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Scanner for hex-input not working as expected inside loop

I need to solve this problem where i need to convert hexadecimal numbers to decimals.


I have compiled it without any errors, but my output came out wrong. Can someone show how to fix these errors?


import java.util.Scanner;
class HexToDecException extends Exception  {
    public HexToDecException(String msg) {

public class Main {
    public static int hexToDec(String hex) throws HexToDecException {
        char currentChar;
        int dec=0;
        int power = 0;
        for(int i=hex.length()+1;i>=0;i--){// by reverse
            currentChar = hex.charAt(i);
            if(currentChar>='0' && currentChar<='9'){
                dec+= Math.pow(16,power)*(currentChar - '0');
            else if(currentChar>='a' && currentChar<='f'){
                dec+= Math.pow(16,power)*(currentChar - 'a' + 10);
            else if(currentChar>='A' && currentChar<='G'){
                dec+= Math.pow(16,power)*(currentChar - 'A' + 10);
                throw new IllegalArgumentException("Invalid Character in Hexadecimal string");
        return power;
    public static void main(String[] args){
        Scanner scan = null;
            scan = new Scanner(;
            int n = scan.nextInt();
            for(int i=0;i<n;i++)
        catch(Exception ex){
            System.out.println("That's all.");



5 6B 5 4AZ A5 9

Expected Output:

107  5 Invalid hexadecimal number 165 9

Output that i got:

That's all



Your solution is close. Here are changes to make so that it runs correctly:

  • modify the return statement of hexToDec() to return the decimal number you’ve constructed, not the power, so change this: return power to this: return dec
  • modify throws clause to this: throw new HexToDecException("Invalid hexadecimal number"); – use the custom exception you defined, and edit the string message to match the output you’re expecting
  • change the for loop in hexToDec() so it that properly checks each position in the input string, so change this: for (int i = hex.length() + 1 to this: for (int i = hex.length() - 1. Maybe read up on how String.length() works in relation to charAt(). If a string has a length N, the corresponding “Nth” position in that string has to be referenced by N-1. N itself would never work, and N+1 would also not work.
  • finally, rewrite the main loop to wrap the call to hexToDec() in a try-catch, and print the exception message if an exception is thrown:
    Scanner scan = new Scanner(;
    int n = scan.nextInt();
    for (int i = 0; i < n; i++) {
        String nextInput =;
        try {
            int x = hexToDec(nextInput);
            System.out.print(x + " ");
        } catch (HexToDecException e) {
            System.out.print(e.getMessage() + " ");

After those changes, this input:

5 6B 5 4AZ A5 9

will produce this output:

107 5 Invalid hexadecimal number 165 9 
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