How to validate user's input, and read-in their input again if it invalidates, using try-catch?

Question

I want to validate user input using the exception handling mechanism.

For example, let's say that I ask the user to enter integer input and they enter a character. In that case, I'd like to tell them that they entered the incorrect input, and in addition to that, I want them to prompt them to read in an integer again, and keep doing that until they enter an acceptable input.

I have seen some similar questions, but they do not take in the user's input again, they just print out that the input is incorrect.

Using do-while, I'd do something like this:

Scanner reader = new Scanner(System.in);  
System.out.println("Please enter an integer: ");
int i = 0;
do {
  i = reader.nextInt();
} while (  ((Object) i).getClass().getName() != Integer  ) {
  System.out.println("You did not enter an int. Please enter an integer: ");
}
System.out.println("Input of type int: " + i);

PROBLEMS:

An InputMismatchException will be raised on the 5th line, before the statement checking the while condition is reached.
I do want to learn to do input validation using the exception handling idioms.

So when the user enters a wrong input, how do I (1) tell them that their input is incorrect and (2) read in their input again (and keep doing that until they enter a correct input), using the try-catch mechanism?

EDIT: @Italhouarne

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);
    }

}

fdsa · Accepted Answer · 2016-03-05 02:28:56Z

3

In Java, it is best to use try/catch for only "exceptional" circumstances. I would use the Scanner class to detect if an int or some other invalid character is entered.

import java.util.Scanner;

public class Test {

    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        boolean gotInt = false;

        while (!gotInt) {
            System.out.print("Enter int: ");
            if (scan.hasNextInt()){
                gotInt = true;
            }
            else {
                scan.next(); //clear current input
                System.out.println("Not an integer");
            }
        }
        int theInt = scan.nextInt();
    }
}

answered Mar 5, 2016 at 2:28

fdsa

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Comments

Darshan Mehta · Accepted Answer · 2016-03-05 02:26:30Z

1

Here you go :

Scanner sc = new Scanner(System.in);
boolean validInput = false;
int value;
do{
    System.out.println("Please enter an integer");
    try{
        value = Integer.parseInt(sc.nextLine());
        validInput = true;
    }catch(IllegalArgumentException e){
        System.out.println("Invalid value");
    }
}while(!validInput);

answered Mar 5, 2016 at 2:26

Darshan Mehta

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Comments

ltalhouarne · Accepted Answer · 2016-03-05 02:28:22Z

1

You can try the following:

Scanner reader = new Scanner(System.in);  
System.out.println("Please enter an integer: ");
int i = 0;

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);

answered Mar 5, 2016 at 2:28

ltalhouarne

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3 Comments

Solace Over a year ago

But while(true) is an infinite loop

ltalhouarne Over a year ago

@Solace No, because break will exit out of it whenever an exception is not thrown (meaning the input is an int).

Solace Over a year ago

I tested your code and it does enter an infinite loop, though not where I had (wrongly) expcted, but in the catch block. It keeps printing You did not enter an int. Please enter an integer:. See edit in question. Though why this is happening is beyond me.

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