Why does the control directly enter the catch block without entering the try block in this SSCCE?

Question

In the following SSCCE, why does the catch block keep executing again and again, resulting in an infinite loop?

When I run this program and was prompted to enter an integer, I entered a.

What I thought would happen was that, an InputMismatchException would be thrown on the first line inside the try block, and from there, the control will go to the catch block, where it would print `` once, and then go the the next iteration of while(true) loop, in which it should have entered the try block again and should have prompted me to enter the integer.

But it seems to have started an unending entering into the catch block as soon as it enters the while(true) loop, because if it was entering the try block, it would have prompted me to enter an integer again.

Where, and what, is the mistake?

import java.util.InputMismatchException;
import java.util.Scanner;


public class WhyThisInfiniteLoop {

    public static void main (String [] args) {
        Scanner reader = new Scanner(System.in); 
        int i = 0; 
        System.out.println("Please enter an integer: ");

        while(true){
          try{
             i = reader.nextInt();
             break;  
          }catch(InputMismatchException ex){
             System.out.println("You did not enter an int. Please enter an integer:");
          }
        }

        System.out.println("Input of type int: " + i);
    }

}

OUTPUT

Please enter an integer:
a
You did not enter an int. Please enter an integer:
You did not enter an int. Please enter an integer:
You did not enter an int. Please enter an integer:
You did not enter an int. Please enter an integer:
You did not enter an int. Please enter an integer:
You did not enter an int. Please enter an integer:
You did not enter an int. Please enter an integer:
You did not enter an int. Please enter an integer:
You did not enter an int. Please enter an integer:
You did not enter an int. Please enter an integer:
You did not enter an int. Please enter an integer:
You did not enter an int. Please enter an integer:
You did not enter an int. Please enter an integer:
You did not enter an int. Please enter an integer:
You did not enter an int. Please enter an integer:
You did not enter an int. Please enter an integer:
You did not enter an int. Please enter an integer:
You did not enter an int. Please enter an integer:
You did not enter an int. Please enter an integer:
You did not enter an int. Please enter an integer:
You did not enter an int. Please enter an integer:
You did not ...
...
...

Given the theory that you were not entering the try block, you should have put a println at the start of the try block. That would have told you something else was wrong. — Patricia Shanahan, Mar 06 '16 at 02:33

score 1 · Accepted Answer · answered Mar 06 '16 at 02:30

Because you aren't reading the thing (a token) that isn't an int in your catch block. Try something like

try {
    i = reader.nextInt();
    break;  
} catch (InputMismatchException ex) {
    System.out.printf("%s is not an int. Please enter an integer:", reader.next());
}

It would be enough to call reader.next() (which will consume the token).

As is, your Scanner says nextInt which results in an error that there isn't an int. And then loops trying to read an int (and failing).

Why does the control directly enter the catch block without entering the try block in this SSCCE?

1 Answers1