public class Pair<K,V>{
    public K key;
    public V value;
    public Pair(K key,V value){
        this.key=key;
        this.value=value;
    }
    public static <K,V> Pair<V,K> swap(Pair<K,V> pair){
        return new Pair<>(pair.value, pair.key);
    }
    @Override
    public String toString() {
        return "(" + key + ", " + value + ")";
    }
    public static void main(String[] args) {
        Pair<String, Integer> p1 = new Pair<>("age", 20);
        System.out.println("before" + p1);
        Pair<Integer, String> p2 = Pair.swap(p1);
        System.out.println("now" + p2);
    }
}