NumberFormatException while reversing a negative number

Question

    Scanner sc=new Scanner(System.in);
    int n=sc.nextInt()
    String s=new StringBuilder().append(n).reverse().toString();

    System.out.println(Integer.parseInt(s));

I am trying to reverse a number but when I try to give negative numbers for example if I give input as -124 it gives me NumberFormatException. Can anyone tell me why this is happening?

The string `-124` reversed is `421-`, which is not a valid number. — Sweeper, Jun 26 '21 at 05:46
Why not print the reversed string to see what you are trying to parse ? — John3136, Jun 26 '21 at 05:50
COBOL supports numeric data items with 'SIGN IS TRAILING SEPARATE' but AFAICS not REVERSE on them -- but you could use overlay or redefinition :-} — dave_thompson_085, Jun 26 '21 at 07:18

score 0 · Answer 1 · answered Jun 26 '21 at 06:05

Reverse of -X is X- hence the - at the end of string is causing the NumberFormatException.
Here is a simple ways to handle both positive & non-positive numbers:

    public static void main (String [] args)
    {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        // Make `n` positive if `n` is negative
        String s = new StringBuilder().append(n<0 ? -n : n).reverse().toString();
        // Print the reverse string along with `-` sign if n<0
        System.out.println((n<0 ? "-" : "") + Integer.parseInt(s));
    }

NumberFormatException while reversing a negative number

1 Answers1