得到了BPP算法的十六进制答案，但是如何将答案转换为十进制？

Question

我想出了十六进制答案，但是我不知道将答案转换为十进制。

import java.io.Console;
import java.math.BigDecimal;
public class Main {
 public static double n = 0;
 public static String ALPHABET = "0123456789ABCDEF";
 public static void main(String[] args) {
  System.out.println("Enter how many decimal places you want for Pi?");
  String line = System.console().readLine();
  n = Integer.parseInt(line);

  double S1 =S(1);
  double S4 = S(4);
  double S5 = S(5);
  double S6 = S(6); 

  double y = 4*S1 -2*S4 - S5 - S6;

  y = Math.abs(y);
  y = y-(int)y; // remove start of fraction
  String HEX = HEX(y);
  System.out.println(HEX);

  //System.out.println(new BigDecimal(y).toPlainString());
 }
 public static double S(double j) {
   double S = 0;
   for(double k = 0; k <= n; k++) {
      S += (Math.pow(16.0,n-k)%(8*k+j))/(8*k+j);
   }
   for(double k = n + 1; k <= (n + 3); k++) {
      S += Math.pow(16.0,n-k)/(8*k+j);
  }
  return S;
 }
 public static String HEX(double y) {
    String HEX1 = "";
    for(int i = 0; i < 4; i++) {
       y = y * 16;
       int z = (int)y; //remove end of fraction
       z = z%16;
       HEX1 += ALPHABET.substring(z,z+1);
    }
    return HEX1;
}

}

是否无法从BBP算法转换十六进制答案？ 我希望有可能。我花了很长时间。