How to save the generated random number on the savings/bank program

im working on an activity right now, it's a savings account and when the user create an account, it will generate a random account number, my question is how do i save that random number that is generate? Random r = new Random(); String accountNumber = String.format("%04d", r.nextInt(1001)); System.out.println("Your Account Number:" + accountNumber); - i use this code, i can't seem to get the account number saved