Rounding a BigDecimal in Java doesn't return the expected number

Question

I have a property called m_FOBST which contains the following number: 1.5776. Here I'm trying to round it:

   this.m_FOBST.setScale(2, BigDecimal.ROUND_HALF_EVEN)

However, I get the number 1.60 when I should be getting 1.58.

Can anyone explain why?

I can't duplicate this, i get: groovy:000> d = new BigDecimal("1.5776") ===> 1.5776 groovy:000> d1 = d.setScale(2, BigDecimal.ROUND_HALF_EVEN) ===> 1.58 — Nathan Hughes, May 09 '13 at 14:57
See http://araklefeistel.blogspot.com/2011/06/javamathbigdecimal-difference-between.html?m=1 — wchargin, May 09 '13 at 14:59
`System.out.println(new BigDecimal("1.5776").setScale(2, BigDecimal.ROUND_HALF_EVEN));` prints `1.58` too. — Keppil, May 09 '13 at 15:01
As another note, you should use `setScale(2, RoundingMode.HALF_EVEN)`, the version you use here is considered a legacy method. — Keppil, May 09 '13 at 15:05

score 4 · Accepted Answer · answered May 09 '13 at 14:58

4

BigDecimal is immutable - make sure you are using the value returned by the setScale() method.

BigDecimal bd = new BigDecimal("1.5776");

bd = bd.setScale(2, BigDecimal.ROUND_HALF_EVEN);

In this case, bd is 1.58

answered May 09 '13 at 14:58

manub

2

He probably is, since he said the answer was 1.60 (and BigDecimal doesn't round anything by default) – Marlon Bernardes May 09 '13 at 14:59
4

You should always use the `String` constructor when creating `BigDecimal` from a floating point number. – Keppil May 09 '13 at 15:00
@Keppil Why do you say that? – sdoca May 09 '13 at 15:01
thanks @Keppil - you're right. You do that because `double` and `float` are not exact representation of floating point numbers, this is the first reason why you want to use `BigDecimal` – manub May 09 '13 at 15:02
2

@sdoca: Floating point numbers are not exact. Run the following line to see what I mean: `System.out.println(new BigDecimal(1.5776));` – Keppil May 09 '13 at 15:02
@sdoca Quoting the javadocs: "[...]Note: the results of this constructor can be somewhat unpredictable [...]" http://docs.oracle.com/javase/1.4.2/docs/api/java/math/BigDecimal.html#BigDecimal(double) – Marlon Bernardes May 09 '13 at 15:03
@Keppil, interesting! Thanks! – sdoca May 09 '13 at 15:05
I copied the exactly same code that you put @manub, and get the correct answer, but i can´t get this using my property. So I did this `BigDecimal bd2 = this.m_FOBST; bd2 = bd.setScale(2, BigDecimal.ROUND_HALF_EVEN);' And it worked, thank you so much. – Talita Albuquerque May 09 '13 at 15:09

1 Answers1