Is it possible to write a java program that connects to a phpBB or another type of message board, logs in with your info, and can post messages or make new topics? I have used google before but i couldnt find any similar apps. What classes would I have to use to get this done?

Sure. You can create a program that mimmicks the actions a human would take.
You'll need to do quite a lot of stuff with HttpURLConnection, URL, and classes like that though.

Beware though that site admins won't like it and may well ban you permanently if you do it.
It's a technique used mainly by spammers...

So I dont have to read the RFC to do it?

well that depends on several things...

phpBB stores its information in a sql database, now you can write a program to query those database, but most webhosts configure them so that only localhost has access to them, so unless you had physical access to the machine AND had the database passwords you couldn't do it.... at least not in this case

could you create a java program that scoures phpBB over http, sure... but the amount of time and bandwidth you would use on a server would be far above normal usage and i'm pretty sure some admin's might block you. Parsing the data this way would also be insane and just outright lame,

in conclusion unless you are administering your own site (and have db access), dont even bother