Incorrect arithmetic in bigdecimal

Question

Hello,
I attempting to do some simple arithmetic (for FX rate conversion) using BigDecimal that is generating incorrect results (while the same calculation with double works correctly).
Can someone please take a look at this sample program and tell me what I may be doing wrong? Or, at least provide some insight into what is going wrong so that I can take steps to avoid?
import static java.math.RoundingMode.HALF_UP;
import java.math.BigDecimal;
import java.math.MathContext;
public class BigDecimalTest {
      public static void main(String[] args) {
            // 15,000,000,000,000
            BigDecimal large_value = new BigDecimal("15000000000000");
            MathContext mc = new MathContext(16, HALF_UP);
            BigDecimal divisor = new BigDecimal("1.5");
            BigDecimal numerator = BigDecimal.ONE.divide(divisor, mc);
            BigDecimal resultOne = large_value.multiply(numerator);
            BigDecimal resultTwo = large_value.divide(divisor, mc);
            System.out.println("Big Decimal arithmetic:");
            System.out.println("15 trillion * (1/1.5) = ["+resultOne.toPlainString()+"]");
            System.out.println("15 trillion / 1.5 = ["+resultTwo.toPlainString()+"]");
            System.out.println("results compare as ["+resultOne.compareTo(resultTwo)+"]\n");
            double large_dbl_value = 15000000000000d;
            double divisorDbl = 1.5;
            double resultOneDbl = large_dbl_value * (1/divisorDbl);
            double resultTwoDbl = large_dbl_value / divisorDbl;
            System.out.println("double arithmetic:");
            System.out.println("15 trillion * (1/1.5) = ["+resultOneDbl+"]");
            System.out.println("15 trillion /1.5 = ["+resultTwoDbl+"]");
            System.out.println("results compare as ["+(resultOneDbl - resultTwoDbl)+"]");
}Thank you,
Ed Bridges

Answer 1

When I look more into the proposed solution -- for example in the code below -- I still see discrepancies in the arithmetic of BigDecimal and double:
import static java.math.RoundingMode.HALF_UP;
import java.math.BigDecimal;
import java.math.MathContext;
public class BigDecimalTest {
     public static void main(String[] args) {
          // 12,345,678,901,234
          BigDecimal large_value = new BigDecimal("12345678901234");
          MathContext mc = new MathContext(16, HALF_UP);
          for (int i = 80; i< 90; i++) {           
               BigDecimal rate = new BigDecimal(i).divide(new BigDecimal("100.0"));
               BigDecimal divisor = rate.setScale(16, HALF_UP);
               BigDecimal numerator = divisor.pow(-1, mc);
               BigDecimal resultOne = large_value.multiply(numerator).round(mc);
               BigDecimal resultTwo = large_value.divide(divisor, mc);
               if (resultOne.compareTo(resultTwo)!= 0) {
                    System.out.println("Big Decimal arithmetic:");
                    System.out.println(large_value + " * (1/" + rate + ") = ["+resultOne.toPlainString()+"]");
                    System.out.println(large_value + " / " + rate + " = ["+resultTwo.toPlainString()+"]");
                    System.out.println("results compare as ["+resultOne.compareTo(resultTwo)+"]\n");
          double large_dbl_value = 12345678901234d;
          for (int i = 80; i< 90; i++) {           
               double divisorDbl = i / 100;
               double resultOneDbl = large_dbl_value * (1/divisorDbl);
               double resultTwoDbl = large_dbl_value / divisorDbl;
               if (resultOneDbl != resultTwoDbl) {
                    System.out.println("double arithmetic:");
                    System.out.println(large_dbl_value + " * (1/" + divisorDbl + ") = ["+resultOneDbl+"]");
                    System.out.println(large_dbl_value + " / " + divisorDbl + " = ["+resultTwoDbl+"]");
                    System.out.println("results compare as ["+(resultOneDbl - resultTwoDbl)+"]");
}This produces the following result:
Big Decimal arithmetic:
12345678901234 * (1/0.86) = [14355440582830.24]
12345678901234 / 0.86 = [14355440582830.23]
results compare as [1]
Big Decimal arithmetic:
12345678901234 * (1/0.89) = [13871549327229.22]
12345678901234 / 0.89 = [13871549327229.21]
results compare as [1]It's these "off-by-one penny" errors that are killing me :-(