Question

Why is good practice to empty the ‘Garbage’ from the input buffer in a block of code like this? What would happen if I didn’t?

try{
    age = scanner.nextInt();
    // If the exception is thrown, the following line will be skipped over.
    // The flow of execution goes directly to the catch statement (Hence, Exception is caught)
    finish = true;
} catch(InputMismatchException e) {
    System.out.println("Invalid input. age must be a number.");
    // The following line empties the "garbage" left in the input buffer
    scanner.next();
}

Answer 1

Assuming you are reading from the scanner in a loop, if you don’t skip the invalid token, you will keep reading it forever. That’s what scanner.next() does: it moves the scanner to the next token. See the below simple example, which outputs:

Found int: 123
Invalid input. age must be a number.
Skipping: abc
Found int: 456

Without the String s = scanner.next() line, it would keep printing “Invalid input” (you can try by commenting out the last 2 lines).

---

public static void main(String[] args) {
    Scanner scanner = new Scanner("123 abc 456");
    while (scanner.hasNext()) {
        try {
            int age = scanner.nextInt();
            System.out.println("Found int: " + age);
        } catch (InputMismatchException e) {
            System.out.println("Invalid input. age must be a number.");
            String s = scanner.next();
            System.out.println("Skipping: " + s);
        }
    }
}