Blob b = Crypto.GenerateAESKey(128); String h = EncodingUtil.ConvertTohex(b); String guid = h.SubString(0,8)+ h.SubString(8,12) + h.SubString(12,15); system.debug(guid);
Other Example
String hashString = '1000' + String.valueOf(Datetime.now().formatGMT('yyyy-MM-dd HH:mm:ss.SSS'));
Blob hash = Crypto.generateDigest('MD5', Blob.valueOf(hashString));
String hexDigest = EncodingUtil.convertToHex(hash);
system.debug('##########' + hexDigest );