当用户输入错误的字符或无效的输入数据时，如何显示“打印”错误？

Question

I want to know if there is an easy way to display a error for a wrong character or invalid input data.

public static void main(String[] args) {

    // Step 1: Create new Scanner object.
    Scanner input = new Scanner(System.in);         

    // Step 2: Prompt the user to enter today's day.
    System.out.print("Enter today’s day as an Integer (0-6): ");
    int Today = input.nextInt();

    // Step 3: Prompt the user to enter the number of days elapsed since today.
    System.out.print("Enter the number of days elapsed since today as an Integer: ");
    int DaysElapsed= input.nextInt();

    // Step 4: Compute the future day.
    int FutureDay = (Today + DaysElapsed) % 7;

    // Step 5: Printing the results.
        // Step 5.1: Today's day result depending the case.
        System.out.print("Today is ");
            // Step 5.2: Future day result depending the case.
        System.out.print(" and the future day is ");

Answer 1

Since you are only expecting 'int' here from the scanner.nextInt() It will throw an InputMismatchException exception. So you can easily validate your input for int here like this -

try {
   int Today = input.nextInt();
   int DaysElapsed= input.nextInt();
} catch (InputMismatchException){
   System.err.println("Input is not an integer");
}   

Scanner.nextInt() also throws NoSuchElementException and IllegalStateException exceptions Moreover you can validate whetehr an input date is valid by using conditions ( today>=1 && today=<31 )

Answer 2

With nextInt() you already filter the allowed values to integers. But if you want the user to enter values in a limited range you could use something like this:

    int Today = 0;

    if (input.hasNextInt()) {
        if (input.nextInt() < 32 && input.nextInt() > 0) { //should be between 0-32

            Today = input.nextInt();

        } else {

            throw new Exception("Number must be between 0-32");
        }
    }

Edit:

If you want to continue on error:

    int Today = 0;
    if(input.hasNextInt()) {

        Today = input.nextInt();
        while (!(Today > 0 && Today < 32)){

            System.out.println("Number must be between 0-32");
            Today = input.nextInt();
        }
    }