Chapter 18 Exercise 24, Introduction to Java Programming, Tenth Edition Y. Daniel LiangY.

18.24  (Hex to decimal)
Write a recursive method that parses a hex number
as a string into a decimal integer. The method header is:

public static int hex2Dec(String hexString)

Write a test program that prompts the user to enter a
hex string and displays its decimal equivalent.

import java.util.Scanner;

public class Exercise_24 {

    public static void main(String[] args) {

        Scanner input = new Scanner(System.in);
        System.out.print("Enter a hexadecimal number: ");
        String hex = input.next();

        System.out.println(hex + " = " + hex2Dec(hex));

    }

    public static int hex2Dec(String hex) {

        if (hex.length() == 1)
            return convertHex(hex.charAt(0));
        else
            return (int)Math.pow(16, hex.length() - 1) * convertHex(hex.charAt(0)) + hex2Dec(hex.substring(1));
    }

    public static int convertHex(char ch) {

        System.out.println(ch + " Text " + (ch - 10));
        if (ch - '0' < 10)
            return ch - '0';
        ch = Character.toUpperCase(ch);
        return ch - 'A' + 10;
    }
}