ExpiringMap.java

class ExpiringMap<K, V> extends HashMap<K, Entry<V>> {
  
  class Entry<V> {
    V value;
    long expiry;
  }
  
  long decayTime = 60_000;
  
  V get(K key) {
    Entry<V> e = super.get(key);
    if (System.currentTimeMillis() >= expiry) {
      remove(key);
      return null;
    }
    return e.value;
  }
  
  void put(K key, V value) {
    super.put(key, new Entry<>(value, System.currentTimeMillis() + decayTime));
  }
  
  void refresh(K key) {
    Entry<V> e = super.get(key);
    e.expiry = System.currentTimeMillis() + decayTime;
  }
}