As @splix mentioned in the comments, doing this using just java.net is… suboptimal. I've never yet encountered a situation where HttpClient wasn't a better option. Event better is his suggestion of twitter4j; unless you're trying to create an alternative, it's almost always better to use an API wrapper like that vs. handling the raw HTTP interactions yourself.
UPDATE:
@Denzil it's odd that you're getting this same error even with twitter4j (I can't test the code until I get some free time to grab the lib, etc.) so I begin to suspect a problem on Twitter's end. If you have Python installed, try the following:
>>> import urllib
>>> urllib.urlopen("http://api.twitter.com/1/users/show/denzil_correa.json").read()
This worked for me.
UPDATE 2:
This definitely sounds like Twitter is intentionally refusing your requests. Possible reasons could include: your IP is on their blacklist for some reason, proxy voodoo, or things I haven't thought of. To elaborate on the proxy voodoo: I don't know what exactly it's doing to your requests, but it's possible it's adding a header or something that the Twitter API doesn't like. I'd recommend contacting Twitter support (if there is such a thing for API problems) or posting to the mailing list.
BTW, here's a thread from the mailing list that mentions ways to see if you're blacklisted.