Java Security

Hi everyone:

I'm quite new to Java and cryptography in general and have a theoretical question. Is the following scenario possible and how would it be implemented:
Two users with two passwords (say, a regular user and a superuser) encrypt, decrypt, read from and write to the same file. The secret key for encryption and decryption should be based on their passwords (generated from their passwords), i.e. not stored anywhere on the system.

I've been racking my brains but can't think of a way. Am I missing an obvious solution?

Can it be done?

Thanks,
Michael