public static long octalADecimal(String octal) {
    long decimal = 0;
    int potencia = 0;
    for (int x = octal.length() - 1; x >= 0; x--) {
        int valorActual = Character.getNumericValue(octal.charAt(x));
        long elevado = (long) Math.pow(8, potencia) * valorActual;
        decimal += elevado;
        potencia++;
    }
    return decimal;
}

/*

    Programado por Luis Cabrera Benito 
  ____          _____               _ _           _       
 |  _ \        |  __ \             (_) |         | |      
 | |_) |_   _  | |__) |_ _ _ __ _____| |__  _   _| |_ ___ 
 |  _ <| | | | |  ___/ _` | '__|_  / | '_ \| | | | __/ _ \
 | |_) | |_| | | |  | (_| | |   / /| | |_) | |_| | ||  __/
 |____/ \__, | |_|   \__,_|_|  /___|_|_.__/ \__, |\__\___|
         __/ |                               __/ |        
        |___/                               |___/         
    
    
    Blog:       https://parzibyte.me/blog
    Ayuda:      https://parzibyte.me/blog/contrataciones-ayuda/
    Contacto:   https://parzibyte.me/blog/contacto/
*/
import java.util.Scanner;

class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.println("Escribe el octal:");
        String octal = sc.nextLine();
        long decimal = octalADecimal(octal);
        System.out.printf("El octal %s es %d en decimal", octal, decimal);
    }

    public static long octalADecimal(String octal) {
        long decimal = 0;
        int potencia = 0;
        for (int x = octal.length() - 1; x >= 0; x--) {
            int valorActual = Character.getNumericValue(octal.charAt(x));
            long elevado = (long) Math.pow(8, potencia) * valorActual;
            decimal += elevado;
            potencia++;
        }
        return decimal;
    }
}