I'm having trouble with rounding. Specifically, after reading all the javadoc, I was expecting the following code:
int n = (integer between 0 and 9, included)
new BigDecimal(n + 0.555d).setScale(2, RoundingMode.HALF_UP).doubleValue()
to return n + 0.56. Instead, these are the return values for n from 0 to 4:
new BigDecimal(0.555d).setScale(2, RoundingMode.HALF_UP).doubleValue()
0.56
new BigDecimal(1.555d).setScale(2, RoundingMode.HALF_UP).doubleValue()
1.55
new BigDecimal(2.555d).setScale(2, RoundingMode.HALF_UP).doubleValue()
2.56
new BigDecimal(3.555d).setScale(2, RoundingMode.HALF_UP).doubleValue()
3.56
new BigDecimal(4.555d).setScale(2, RoundingMode.HALF_UP).doubleValue()
4.55
I have also tried to change the rounding mode:
int n = (integer between 0 and 9, included)
new BigDecimal(n + 0.555d).setScale(2, RoundingMode.HALF_DOWN).doubleValue()
expecting n + 0.55 as a result for each and every n. Instead, the return values are exactly the same as the previous example:
new BigDecimal(0.555d).setScale(2, RoundingMode.HALF_DOWN).doubleValue()
0.56
new BigDecimal(1.555d).setScale(2, RoundingMode.HALF_DOWN).doubleValue()
1.55
new BigDecimal(2.555d).setScale(2, RoundingMode.HALF_DOWN).doubleValue()
2.56
new BigDecimal(3.555d).setScale(2, RoundingMode.HALF_DOWN).doubleValue()
3.56
new BigDecimal(4.555d).setScale(2, RoundingMode.HALF_DOWN).doubleValue()
4.55
Am I missing something?
The problem you have is that double is not a precise representation and you are round based on this imprecise number.
BigDecimal bd = new BigDecimal(1.555d);
System.out.println("bd=" + bd);
bd = bd.setScale(2, RoundingMode.HALF_UP);
System.out.println("after rounding bd=" + bd);
double d = bd.doubleValue();
System.out.println("after rounding d=" + d);
prints
bd=1.5549999999999999378275106209912337362766265869140625
after rounding bd=1.55
after rounding d=1.55
however
BigDecimal bd = BigDecimal.valueOf(1.555d);
System.out.println("bd=" + bd);
bd = bd.setScale(2, RoundingMode.HALF_UP);
System.out.println("after rounding bd=" + bd);
double d = bd.doubleValue();
System.out.println("after rounding d=" + d);
prints
bd=1.555
after rounding bd=1.56
after rounding d=1.56
This works because BigDecimal.valueOf does some extra rounding based on how double would appear if you printed it.
However I wouldn't use BigDecimal unless performance/simplicity is not an issue.
double d = 1.555d;
System.out.println("d=" + d);
d = roundToTwoPlaces(d);
System.out.println("after rounding d=" + d);
public static double roundToTwoPlaces(double d) {
return ((long) (d < 0 ? d * 100 - 0.5 : d * 100 + 0.5)) / 100.0;
}
prints
d=1.555
after rounding d=1.56
For more details Double your money again compares the performance of different ways of rounding.