GJ

class Pair<A,B> {
    A fst;
    B snd;

    Pair(A fst, B snd) {
        this.fst = fst;
        this.snd = snd;
    }

    public String toString() {
        return "(" + fst + ", " + snd + ")";
    }
}

class Swap {
    static <A,B> Pair<B,A> swap(Pair<A,B> p) {
        return new Pair<B,A>(p.snd, p.fst);
    }

    public static void main(String[] args) {
        Pair<String,Integer> p = new Pair<String,Integer>("hello", 42);
        System.out.println("Before: " + p);
        Pair<Integer,String> q = swap(p);
        System.out.println("After:  " + q);
    }
}