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How to round a number to n decimal places in Java

What I would like is a method to convert a double to a string which rounds using the half-up method - i.e. if the decimal to be rounded is 5, it always rounds up to the next number. This is the standard method of rounding most people expect in most situations.

I also would like only significant digits to be displayed - i.e. there should not be any trailing zeroes.

I know one method of doing this is to use the String.format method:

String.format("%.5g%n", 0.912385);

returns:

0.91239

which is great, however it always displays numbers with 5 decimal places even if they are not significant:

String.format("%.5g%n", 0.912300);

returns:

0.91230

Another method is to use the DecimalFormatter:

DecimalFormat df = new DecimalFormat("#.#####");
df.format(0.912385);

returns:

0.91238

However as you can see this uses half-even rounding. That is it will round down if the previous digit is even. What I'd like is this:

0.912385 -> 0.91239
0.912300 -> 0.9123

What is the best way to achieve this in Java?

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27

I believe one of the goals of the question was that "there should not be any trailing zeroes".

Lunchbox
– Lunchbox

2012-11-20 20:10:16 +00:00
Commented Nov 20, 2012 at 20:10
9

For this question, the op didn't want zeros, but this is exactly what I wanted. If you have a list of numbers with 3 decimal places, you want them to all have the same digits even if it's 0.

Tom Kincaid
– Tom Kincaid

2014-04-08 17:06:39 +00:00
Commented Apr 8, 2014 at 17:06
You forgot to specify RoundingMode.

IgorGanapolsky
– IgorGanapolsky

2015-09-29 21:27:08 +00:00
Commented Sep 29, 2015 at 21:27
2

@IgorGanapolsky by default Decimal mode uses RoundingMode.HALF_EVEN.

EndermanAPM
– EndermanAPM

2016-09-27 15:40:01 +00:00
Commented Sep 27, 2016 at 15:40
Or this (no check with instanceof etc). First create a suitable DecimalFormat. DecimalFormat df = (DecimalFormat) NumberFormat.getNumberInstance(); then you can specify grouping and pattern. Try df.toPattern(); and df.applyPattern("#,##0.00000;(#)"); and then you can use df.parse(String); and ` df.format(double);` do format after the pattern. This case negative numbers will be within parenthesis.

Anders
– Anders

2024-10-30 18:56:49 +00:00
Commented Oct 30, 2024 at 18:56

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How to round a number to n decimal places in Java

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