如果用户输入String而不是Int，有什么可能的例外情况？

Question

I am just playing with Java.I'm trying to force my program to only accept number digits 1 and 2. I believe I have successfully done this using a while loop (please correct me if I'm wrong). But how do I go about printing an error statement if the user enters a string. eg: "abc".

My code:

    while (response != 1 && response != 2) {
        System.out.println("Please enter 1 for Car or 2 for Van: ");
        response = scan.nextInt();
    }

    if (response == 1) {
        vehicleType = VehicleType.CAR;
        while (numPassengerSeats < 4 || numPassengerSeats > 7) {
            System.out.println("Please enter the number of Passengers: ");
            numPassengerSeats = scan.nextInt();
        }
    } else {
        vehicleType = VehicleType.VAN;
        while (true) {
            System.out.println("Please enter the last maintenance date (dd/mm/yyyy): ");
            String formattedDate = scan.next();
            lastMaintenanceDate = formatDate(formattedDate);
            if (lastMaintenanceDate != null)
                break;
        }
    }

Answer 1

Let's have a look at javadoc for nextInt() :

Scans the next token of the input as an int. An invocation of this method of the form nextInt() behaves in exactly the same way as the invocation nextInt(radix), where radix is the default radix of this scanner.

Returns : the int scanned from the input

Throws :

InputMismatchException - if the next token does not match the Integer regular expression, or is out of range

NoSuchElementException - if input is exhausted

IllegalStateException - if this scanner is closed

As per javadoc, it throws InputMismatchException if the user enters a String instead of an int . So, we need to handle it.

Answer 2

I think you have not successfully force your program to accept just integers since by using java.util.Scanner.nextInt() , user can still be able to input non integers, however java.util.Scanner.nextInt() will just throw an exception. Refer to this for possible exception thrown.
I have made a solution to force your program to accept just integers. Just follow the sample code below:

Sample code:

package main;

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        int response = 0;
        Scanner scan = new Scanner(System.in);
        while (response != 1 && response != 2) {
            System.out.println("Please enter 1 for Car or 2 for Van: ");
            try {
                response = Integer.parseInt(scan.nextLine()); 
                if (response != 1 && response != 2) {
                    System.out.println("Input is not in choices!");
                }
            } catch (NumberFormatException e) {
                System.out.println("Input is invalid!");
            }
        }
        scan.close();
    }

}