Question

Full working now and code is complete Thanks for the help.

Answer 1

You can restrict user to enter another value by this: (This program is for if You are taking values from user). This will asks for number until you enter number within 0 to 9.

You can make your code according to this. (This is just for your reference, How can you restrict user to enter wrong thing)

Scanner scan=new Scanner(System.in);
int i=-1;
i=scan.nextInt();
while(i<=0 && i>=9){
    i=scan.nextInt();
}

EDIT

As per your comment, In that case you need to change this as:

String s="";
while(!s.matches("^[0-9A-F]+$")){
    s=scan.nextLine();
}