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•Created by austrianpainter on 1/17/2025 in #java-help

Scanner Bug (Tried Everything)

I am guessing that's what is going on anyway

9 replies

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•Created by austrianpainter on 1/17/2025 in #java-help

Scanner Bug (Tried Everything)

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.

9 replies

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•Created by Thorinwasher on 1/12/2025 in #java-help

How to access generic info on class

it is, but you have to do it during runtime

19 replies

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•Created by Thorinwasher on 1/12/2025 in #java-help

How to access generic info on class

welcome to the world of type erasure. If you wanna submit a complaint get in line.

19 replies

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