Javadoc
Subclasses must implement to have automatic creation of new instances
or alternatively can use put to add new items to the cache.
Implementing this method is prefered to guarantee that there will be only
one instance per key ever created. Calling put() to add items in a multi-
threaded situation will require external synchronization to prevent two
instances for the same key, which defeats the purpose of this cache
(put() is useful when initialization is done during startup and items
are not added during execution or when (temporarily) having possibly two
or more instances of the same key is not of concern).