Rounding BigDecimal Calculation Yielded Wrong Result

[James Gordon]

Hello All,

BigDecimal total = new BigDecimal("415");
BigDecimal rate = new BigDecimal("1.2");
System.out.println(
  total.multiply(
    (rate.divide(new BigDecimal(100), 2, BigDecimal.ROUND_HALF_UP))));
 
System.out.println(
  total.multiply(
    (rate.divide(new BigDecimal(100), 3, BigDecimal.ROUND_HALF_UP))));

The first println yielded 4.15
The second println yielded 4.980

Doing it manually, the result should be indeed 4.98

Does this imply that we should not use BigDecimal for calculations?


Thanks in advance.
[ September 22, 2004: Message edited by: James Gordon ]

[Joyce Lee]

Hi James,

I think it's still possible to use BigDecimal for the calculation if the scale value is set higher. Then round off the final value to 2 or 3 decimal places.

  
BigDecimal total = new BigDecimal("415");
BigDecimal rate = new BigDecimal("1.2");
 
BigDecimal value = rate.divide(new BigDecimal(100), 2, BigDecimal.ROUND_HALF_UP);       
System.out.println(value); // output is 0.01 when scale set to 2
System.out.println(total.multiply(value)); // output is 4.15
        
value = rate.divide(new BigDecimal(100), 3, BigDecimal.ROUND_HALF_UP); 
System.out.println(value); // output is 0.012 when scale set to 3
System.out.println(total.multiply(value)); // output is 4.980
 
value = rate.divide(new BigDecimal(100), 10, BigDecimal.ROUND_HALF_UP); 
System.out.println(value); output is 0.0120000000 when scale set to 10
System.out.println(total.multiply(value).setScale(3,
    BigDecimal.ROUND_HALF_UP)); // output is 4.980

Joyce
[ September 22, 2004: Message edited by: Joyce Lee ]

[David Hibbs]

Keep in mind that you have yourself specified the number of significant digits to carry. BigDecimal has another divide method that does not require a scale:

public BigDecimal divide(BigDecimal val,
int roundingMode)Returns a BigDecimal whose value is (this / val), and whose scale is this.scale(). If rounding must be performed to generate a result with the given scale, the specified rounding mode is applied.

I have yet to see this method fail in standard calculations with a known-to-be-precise answer.

HOWEVER...

*You can always cover yourself with a NumberFormat that only allows 2 decimal places.
*BigDecimal also provides many rounding modes to choose from,
*You can always modify operator order to clarify what the result should be
*Your operands define the inital scale of the result

Play around with the following code some. Note that I don't specify a scale.

double rawTotal = 415;
BigDecimal total = new BigDecimal(rawTotal); // "415.0" is equivalent
BigDecimal rate = new BigDecimal("1.2");
BigDecimal oneHundred = new BigDecimal(100);
 
BigDecimal result = rate.multiply(total);
rate.divide(oneHundred, BigDecimal.ROUND_CEILING);
System.out.println( "First result=" + result.doubleValue() );
 
result = result.divide(oneHundred, BigDecimal.ROUND_CEILING);
System.out.println( "Second result=" + result.toString() );

If you pass "415" without the ".0" you'll get 5 as a result -- one of the operands had a zero scale.

Anyway, good luck and have fun.