How to use generic class with specific objects in a static context?

Question

I'm gonna try to explain at my best.

I use Play Framework 2, and I will do a lot of CRUD actions. Some of them will be identitcal, so I'd like to KISS and DRY so at first I was thinking about an abstract class containing the list, details, create, update and delete methods, with generic object, and extend this class by specifying which object to use (Model & Form) :

public abstract class CrudController extends Controller {
    protected static Model.Finder<Long, Model> finder = null;
    protected static Form<Model> form = null;

    public static Result list() {
        // some code here
    }

    public static Result details(Long id) {
        // some code here
    }

    public static Result create() {
        // some code here
    }

    public static Result update(Long id) {
        // some code here
    }

    public static Result delete(Long id) {
        // some code here
    }
}

And a class that will use CRUD :

public class Cities extends CrudController {
    protected static Model.Finder<Long, City> finder = City.find;
    protected static Form<City> form = form(City.class);

    // I can override a method in order to change it's behavior :
    public static Result list() {
        // some different code here, like adding some where condition
    }
}

This would work if I wasn't in a static context.

But since it's the case, how can I do ?

Answer 1

This can be achieved using delegation: define a regular Java class containing the CRUD actions logic:

public class Crud<T extends Model> {

    private final Model.Finder<Long, T> find;
    private final Form<T> form;

    public Crud(Model.Finder<Long, T> find, Form<T> form) {
        this.find = find;
        this.form = form;
    }

    public Result list() {
        return ok(Json.toJson(find.all()));
    }

    public Result create() {
        Form<T> createForm = form.bindFromRequest();
        if (createForm.hasErrors()) {
            return badRequest();
        } else {
            createForm.get().save();
            return ok();
        }
    }

    public Result read(Long id) {
        T t = find.byId(id);
        if (t == null) {
            return notFound();
        }
        return ok(Json.toJson(t));
    }

    // … same for update and delete
}

Then you can define a Play controller having a static field containing an instance of Crud<City>:

public class Cities extends Controller {
    public final static Crud<City> crud = new Crud<City>(City.find, form(City.class));
}

And you’re almost done: you just need to define the routes for the Crud actions:

GET     /                     controllers.Cities.crud.list()
POST    /                     controllers.Cities.crud.create()
GET     /:id                  controllers.Cities.crud.read(id: Long)

Note: this example produces JSON responses for brevety but it’s possible to render HTML templates. However, since Play 2 templates are statically typed you’ll need to pass all of them as parameters of the Crud class.