Marc: We can write our own getKey(URL) which does the same thing as toString() with a StringBuilder instead of a StringBuffer. As far as I can tell, however, customizing the handler requires either the use of one of the more involved URL constructors, or a JVM-global URLStreamHandlerFactory change — which I’m not sure a third party library should be doing. I’m also unsure how much benefit the hashCode caching does (once you avoid doing DNS lookups!).

The most efficient way would be probably to provide our own URLStreamHandler to allow a custom implementation of hashCode() which result value is cached by URL. Indeed the result of URL.toString() is not cached (and uses a StringBuffer rather than a StringBuilder).