInstance = gson.fromJson(json, model);
    ((IModel)modelInstance).onCreateFromJson();
    return modelInstance;
}

As you can see I cast the instance of the model (previously given as class method parameter) to an interface, so I can call specific methods.
Further the generic return type works nice as well, since the caller must not cast any object. It’s simply enough to call:

MyModel myModel = ModelFactory.getInstance().createFromJson(json, MyModel.class);

But, unfortunately, I can call the factory method with ANY kind of object, like:

Integer myTest = ModelFactory.getInstance().createFromJson(json, Integer.class);

So I would like to define the factory method parameter to be of type IModel. But I can’t figure out how, since I’m passing a class, and not an instance of a Model (IModel).

So, actually I am searching for something like this:

public <Model<? extends IModel>> Model 
    createFromJson(String json, final Class<Model<? extends IModel>> model) 

Can anyone give me advice?

Answer 1

Using Model as name for your type parameter is confusing since you also seem to have a class named Model. Instead use M or some other letter.

To specify which class a type must extend you need to use this:

M extends IModel

Or using it for your example code:

public <M extends IModel> M createFromJson(String json, final Class<M> model) {
  // Method code
}