HTTPS provides a number of security benefits.  These include at least
(a) encryption and (b) verification of authenticity.  The latter avoids
at least some of the need for checking MD5 checksums and the like; that
is, if someone were to hijack a router between Sun and you, you could
tell that it's not Sun that is serving the pages at the other end.  You
would see a security warning in your browser, because either the web
page you requested (java.sun.com) is not the name on the server
certificate, or else the certificate will not be signed by a trusted CA.

The encryption/decryption process would also ensure that data corruption
during transfer would probably result in a failure to decrypt content,
rather than a corrupted file on disk.  So you'd find out sooner if there
were a problem with the download, and the user agent would probably
attempt to re-request the content and clear things up.

Hope that clears things up.