This prints null:

System.out.println(new URI("http://a.1a/").getHost());

But this prints a.1a:

System.out.println(new URL("http://a.1a/").getHost());

If all URLs are URIs (but not all URIs are URLs) shouldn't a valid URL that has a host component also have the same host component (instead of null) as a URI?

CodePudding user response：

Look at the Javadoc:

https://docs.oracle.com/javase/8/docs/api/java/net/URI.html:

Returns: The host component of this URI, or null if the host is undefined"

OK, so why is the host part of your particular URI ("http://a.1a/") undefined? Look at the RFC:

https://www.ietf.org/rfc/rfc2396.txt

Hostnames take the form described in Section 3 of [RFC1034] and
Section 2.1 of [RFC1123]: a sequence of domain labels separated by
".", each domain label starting and ending with an alphanumeric
character and possibly also containing "-" characters. The rightmost domain label of a fully qualified domain name will never start > with a digit... To actually be "Uniform" as a resource locator, a URL hostname should be a fully qualified domain name.