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; }

    // swap：交换后原key变value，原value变key，返回新Pair
    public Pair<V, K> swap() {
        return new Pair<>(value, key);
    }
}


import java.util.concurrent.ConcurrentHashMap;

public class Cache<K, V> {
    private final ConcurrentHashMap<K, V> cache = new ConcurrentHashMap<>();

    public void put(K key, V value) { cache.put(key, value); }
    public V get(K key) { return cache.get(key); }
    public V remove(K key) { return cache.remove(key); }
    public void clear() { cache.clear(); }
    public int size() { return cache.size(); }
    public boolean containsKey(K key) { return cache.containsKey(key); }
}