Salesforce Apex Test Class

Salesforce

How to create an user with unique username in test class in Salesforce?

Sample Code: public static User createTestempUser(Id roleId, Id profID, String fName, String lName) {          String orgId = UserInfo.getOrganizationId();       String dateString =        String.valueof(Datetime.now()).replace(' ','').replace(':','').replace('-','');       Integer randomInt = Integer.valueOf(math.rint(math.random()*1000000));       String uniqueName = orgId + dateString + randomInt;       User tempUser = new User(  FirstName = fName,                               LastName = lName,                               email = uniqueName + '@test' + orgId + '.org',                               Username = uniqueName + '@test' + orgId + '.org',                               EmailEncodingKey = 'ISO-8859-1',                               Alias = uniqueName.substring(18, 23),                               TimeZoneSidKey = 'America/Los_Angeles',                               LocaleSidKey = 'en_US',                               LanguageLocaleKey = 'en_US',                               ProfileId = profId,                               UserRoleId = roleId );       insert tempUser;       return tempUser;          }