Java Question

I'm going to try to make a nice little password checker, just for fun. I need to know how to make java actually interact with websites, or pop ups, such as steam login , so it could match up the password/usernames i give it and return either it works or it doesnt.

Can anyone give me some example code of how i can get java, or anyother program to do this? THNX!

 

id like to know this too.