Oh god this works

temp.java

public class Temp {
    public static int myf(int i, int j, Function<List<Integer>,Integer> fn) {
        System.out.println("start");
        List<Integer> myi= new ArrayList<>();
        myi.add(i);
        myi.add(j);

        int retval = fn.apply(myi);
        System.out.println(retval);
        System.out.println("end");
        return retval;
    }

    public static void main(String[] args) {
        myf(13, 12, list ->  list.get(0) + list.get(1)) ;
    }
}