Using Exceptions when preventing errors in input

Question

Hi im trying to create a code where i get user to input date. I will then manipulate this date to create a cost of travel for each day. Im struggling to add Exceptions to prevent errors from being entered. Can anyone give me some tips on how to do this? My Code:

import java.util.Calendar;
import java.util.Date;
import java.util.Scanner;

public class Price
{
  public static void main (String [] args)
  {
    userInput();         
  }

  public static void userInput()
  {
    Scanner scan = new Scanner(System.in);

    int month, day, year;

    System.out.println("Please enter a month MM: ");
    month = scan.nextInt();
    System.out.println("Please enter a day DD: ");
    day = scan.nextInt();
    System.out.println("Please enter a year YYYY: ");
    year = scan.nextInt();
    System.out.println("You chose: " + month + " /" + day +  " /" + year);  
  }
}

This shouldn't be handled by exceptions, but by simple tests/loops. See hasNextInt() in the Scanner javadoc. — JB Nizet
– JB Nizet, Commented Mar 21, 2013 at 22:27

Mel Nicholson · Accepted Answer · 2013-03-21 22:51:57Z

1

Hide the exception handling inside a method...

public static int inputInteger(Scanner in, String msg, int min, int max) {
  int tries = 0;
  while (tries < maxTries) {
    System.out.println(msg);
    try {
      int result = in.nextInt();
      if (result < min || result > max) {
        System.err.println("Input out of range:" + result);
        continue;
      }
      return result;
    } catch (Exception ex) {
      System.err.println("Problem getting input: "+ ex.getMessage());
    }
  }
  throw new Error("Max Retries reached, giving up");
}

This is a little simplistic, but it's a good start for simple apps. The same sort of loop can allow you to validate the input (e.g. don't take 35 as a date)

edited Mar 21, 2013 at 22:51

answered Mar 21, 2013 at 22:30

Mel Nicholson

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Comments

hotkey · Accepted Answer · 2013-03-21 22:39:54Z

0

May be you should use IllegalArgumentException like this:

if (month < 1 || month > 12 || day < 1 || day > 31)
    throw new IllegalArgumentException("Wrong date input");

or Exception base class:

if (month < 1 || month > 12 || day < 1 || day > 31)
    throw new Exception("Wrong date input");

Also, you can create your own subclass of Exception:

class WrongDateException extends Exception
{
   //You can store as much exception info as you need
}

and then catch it by

try {
    if (!everythingIsOk)
        throw new WrongDateException();
}
catch (WrongDateException e) { 
    ... 
}

edited Mar 21, 2013 at 22:39

answered Mar 21, 2013 at 22:34

hotkey

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Comments

Azad · Accepted Answer · 2013-03-21 22:42:40Z

0

I will just put the first loop which asking for month and the others is the same steps and the same idea see this :

int month, day, year;
while(true){
    try{
System.out.println("Please enter a month MM: ");
month=scan.nextInt();
if(month>0&&month<=12)
    break;
else System.err.println("Month should be between 1 and 12");
}catch(InputMismatchException ex){
   System.err.println(ex.getMessage());
     }
}
System.out.println("You chose: " + month );

edited Mar 21, 2013 at 22:42

answered Mar 21, 2013 at 22:36

Azad

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2 Comments

Mel Nicholson Over a year ago

Scanners throw WrongInputException.

Azad Over a year ago

Any way NumberFormatException or InputMismatchException must be handled here for not breaking the loop due to the mistmatch disaster error :)

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