Question

Again I’m facing issues with generics, I thought I now know how to do this. To illustrate I created this simple example.

Given this interface

public interface Converter<A, B> {
    B encode( A a);
}

why does the following error:

void onRequest(ArrayList<String> a,Converter <? extends List<String>,String> conv) {
    conv.encode(a);
}

I expected that, with this notation, conv.encode would accept any param that extends List

any ideas?

Answer 1

Change it to:

void onRequest(ArrayList<String> a,Converter <? super List<String>,String> conv) {
    conv.encode(a);
}

You want to call encode with an ArrayList, therefore ArrayList has to be a subclass of the furst type parameter of Converter. Therefore this first type parameter has to be a super class of ArrayList. By making it be super of List this is automatically guaranteed.