Java Scanner.hasNextInt() 不等待输入

Question

I am trying to read only valid integers from the console. I got the example below from another Stackoverflow answer and it should work. If I call the method once it works but on the second call it somehow ignores the sc.hasNextInt() statement and doesn't wait for an input here or thinks there is already an input.

   Scanner sc = new Scanner(System.in);
    int input;
    
    do {
        System.out.println("Please enter a number [0-200]!");
        while (!sc.hasNextInt()) {
            System.out.println("That's not a number!");
            sc.next();
        }
        input = sc.nextInt();
    } while (input >= 0 && input <= 200);
    
    sc.close();
    return input;

Example of input:

Please enter a number [0-200]!
d
That's not a number!
-1
Please enter a number [0-200]!
That's not a number!
Exception in thread "main" java.util.NoSuchElementException
    at java.base/java.util.Scanner.throwFor(Scanner.java:937)
    at java.base/java.util.Scanner.next(Scanner.java:1478)
    at Aufgabe5.getInput(Aufgabe5.java:44)
    at Aufgabe5.main(Aufgabe5.java:112)

Answer 1

nextInt() works in a way that if you input a number and then press Return "The enter key" that key is registered and waiting, when the nextInt() is called again, that registered key will then get consumed by the nextInt() and program will continue without waiting an input, cos it assumed that there was an input. to get around this you can follow the scanner.nextInt() with a scanner.nextLine() to consume that Return key.

other way it can be done by saving a line of code is by replacing scanner.nextInt() with Integer.valueOf(scanner.nextLine()) . or better yet, Integer.parseInt(scanner.nextLine())

Answer 2

If you are looking for code, below method will work for your situation.

public static int getNumber() {

    Scanner sc = new Scanner(System.in);
    int input;

    System.out.println("Please enter a number [0-200]!");

    while (true){
        try {
            input = Integer.parseInt(sc.nextLine());


            if (input >= 0 && input <= 200){
                return input;
            }else {
                System.out.println("Please enter a number [0-200]!");
            }



        } catch (NumberFormatException e) {
            System.out.println("That's not a number!");
            System.out.println("Please enter a number [0-200]!");
        }

    }

}

Answer 3

So apparently removing the sc.close() statement made it work. Idk why though