Java - Generate random user name If the user doesn't enter input

Very new to coding and starting off with Java.

I am trying to create a scenario Where a user Is creating a Username and password. I need the program to accept the users input for username and password. If the user does not enter a username or password then I need the program to generate a random username and a random number for password.

I have tried searching on google and youtube however I have had no success so far with either.