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: []

more weird... why does T.list() work, but T.count() doesn't? If it's failing in a unit test, there was something about .count() not being mocked well. — billjamesdev
– billjamesdev, Commented Mar 5, 2015 at 0:09
This is failing when I actually run the app... Haven't tried writing a test for it yet. — Thaldin
– Thaldin, Commented Mar 5, 2015 at 0:12

Community · Accepted Answer · 2017-05-23 12:26:13Z

3

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"

edited May 23, 2017 at 12:26

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answered Mar 5, 2015 at 10:17

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2 Comments

Thaldin Over a year ago

This indeed was my issue. Now on to new ones that I am running into!

Thaldin Over a year ago

So as a follow up to this. I've gotten the create method to work as well as the index now using this method, but I can't seem to get anything working with a method that accepts an "instance" def show(Team instance) { respond instance } I have tried replacing Team with Class<T>, T, Object... and I am just not getting it I guess with Groovy. When I debug, it never seems to even hit the method at all.

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