I'm not exactly how you're writing to the URLStream...
When talking to the server, the 'writing' should be through either GET or POST args (GET are the ones that can be seen in the URL, POST are in the HTTP header).
I'd think Java would throw an exception there
Way I'm parsing this, it's trying to write to a URL. I'm a bit rusty on HTTP's exact format, but I was almost certain it was a one-way street
What I suggest is either adding the username and score to the GET arguements, or the POST arguements.
You could even do a simple cypher to discourage those that may want to post fake high scores.
Seeing that you already have multiple GET arguements there, I doubt I need to explain them.
I'm not sure how to do an HTTP POST with Java, but I'm sure there's a way somewhere