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From: Robert Klemme <shortcutter@googlemail.com>
Newsgroups: comp.lang.java.programmer
Subject: Re: Fixed-point arithmetic library
Date: Wed, 22 Feb 2012 23:13:06 +0100
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On 22.02.2012 22:39, Tom Anderson wrote:
> Evening all,
>
> I would quite like to represent some numbers in fixed point, and do
> arithmetic with them.

Fixed point math is susceptible to precision issues which can be more 
severe than those of float and double: 0.01 * 0.2 -> 0.00

http://en.wikipedia.org/wiki/Fixed_point_arithmetic#Precision_loss_and_overflow

Out of curiosity: What do you want to do with that?

> These numbers will mostly not be more than a bilion, and will probably
> never be more than a hundred billion. Some of them, i will need to
> represent to eight decimal places. I'd like to be able to multiply two
> large numbers, but i suspect will not need to multiply three. 11 * 2 + 8
> = 30 decimal digits; that's about 100 bits, so 128 bits would be big
> enough (about 5 decimal digits of headroom).
>
> Can anyone suggest an existing library for doing fixed-point arithmetic
> which would cover this?
>
> I have googled, but not found anything. There are a few fixed-precision
> libraries aimed at J2ME (i assume because old phones didn't have
> floating-point units?). There's something called jExigo, but it's 64-bit
> and the code doesn't look great.
>
> It's quite possible that there is no such library. But i thought it
> prudent to ask before writing one myself.

What about BigDecimal?
http://docs.oracle.com/javase/6/docs/api/java/math/BigDecimal.html

You could either add the precision loss after operations or just apply 
it for output.

package math;

import java.math.BigDecimal;
import java.math.RoundingMode;

public class FixedPointMath {

     public static void main(String[] args) {
         BigDecimal bd1 = new BigDecimal("0.01");
         BigDecimal bd2 = new BigDecimal("0.2");
         BigDecimal bd3 = bd1.multiply(bd2);
         BigDecimal bd4 = bd3.setScale(2, RoundingMode.HALF_UP);
         System.out.println(bd1 + " * " + bd2 + " -> " + bd3 + " -> " + 
bd4);
     }

}

Alternatively you could use BigInteger and add the scaling logic 
yourself when cooking your fixed math lib.

http://docs.oracle.com/javase/6/docs/api/java/math/BigInteger.html

Kind regards

	robert

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