public class Pair<K, V> {
    private K key;
    private V value;

    public Pair(K key, V value) {
        this.key = key;
        this.value = value;
    }

    public K getKey() { return key; }
    public V getValue() { return value; }

    // 交换后 key/value 类型可能变化，所以返回新的 Pair<V, K>
    public Pair<V, K> swap() {
        return new Pair<>(this.value, this.key);
    }

    public static void main(String[] args) {
        Pair<String, Integer> p = new Pair<>("Age", 25);
        Pair<Integer, String> swapped = p.swap();
        System.out.println(swapped.getKey() + "=" + swapped.getValue()); // 25=Age
    }
}