Main.java

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;
}