16 -> 10

package com.company;
import java.util.Scanner;

public class Main {

    public static String inputLine() {
        Scanner scan = new Scanner(System.in);
        boolean isIncorrect;
        String line;
        System.out.println("Enter the number in hexadecimal notation");
        do {
            isIncorrect = false;
            line = scan.nextLine();
            isIncorrect = checkLine(line);
        }while (isIncorrect);
        return line;
    }

    public static boolean checkLine (String line) {
        boolean flag = false;
        if (line.length() == 0){
            System.out.println("You entered an empty line!\nRepeat enter");
            flag = true;
        }
        return flag;
    }

    public static void changeHexadecimalNumber(String hexadecimalNumber, int[] sourceNumber) {
        try {
            for (int i = 0; i < hexadecimalNumber.length(); i++) {
                if (hexadecimalNumber.charAt(i) == 'A'){
                    sourceNumber[i] = 10;
                } else if (hexadecimalNumber.charAt(i) == 'B'){
                    sourceNumber[i] = 11;
                } else if (hexadecimalNumber.charAt(i) == 'C'){
                    sourceNumber[i] = 12;
                } else  if (hexadecimalNumber.charAt(i) == 'D'){
                    sourceNumber[i] = 13;
                } else if (hexadecimalNumber.charAt(i) == 'E'){
                    sourceNumber[i] = 14;
                } else if (hexadecimalNumber.charAt(i) == 'F'){
                    sourceNumber[i] = 15;
                } else {
                    sourceNumber[i] = Integer.parseInt(String.valueOf(hexadecimalNumber.charAt(i)));
                }
            }
        }catch (NumberFormatException e){
            hexadecimalNumber = inputLine();
            changeHexadecimalNumber(hexadecimalNumber, sourceNumber);
        }
    }

    public static int convertToDecimalNumberSystem(int[] sourceNumber){
        int decimalNumber = 0;
        for (int i = 0; i < sourceNumber.length; i++) {
            decimalNumber += sourceNumber[i] * Math.pow(16, sourceNumber.length - (i + 1));
        }
        return decimalNumber;
    }

    public static void outputResult(int decimalNumber) {
        System.out.println("Your decimal number: " + decimalNumber);
    }

    public static void main(String[] args) {
        System.out.println("This program converts a number from hexadecimal to decimal");
        String hexadecimalNumber = inputLine();
        int[] sourceNumber = new int[hexadecimalNumber.length()];
        changeHexadecimalNumber(hexadecimalNumber, sourceNumber);
        int decimalNumber = convertToDecimalNumberSystem(sourceNumber);
        outputResult(decimalNumber);
    }
}