Working with BigIntegers

Question

I creating a program where it takes a base unit in the metric system. (Say grams.) And then when you select a prefix changes it to the equivalent amount. (Such as 1000 grams when you select Kilo would change it to 1 Kilogram.)

Problem is when I run the code it'll always end up as zero, which makes me think I'm manipulating the BigIntegers wrong. (I'm using VERY large numbers due to some prefixes being very small or very large beyond the usual long number.)

Here is the code:

import java.util.Scanner;
import java.math.BigInteger;

public class BaseMetricUnits {

public static void main(String[] args) {
    // TODO Auto-generated method stub
    System.out.println("Hello, World!");
    float meter = 1;
    float kilogram = 1;
    float second = 1;
    float ampere = 1;
    float kelvin = 1;
    float mole = 1;
    float candela = 1;
    PrefixConverter();
}
public static void PrefixConverter() 
{

    BigInteger Yocto = BigInteger.valueOf((long) 0.00000000000000000000000);
    BigInteger Zepto =  BigInteger.valueOf((long) 0.000000000000000000001);
    BigInteger Atto = BigInteger.valueOf((long) 0.000000000000000001);
    BigInteger Femto = BigInteger.valueOf((long) 0.000000000000001);
    BigInteger Pico = BigInteger.valueOf((long)0.000000000001);
    BigInteger Nano = BigInteger.valueOf((long)0.000000001);
    BigInteger Micro = BigInteger.valueOf((long)0.000001);
    BigInteger Milli = BigInteger.valueOf((long)0.001);
    BigInteger Centi = BigInteger.valueOf((long)0.01);
    BigInteger Deci = BigInteger.valueOf((long)0.1);
    BigInteger Deca = BigInteger.valueOf((long)10);
    BigInteger Hecto = BigInteger.valueOf((long)100);
    BigInteger Kilo = BigInteger.valueOf((long)1000);
    BigInteger Mega = BigInteger.valueOf((long)1000000);
    BigInteger Giga = BigInteger.valueOf((long)1000000000);
    BigInteger Tera = new BigInteger("1000000000000");
    BigInteger Peta = new BigInteger("1000000000000000");
    BigInteger Exa = new BigInteger("1000000000000000000");
    BigInteger Zetta = new BigInteger("1000000000000000000000");
    BigInteger Yotta = new BigInteger("1000000000000000000000000");

    long Amount;
    double Prefix; 
    String Units = "";
    BigInteger translatedResult;
    BigInteger Result;

    Scanner inputDevice = new Scanner(System.in);
    System.out.print("Please enter the type of unit to be used. (meters, grams, etc.) >> ");
    Units = inputDevice.next();
    System.out.print("Please enter an amount to be translated >> ");
    Amount = inputDevice.nextLong();
    System.out.print("Please choose one of the following Prefixes to translate to. ");
    System.out.print(" 1 - Yocto ");
    System.out.print(" 2 - Zepto ");
    System.out.print(" 3 - Atto ");
    System.out.print(" 4 - Femto ");
    System.out.print(" 5 - Pico ");
    System.out.print(" 6 - Nano ");
    System.out.print(" 7 - Micro ");
    System.out.print(" 8 - Milli ");
    System.out.print(" 9 - Centi ");
    System.out.print(" 10 - Deci ");
    System.out.print(" 11 - Deca ");
    System.out.print(" 12 - Hecto ");
    System.out.print(" 13 - Kilo ");
    System.out.print(" 14 - Mega ");
    System.out.print(" 15 - Giga ");
    System.out.print(" 16 - Tera ");
    System.out.print(" 17 - Peta ");
    System.out.print(" 18 - Exa ");
    System.out.print(" 19 - Zetta ");
    System.out.print(" 20 - Yotta ") ;
    Prefix = inputDevice.nextDouble();

     if(Prefix == 1)
     {
        Result = Yocto.multiply(BigInteger.valueOf(Amount));
        translatedResult = Yocto.divide(BigInteger.valueOf(Amount));
        System.out.println("You have " + Result + " " + Units + " which translates to " + translatedResult + "Yocto" + Units + ".");
     }
     if(Prefix == 2)
     {
        Result = Zepto.multiply(BigInteger.valueOf(Amount));
        translatedResult = Zepto.divide(BigInteger.valueOf(Amount));
        System.out.println("You have " + Result + " " + Units + " which translates to " + translatedResult + "Zepto" + Units + ".");
     }
     if(Prefix == 3)
     {
        Result = Atto.multiply(BigInteger.valueOf(Amount));
        translatedResult = Atto.divide(BigInteger.valueOf(Amount));
        System.out.println("You have " + Result + " " + Units + " which translates to " + translatedResult + "Atto" + Units + ".");
     }
     if(Prefix == 4)
     {
        Result = Femto.multiply(BigInteger.valueOf(Amount));
        translatedResult = Femto.divide(BigInteger.valueOf(Amount));
        System.out.println("You have " + Result + " " + Units + " which translates to " + translatedResult + "Femto" + Units + ".");
     }
     if(Prefix == 5)
     {
        Result = Pico.multiply(BigInteger.valueOf(Amount));
        translatedResult = Pico.divide(BigInteger.valueOf(Amount));
        System.out.println("You have " + Result + " " + Units + " which translates to " + translatedResult + "Pico" + Units + ".");
     }
     if(Prefix == 6)
     {
        Result = Nano.multiply(BigInteger.valueOf(Amount));
        translatedResult = Nano.divide(BigInteger.valueOf(Amount));
        System.out.println("You have " + Result + " " + Units + " which translates to " + translatedResult + "Nano" + Units + ".");
     }
     if(Prefix == 7)
     {
        Result = Micro.multiply(BigInteger.valueOf(Amount));
        translatedResult = Micro.divide(BigInteger.valueOf(Amount));
        System.out.println("You have " + Result + " " + Units + " which translates to " + translatedResult + "Micro" + Units + ".");
     }
     if(Prefix == 8)
     {
        Result = Milli.multiply(BigInteger.valueOf(Amount));
        translatedResult = Milli.divide(BigInteger.valueOf(Amount));
        System.out.println("You have " + Result + " " + Units + " which translates to " + translatedResult + "Milli" + Units + ".");
     }
     if(Prefix == 9)
     {
        Result = Centi.multiply(BigInteger.valueOf(Amount));
        translatedResult = Centi.divide(BigInteger.valueOf(Amount));
        System.out.println("You have " + Result + " " + Units + " which translates to " + translatedResult + "Centi" + Units + ".");
     }
     if(Prefix == 10)
     {
        Result = Deci.multiply(BigInteger.valueOf(Amount));
        translatedResult = Deci.divide(BigInteger.valueOf(Amount));
        System.out.println("You have " + Result + " " + Units + " which translates to " + translatedResult + "Deci" + Units + ".");
     }
     if(Prefix == 11)
     {
        Result = Deca.multiply(BigInteger.valueOf(Amount));
        translatedResult = Deca.divide(BigInteger.valueOf(Amount));
        System.out.println("You have " + Result + " " + Units + " which translates to " + translatedResult + "Deca" + Units + ".");
     }
     if(Prefix == 12)
     {
        Result = Hecto.multiply(BigInteger.valueOf(Amount));
        translatedResult = Hecto.divide(BigInteger.valueOf(Amount));
        System.out.println("You have " + Result + " " + Units + " which translates to " + translatedResult + "Hecto" + Units + ".");
     }
     if(Prefix == 13)
     {
        Result = Kilo.multiply(BigInteger.valueOf(Amount));
        translatedResult = Kilo.divide(BigInteger.valueOf(Amount));
        System.out.println("You have " + Result + " " + Units + " which translates to " + translatedResult + "Kilo" + Units + ".");
     }
     if(Prefix == 14)
     {
        Result = Mega.multiply(BigInteger.valueOf(Amount));
        translatedResult = Mega.divide(BigInteger.valueOf(Amount));
        System.out.println("You have " + Result + " " + Units + " which translates to " + translatedResult + "Mega" + Units + ".");
     }
     if(Prefix == 15)
     {
        Result = Giga.multiply(BigInteger.valueOf(Amount));
        translatedResult = Giga.divide(BigInteger.valueOf(Amount));
        System.out.println("You have " + Result + " " + Units +  " which translates to " + translatedResult + "Giga" + Units + ".");
     }
     if(Prefix == 16)
     {
        Result = Tera.multiply(BigInteger.valueOf(Amount));
        translatedResult = Tera.divide(BigInteger.valueOf(Amount));
        System.out.println("You have " + Result + " " + Units + " which translates to " + translatedResult + "Tera" + Units + ".");
     }  
     if(Prefix == 17)
     {
        Result = Peta.multiply(BigInteger.valueOf(Amount));
        translatedResult = Peta.divide(BigInteger.valueOf(Amount));
        System.out.println("You have " + Result + " " + Units + " which translates to " + translatedResult + "Peta" + Units + ".");
     }
     if(Prefix == 18)
     {
        Result = Exa.multiply(BigInteger.valueOf(Amount));
        translatedResult = Exa.divide(BigInteger.valueOf(Amount));
        System.out.println("You have " + Result + " " + Units + " which translates to " + translatedResult + "Exa" + Units + ".");
     }
     if(Prefix == 19)
     {
        Result = Zetta.multiply(BigInteger.valueOf(Amount));
        translatedResult = Zetta.divide(BigInteger.valueOf(Amount));
        System.out.println("You have " + Result + " " + Units + " which translates to " + translatedResult + "Zetta" + Units + ".");
     }
     if(Prefix == 20)
     {
        Result = Yotta.multiply(BigInteger.valueOf(Amount));
        translatedResult = Yotta.divide(BigInteger.valueOf(Amount));
        System.out.println("You have " + Result + " " + Units + " which translates to " + translatedResult + "Yotta" + Units + ".");
     }
     else
     {
        System.out.println("Not a valid input.");
     }
    }
}

Thanks for your help.

Answer 1

BigInteger can only store integers, and so is not a suitable data type for this application. I strongly recommend replacing it with BigDecimal.

You should also use string representations of the fractions to initialize:

  BigDecimal Zepto =  new BigDecimal("0.000000000000000000001");
  BigDecimal Atto = new BigDecimal("0.000000000000000001");
  BigDecimal Femto = new BigDecimal("0.000000000000001");
  BigDecimal Pico = new BigDecimal("0.000000000001");
  BigDecimal Nano = new BigDecimal("0.000000001");

Comments on this answer and on the question indicate a concern with using BigDecimal to store the larger numbers. This program illustrates the fact that it is not an issue:

import java.math.BigDecimal;

public class Test {
  public static void main(String[] args) {
    BigDecimal googol = new BigDecimal("1e100");
    System.out.println(googol);
    System.out.println(googol.add(BigDecimal.ONE));
  }
}

Output:

1E+100
10000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001