Ah well then doesn't that answer your question about logging in? Comparing input with predefined values in your program seems perfectly acceptable for your application. I guess what you want to know is how to change from the log in screen to the content/menu screen?

What you could do is have multiple panels on your applet and make them visible in correspondence to who logs in. In that way, when a worker enters his details correctly, the log in screen is made invisible and the worker screen is made visible. Not the most elaborate solution but rather simple don't you think? Perhaps you should disable the panel when you make it invisible... and enable it when made visible.

Hope this makes sense, otherwise give us a shout.