Java Community | Help. Code. Learn.•6d ago

Scanner Bug (Tried Everything)

So I ask users through input through these two methods:

public static String whileLoopInputString (Predicate<String> checkCase, String inputMessage, String errorMessage, Scanner scanner) {
        String input = "";

        while (!checkCase.test(input)) {
            System.out.println(inputMessage);
            input = scanner.nextLine();
            if (!checkCase.test(input)) {
                System.out.println(errorMessage);
            }
        }

        return input;
    }


    public static byte whileLoopInputByte (Predicate<Byte> checkCase, String inputMessage, String errorMessage, Scanner scanner) {
        byte input = -1;

        while (!checkCase.test(input)) {
            try {
                System.out.println(inputMessage);
                input = scanner.nextByte();
                scanner.nextLine();
                if (!checkCase.test(input)) {
                    System.out.println(errorMessage);
                }
            } catch (InputMismatchException e) {
                System.out.println(errorMessage);
                scanner.nextLine();
            }
        }

        return input;
    }

public static String whileLoopInputString (Predicate<String> checkCase, String inputMessage, String errorMessage, Scanner scanner) {
        String input = "";

        while (!checkCase.test(input)) {
            System.out.println(inputMessage);
            input = scanner.nextLine();
            if (!checkCase.test(input)) {
                System.out.println(errorMessage);
            }
        }

        return input;
    }


    public static byte whileLoopInputByte (Predicate<Byte> checkCase, String inputMessage, String errorMessage, Scanner scanner) {
        byte input = -1;

        while (!checkCase.test(input)) {
            try {
                System.out.println(inputMessage);
                input = scanner.nextByte();
                scanner.nextLine();
                if (!checkCase.test(input)) {
                    System.out.println(errorMessage);
                }
            } catch (InputMismatchException e) {
                System.out.println(errorMessage);
                scanner.nextLine();
            }
        }

        return input;
    }

The error occurs in the switch statement below, I think it happens when I ask for a byte, then a string, or vice versa (I forget):

            switch (bytInput) {
                case 1:
                    System.out.println("Rules");
                    break;
                case 2:
                    strCreatedPlayerName = Logic.whileLoopInputString(
                            conditions);
                    break;
                
                case 3:
                    strUsernameToLoadIn = Logic.whileLoopInputString(conditions);
        
                case 4:
                    method();
                    break;
                case 5:
                    bolIsInProgram = false;
                    break;

            }

            switch (bytInput) {
                case 1:
                    System.out.println("Rules");
                    break;
                case 2:
                    strCreatedPlayerName = Logic.whileLoopInputString(
                            conditions);
                    break;
                
                case 3:
                    strUsernameToLoadIn = Logic.whileLoopInputString(conditions);
        
                case 4:
                    method();
                    break;
                case 5:
                    bolIsInProgram = false;
                    break;

            }

bytInput also used a method switch is in a loop, if you need code more pls ask

6 Replies

JavaBot•6d ago

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dan1st•6d ago

Can you elaborate on what happens?

nikcho-kouhai•6d ago

It's probably because of the scanner.next stuff. Generally speaking that can cause issues because the non-parsed data stays in the input stream and it has to be discarded via a call to nextLine. You can confirm this by running this piece of code:

        Scanner scan = new Scanner(System.in);
        try {
            scan.nextByte();
        }catch (InputMismatchException e){

        }
        System.out.println(scan.nextLine());

        Scanner scan = new Scanner(System.in);
        try {
            scan.nextByte();
        }catch (InputMismatchException e){

        }
        System.out.println(scan.nextLine());

I believe the best way to avoid this is to just use nextLine() on your scanner and then parsing to the appropriate type as that advances the scanner past the line no matter what.

        Scanner scan = new Scanner(System.in);
        try{
            String in = scan.nextLine();
            byte b = Byte.parseByte(in);
        }catch (NumberFormatException e){

        }
        //this hangs, because there is nothing in the input stream.
        System.out.println(scan.nextLine());

        Scanner scan = new Scanner(System.in);
        try{
            String in = scan.nextLine();
            byte b = Byte.parseByte(in);
        }catch (NumberFormatException e){

        }
        //this hangs, because there is nothing in the input stream.
        System.out.println(scan.nextLine());

For this you can make a utility function that looks something like this and should work for all number types:

    public static <T> T getInput(Function<String,T> c,String inputMessage,String errorMessage,Scanner scanner){
        while (true){
            try{
                System.out.println(inputMessage);
                return c.apply(scanner.nextLine());
            }catch (NumberFormatException e){
                System.out.println(errorMessage);
            }
        }
    }
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        byte i = getInput(Byte::parseByte,"Some input message","Some error message",scan);
    }

    public static <T> T getInput(Function<String,T> c,String inputMessage,String errorMessage,Scanner scanner){
        while (true){
            try{
                System.out.println(inputMessage);
                return c.apply(scanner.nextLine());
            }catch (NumberFormatException e){
                System.out.println(errorMessage);
            }
        }
    }
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        byte i = getInput(Byte::parseByte,"Some input message","Some error message",scan);
    }

Of course if you don't want to do this you can just do it like you have so far, except replace the calls to nextByte or next whatever with nextLine and parse after that. I am guessing that's what is going on anyway

austrianpainterOP•6d ago

Thank you so much! Sorry for the little detail it was 3am for me when I asked. I might do that (which my teacher kind of dissaproves of) or just make an entirely new Scanner object in the method

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Scanner Bug (Tried Everything)

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