Forcing a user to only input numbers

Question

Hey guys! I am writing the code for a game of NIM as an assignment. However, i am having trouble in forcing the user to only input certain things, in this case they are only allowed to enter a number that is less than the total ammount of stones in the pile, but greater than 0 (i.e 1 or more). Now this works fine, and if they enter a letter the NumberFormatException catch handles it fine, but what i am having trouble with is when they enter nothing. They just hit return without actually entering a number. This causes they game to accept that as their turn and go to the next players turn. How would i go about forcing the user to input something. Also if you can give me any clues as to how to make this wny more efficent, it would be very much appreciated! Thank you for your time!
               do {
                    if ((userStoneChoice > pileTotal) || (userStoneChoice < 1)) {
                         // if the users choice is greater than the ammount of stones in the pile, force them to enter a new number.
                         System.out
                                   .println("Sorry, but there are "+ pileTotal+ " stones in pile "+ pileNumber+
".Please ensure you choose a number with that range.");
                         userStoneChoice = Integer.parseInt(in.readLine());
                    } else {
               } while ((userStoneChoice > pileTotal) || (userStoneChoice < 1));

Answer 1

    private void doUserMove(){
        try{
            userStoneChoice = getUserStoneChoice();
        }catch(IOException ex){
            ex.printStackTrace();
    private int getUserStoneChoice() throws IOException{
        int choice;
        do{
            choice = 0;
            String str = in.readLine();
            try{
                choice = Integer.parseInt(str);
            }catch( NumberFormatException ex ){}
            if ( choice > pileTotal || choice < 1 ){
                // if the user's choice is greater than the ammount of stones in the pile,
                // force them to enter a new number.
                choice = 0;
                System.out
                        .println("Sorry, but there are "+ pileTotal+ " stones in pile "+ pileNumber+
                        ".Please ensure you choose a number with that range.");
        } while( choice == 0) ;
        return choice;
    }