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.

Issue

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

Code

import java.util.Scanner;
import java.io.*;
class HexToDecException extends Exception  {
   
    public HexToDecException(String msg) {
        super(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');
                power++;
            }
            else if(currentChar>='a' && currentChar<='f'){
                dec+= Math.pow(16,power)*(currentChar - 'a' + 10);
                power++;
            }
            else if(currentChar>='A' && currentChar<='G'){
                dec+= Math.pow(16,power)*(currentChar - 'A' + 10);
                power++;
            }
            else
                throw new IllegalArgumentException("Invalid Character in Hexadecimal string");
        }
        
        return power;
    }
    public static void main(String[] args){
        Scanner scan = null;
        try{
            scan = new Scanner(System.in);
            int n = scan.nextInt();
            for(int i=0;i<n;i++)
                System.out.println(hexToDec(scan.next()));
        }
        catch(Exception ex){
            ex.printStackTrace();
        }
        
        finally{
            System.out.println("That's all.");
        }
    }
}

Example

Input:

5 6B 5 4AZ A5 9

Expected Output:

107  5 Invalid hexadecimal number 165 9

Output that i got:

That's all

Answer

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(System.in);
  int n = scan.nextInt();
  for (int i = 0; i < n; i++) {
      String nextInput = scan.next();
      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