Why is there no parameter contra-variance for overriding?

Question

C++ and Java support return-type covariance when overriding methods.

Neither, however, support contra-variance in parameter types - instead, it translates to overl

Answer 1

Thanks to Donroby for his answer above - I'm just extending on it.

interface Alpha
interface Beta
interface Gamma extends Alpha, Beta
class A {
    public void f(Alpha a)
    public void f(Beta b)
}
class B extends A {
    public void f(Object o) {
        super.f(o); // What happens when o implements Gamma?
    }
}

You're falling on a problem akin to the reason multiple implementation inheritance is discouraged. (If you try to invoke A.f(g) directly, you'll get a compile error.)