Why doesn't this generic usage work in groovy?

Question

Learning Groovy and Grails and I am trying to simplify some controllers by making a BaseController.

I define the following:

class BaseController<T> {

    public def index(Integer max) {
        params.max = Math.min(max ?: 10, 100)
        respond T.list(params), model:[instanceCount: T.count()]
    }
}

Then I have the following:

class TeamController extends BaseController<Team> {
    static allowedMethods = [save: "POST", update: "PUT", delete: "DELETE"]

    /*
    def index(Integer max) {
        params.max = Math.min(max ?: 10, 100)
        respond Team.list(params), model:[teamInstanceCount: Team.count()]
    }
    */
}

Every time I try to make a call to this, I get a MethodMissingException on T.count(). Why does Team.count() work, but when I try to use generics T.count() fail?

-- edit -- (Adding exception) No signature of method: static java.lang.Object.count() is applicable for argument types: () values: []

Answer 1

T is a type. So this does not work as you expect it to be. You have to hold a concrete class (see Calling a static method using generic type).

So in your case it's easiest to also pass down the real class. E.g.:

class A<T> {
    private Class<T> clazz
    A(Class<T> clazz) { this.clazz = clazz }
    String getT() { T.getClass().toString() }
    // wrong! String getX() { T.x }
    String getX() { clazz.x }
}

class B {
    static String getX() { return "x marks the place" }
}

class C extends A<B> {
    C() { super(B) }
}

assert new C().x=="x marks the place"