Looping Try Statement

Question

I am trying to loop a try block every time an exception is thrown.

An example might be when the program prompts for a double and the user enters a string, so, a NumberFormatException is thrown. So the program will request the user to re-enter.

Here's what I am doing currently. Is this the proper way to do so or is there better way?

// infinite loop
for (;;)
{     
    try 
    {
        //do something
        break; // if an exception is not thrown. it breaks the loop.
    }
    catch (Exception e)
    {
        //display the stack trace.
    }

    // restarts the for loop
}

Heh, I dealt with a similar problem in my [blog](http://blog.engineering.vayana.in/case-study-in-dsl-development-with-scala-part-1/) recently, albeit in a different language. (see bullet 4) — missingfaktor, Feb 03 '12 at 06:02
Ah, ok. This snippet is fully functional. I just want to make sure my method is a generally accepted way of doing it. — Nyx, Feb 03 '12 at 06:04
Hi Marv, you can take a look at my example code for you. If you are gonna collect input many times, you can display input constraints once at the beginning or warning the user every time. It is a good alternative to catching exceptions. With this technique you are able to force users obey your input constraints as well, for instance it can be used for controlling a text field that is desired to accept just a particular string/numerical pattern. — Juvanis, Feb 03 '12 at 06:24

Juvanis · Accepted Answer · 2012-02-03T06:20:44.327

Instead of throwing exceptions according to the input, keep your restrictions on the user input by getting use of the regular expressions. Java regular expressions will help you at this point.

import java.util.Scanner;
import java.util.regex.Pattern;

public class Sample
{
    private final static Pattern DIGITS = Pattern.compile( "\\d*" );

    public static void main ( String [] args )
    {
        Scanner scanner = new Scanner( System.in );
        while ( true )
        {
            String input = scanner.nextLine();
            if ( evalInput( input ) )
                process( input );
            else
                System.out.println("Input constraints: it must be just numerical.");
        }
    }

    public static void process ( String str )
    {
        // Whatever you wanna do with the accepted input.
    }

    public static boolean evalInput ( String str )
    {
        if ( str != null && DIGITS.matcher( str ).matches() )
            return true;
        return false;
    }
}

I hope it helps a lot. You can check for the samples of the Java Regex and Pattern for complex or more specific inputs. — Juvanis, Feb 03 '12 at 06:31

score 3 · Answer 2 · answered Feb 03 '12 at 06:04

3

I'd do it just like you did, perhaps adding a prompt to re-enter.

while(true) {
  try {
    String s = read.nextLine();
    i = Integer.parseInt(s);
    break;
  } catch(NumberFormatException) {
    System.out.println("Please try again.");
    continue;
  }
}

answered Feb 03 '12 at 06:04

missingfaktor

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Thank you. Is there a particular reason why a while loop is preferred over a for loop? – Nyx Feb 03 '12 at 06:07
Not really. Both `while(true)` and `for(;;)` look equally readable to me, though I prefer the former. – missingfaktor Feb 03 '12 at 06:09
@Marv: beacuse it is more readable than for loop. Take a look here http://stackoverflow.com/questions/885908/while-1-vs-for-is-there-a-speed-difference. – RanRag Feb 03 '12 at 06:12
Then why do forever loops exist? Shouldn't a while (true) loop suffice? – Nyx Feb 11 '12 at 03:08
@Marv, what do you mean by that? – missingfaktor Feb 11 '12 at 05:17
I meant forever loops as for(;;) loops. If while (true) can be used, why is for(;;) necessary? – Nyx Feb 11 '12 at 05:39
@Marv, they're just two ways of doing the same thing. Use the one you like better. – missingfaktor Feb 11 '12 at 05:54
@Marv, please make up your mind on one answer. Don't keep moving green arrow here and there. – missingfaktor Feb 11 '12 at 07:18
@missingfaktor It's just that I like both answers. And both helped me equally. – Nyx Feb 12 '12 at 22:14

score 0 · Answer 3 · answered Feb 03 '12 at 06:22

In this case I'd rather put the whole try-block inside the loop. I think it's easier to read than breaking out of try:

while (true) {
    try {
        ...
    } catch (...) {
        ...
    }
}

Also, I fin it clearer to write while (true) for an infinite loop, and wouldn't normaly use exceptions to handle user input.

Looping Try Statement

3 Answers3