code: public V put(K key, V value)
    {
      V oldValue = compareAndPut(null, key, value, false);
  
      return oldValue;
    }
docs: Puts a new item in the cache.  If the cache is full, remove the
  LRU item.
  
  @param key key to store data
  @param value value to be stored
  
  @return old value stored under the key