Programming :: Write Programs That Use Electronic Signatures In Java?

Do you know how to write programs that use electronic signatures in JAVA (although I am not pretentious about the language in any way).

When I say e-signatures I mean those USB smart cards(security tokens) that are capable of doing encryption by themselves and store securely the private key. I am particularly interested in how authentication works with a digital signature, how to sign data, and most importantly how to check that a digital certificate is valid. What do I need to store in the Database for authentication for example -- is it the serial or the Hash?