Calling method within itself

Question

I wanted to make an input which accepts numbers from 1 - 10 and prints the range.

I need to check if the input is an integer (check), check if the range is 0-10 (check), and if it's not any of those things, to ask the user again. So, a recursive method?

Currently I have this:

import java.util.Scanner; import java.util.InputMismatchException; public class FinalTest { public static void main (String [] args) { Scanner in = new Scanner(System.in); int k = 0; System.out.print("int - "); try { k = in.nextInt(); } catch (InputMismatchException e) { System.out.println("ERR: Input"); System.exit(1); } if(k <= 10 && k > 0) { for(int j=1; j <= k; j++) { System.out.println(j); } } else { System.out.println("ERR: Oob"); System.exit(1); } } }

I would like to replace the "System.exit()" so that it re attempts to ask the user for input again.

calling main(); produces an error.

How do I correctly call the main method in this case?

You don't need recursion for this. Just use a loop. See: The while and do-while Statements — Jesper
– Jesper, Commented Aug 4, 2017 at 11:17
Stick it in a loop. Or alternatively put the logic into a function and call the function recursively. — Scott
– Scott, Commented Aug 4, 2017 at 11:17
For what it's worth you can use a recursive method for this but you shouldn't. A simple loop would suffice while(don't have valid answer){ask for answer} — Richard Tingle
– Richard Tingle, Commented Aug 4, 2017 at 11:18

GhostCat · Accepted Answer · 2017-08-04 12:44:39Z

Two choices here:

actually create a method and call that
simply use a loop

Loop could go like:

boolean askForInput = true; while ( askForInput ) { try { k = in.nextInt(); askForInput = false; } catch ... print "not a number try again" }

But beyond that: you still want to put this code into its own method. Not because that code should call itself, but for clarity reasons. Like:

public static int askForNumber(Scanner in) { ... code from above return k; }

And to answer your question: you do not want to use recursion here. You want to loop; and yes, recursion is one way to implement looping, but that is simply overkill given the requirement you want to implement here.

And for the record: when creating that helper method, you can actually simplify it to:

public static int askForNumber() { while ( askForInput ) { try ... return in.nextInt(); } catch ... print "not a number try again" } }

Beyond that: you typically use recursion for computational tasks, such as computing a factorial, or fibonacci number, ... see here for example.

Hey, appreciate the answer. Very helpful. Just one thing - why isn't this working? boolean askForInput = true; while ( askForInput ) { System.out.print("int - "); try { k = in.nextInt(); askForInput = false; } catch (InputMismatchException e) { System.out.println("ERR: Input"); } }
It just keeps printing ERR. Shouldn't it prompt for input every time? Heres the pastebin url, for easier reading: pastebin.com/vPrXw3MX
@py9 I followed that link and put it into a simple main method ... it prints "int-" ... I enter a number, hit enter ... program ends. Thus: please put a minimal reproducible example there - something I can download as a whole and run to repro the problem.
the issue is when you dont input a integer, but a string. Shouldn't it go to the beginning of the loop and ask for an input again? (since the askForInput is still true). Here's the full code: pastebin.com/GAykyrwj
@py9 OK. I have to admit I simply had a wrong understanding how nextInt() works. So you might want to go with your previous try/catch code. I am honestly to busy today to start making experiments with that scanner myself.

user3335083 · Accepted Answer · 2017-08-04 11:22:12Z

for the part of the recursive method printing a range:

public void printAscending(int n) { if (n > 0) { printAscending(n - 1); System.out.println(n); } }

Maaaatt · Accepted Answer · 2017-08-04 11:23:22Z

I think using recursion is just too much for something that simple and would probably be more expensive. You can add a while loop around your scanning bit until the entered value is valid. I would also put the printing loop out of the while to not have to test a condition before printing since if you get out of the while loop, it means number if valid. You could test just the -1 value to exit process.

public class FinalTest { public static void main (String [] args) { Scanner in = new Scanner(System.in); int k = 0; do { System.out.print("int - "); try { k = in.nextInt(); } catch (InputMismatchException e) { System.out.println("ERR: Input"); System.exit(1); } } while(!(k>0 && k<=10) && k!=-1); if(k!=-1) { for(int j=1; j<=k; j++) { System.out.println(j); } } else { System.out.println("Bye Bye."); } } }

Flare Flax · Accepted Answer · 2017-08-04 11:34:47Z

Okay, so what I personally do when I need to use recursion is I create a separate function/method for it. And when I need to restart the method, I just call it within itself. So it would be something like this:

private void recursiveMethod() { // do stuff . . . if (yourCondition) { //continue to next piece of code } else { recursiveMethod(); }

}

But in big projects, try to stay away from recursion because if you mess up, it can

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