Question

I have three Groovy classes: M, A, & B. B exists as a mixin of M and an extension of the abstract class A:

class M {
    def foo = 11
    def bar = 12
}

abstract class A {
    abstract foo
}

@Mixin(M)
class B extends A {
}

def b = new B()
print "${b.foo}\n"
print "${b.bar}\n"

Attempting to run this causes Groovy to complain with:
Can't have an abstract method in a non-abstract class. The class 'B' must be declared abstract or the method 'void setFoo(java.lang.Object)' must be implemented.
However, the method is implemented by the mixin M.

Furthermore, if I change B to be:

@Mixin(M)
class B extends A {
    def foo = 13
}

Then I get the printout:

11

12

And not:

13

12

Which is what I expect and seems to prove that M provides an acceptable implementation of the abstract methods of A.

So, why isn’t groovy happy with using the mixin M to satisfy the abstract class A, what am I doing wrong?

Answer 1

You’re doing nothing wrong. It’s the problem of the compiler performing this check before actually applying the AST transformations. You better post this on their issue tracker at http://jira.codehaus.org/secure/BrowseProject.jspa?id=10242.

Since @Mixin transformation was actually created by the author of Groovy++ AFAIK, and since Groovy++ has much more extended support for categories, mixins and traits you could expect this to be a valid code there. You should try it.