Thank you very much for your help.
I have tried with your data and the signature comes out different from yours. Strange that it was ok with the python code on Bitstamp website. Maybe I'm malforming the message?
So here's the code I'm using:
key = "apiKeyDEMO";
secret="apiSecretDEMO";
nonce = 1385390835;
String message = nonce + 12345 + key;
Mac sha256_HMAC = Mac.getInstance("HmacSHA256");
SecretKeySpec secretKey = new SecretKeySpec(this.secret.getBytes(), "HmacSHA256");
sha256_HMAC.init(secretKey);
byte[] hash = sha256_HMAC.doFinal(message.getBytes());
String signature = Hex.encodeHexString(hash).toUpperCase();
my signature comes out to be
CC78BD37757238B803924846D32742477B3B091C48C21AB05949A7515C3EB7A1