code: @SuppressWarnings("unchecked")
      public synchronized boolean containsKey(Object key) {
          return cache.containsKey(key) || (isDiskCacheEnabled && getFileForKey((KeyT) key).exists());
      }
docs: Checks if a value is present in the cache. If the disk cached is enabled, this will also
  check whether the value has been persisted to disk.
  
  @param key
  the cache key
  @return true if the value is cached in memory or on disk, false otherwise