Reject Non numeric characters

Question

Well this little thing is killing me. I am quite a rookie in Java and, have been trying every possible possibility out there and nothing seems to work.

I just want the program to either reject non numerical strings, or ignore letters when pressed on keyboard.

import java.util.Scanner;

public class practice7 {


    public static void main(String[] args) {


        System.out.println("    Wlecome to the Magellan School student assistant \n\n");
        System.out.print("Please Enter your Student ID:  ");
        Scanner sc = new Scanner(System.in);
        Scanner NS = new Scanner(System.in);
        int ID = sc.nextInt();

//PRETTY SURE IT GOES HERE...

//rest of program

I have tried all the answers given here, but every time I write letters I get this:

Please Enter your Student ID:  ee

Exception in thread "main" java.util.InputMismatchException
    at java.util.Scanner.throwFor(Scanner.java:909)
    at java.util.Scanner.next(Scanner.java:1530)
    at java.util.Scanner.nextInt(Scanner.java:2160)
    at java.util.Scanner.nextInt(Scanner.java:2119)
    at practice7.main(practice7.java:11)
Java Result: 1
BUILD SUCCESSFUL (total time: 4 seconds)***

Just take a look on http://stackoverflow.com/questions/2912817/how-to-use-scanner-to-accept-only-valid-int-as-input question — amukhachov, Dec 12 '13 at 14:39

The Guy with The Hat · Answer 1 · 2013-12-12T14:44:42.117

2

You can use

while (!input.matches("[0-9]+"))
{ System.out.print("Error, invalid input. Try Again:  "); input = sc.nextInt(); }

Which uses String's .matches(String regex) method. [0-9]+ is a regex, which you can find an explanation and demonstration of here.

edited Dec 12 '13 at 14:44

answered Dec 12 '13 at 14:37

The Guy with The Hat

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wrossmck · Accepted Answer · 2013-12-12T14:50:13.007

Using exceptions:

int input = 0;
Boolean ready = false;
while(!ready){
    try{
        System.out.print("Please Enter your Student ID (Numbers Only):  ");
        Scanner in = new Scanner(System.in);
        input = in.nextInt();
        ready=true;
    }catch(IOException e){
        System.out.err("that was not a number");
    }
}

// now we have input of integer numbers
doRestOfProgram();

you could even do

String input = "nothing";

while(!input.matches("[0-9]+"))
{
    System.out.print("Please Enter your Student ID (Numbers Only):  ");
    Scanner in = new Scanner(System.in);
    input = in.nextLine();
}

// now we have input of string numbers
doRestOfProgram();

F***ng H*ll!! This could have taken me like forever! IT WORKS, and it is so sinple. Love it, THANKS — eneko, Dec 12 '13 at 15:09

score 0 · Answer 3 · answered Dec 12 '13 at 14:31

0

// Will throw an exception if String id cannot be parsed into an integer.
int idNum = Integer.valueOf(id);

answered Dec 12 '13 at 14:31

Tim B

38,707
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handle it in a try{}catch(IOException e){} block – wrossmck Dec 12 '13 at 14:37

Reject Non numeric characters

3 Answers3