Java round decimal numbers

Question

I am trying to get a discount of an amount given

lets say: amount = "1.2" and discountPercentage = "17.3"

Code:

orig = Double.parseDouble("amount");
discount = orig*(discountPercentage/100);
discount = Math.round(discount);

discountedprice =  orig - discount;

On the code above: when I round the discount using Math.round(0.2076) i am getting Zero(0) result.

What I want to happen is like this:

0.2076 = should get 0.21 when rounded up

score 0 · Answer 1 · answered Mar 02 '14 at 10:14

0

Try this:

...
discount *= 100;
discount = Math.round(discount);
discount /= 100;
...

answered Mar 02 '14 at 10:14

Hungry Blue Dev

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[Refutation here](http://stackoverflow.com/a/12684082/207421). – user207421 Mar 02 '14 at 10:23
Refutation, refuted here. Using Math.round for this range of `double` as no errors http://ideone.com/BVCHh3 – Peter Lawrey Mar 02 '14 at 11:43

score 0 · Answer 2 · answered Mar 02 '14 at 10:34

Error:

orig = Double.parseDouble("amount");

does not use the variable, but is a string with the content a-m-o-u-n-t.

String amount = "1.2";
double orig = Double.parseDouble(amount);
double discount = orig*(discountPercentage/100);
discount = Math.round(discount);

However be warned, that a double value can only be an approximation of a decimal number, as it is a sum of powers of 2; 0.2 being 2^-3 ... Hence 100 * 0.01 is not precisely equal to 1.0.

For financial software it is better to use BigDecimal, which however is more circumstantial though. For a taste:

BigBecimal amount = new BigDecimal("1.20"); // Precision of 2 decimals.
BigDecimal discount = amount.multiply(discountPercentage.movePointLeft(2));

score 0 · Answer 3 · answered Mar 02 '14 at 10:37

your code will throw a NumberFormatException because of

orig = Double.parseDouble("amount");

You are trying to parse a number from the acutal String "amount". If you already have the amount variable initalized, you should use that variable (remove the quotes in the parseDouble method. E.g.

    String amount = "7.0";
    System.out.println(Double.parseDouble(amount));

Also the types of your variables are not clear: discount, discountPercentage, discountedPrice. Based on the code you provided in your question, a lot is not clear and therefore could be part of your problem.

Peter Keller · Answer 4 · 2014-07-11T11:23:24.007

0

Don't use Float or Double for money calculations. Use BigDecimal instead as e.g. stated here or here.

Some code:

final BigDecimal orig = new BigDecimal(1.2);
final BigDecimal discountPercentage = new BigDecimal(0.173);
final BigDecimal discount = orig.multiply(discountPercentage);
System.out.println("discount = " + discount.setScale(2, BigDecimal.ROUND_HALF_UP));

Prints:

discount = 0.21

edited Jul 11 '14 at 11:23

answered Mar 02 '14 at 11:10

Peter Keller

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I agree, never use `Float`, or `Double` or `float`, but you can use `BigDecimal`, `double` with rounding and `long` with fixed precision. – Peter Lawrey Mar 02 '14 at 11:44
@PeterLawrey I should have added a link to your post as well: http://vanillajava.blogspot.ch/2011/08/double-your-money-again.html. However I think for the normal use `BigDecimal` is the better solution. – Peter Keller Mar 02 '14 at 12:16
You can argue either way in terms of technology, but really what matters is what do you feel is the more error prone. Many prefer the explicit scale & rounding functions and for them this produces less errors in code. – Peter Lawrey Mar 02 '14 at 12:52

score -1 · Accepted Answer · answered Mar 02 '14 at 10:22

-1

float round(float f,float prec)
{
    return (float) (Math.floor(f*(1.0f/prec) + 0.5)/(1.0f/prec));
}

use

round(0.2076f,0.01f)

result

0.21

answered Mar 02 '14 at 10:22

SteveL

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care to explain the down vote? it works perfectly – SteveL Mar 02 '14 at 10:25
Almost perfectly, 0.01f cannot be represented exactly and introduces an error. Use 100 instead as this can be represented exactly. – Peter Lawrey Mar 02 '14 at 11:18

Java round decimal numbers

Code:

5 Answers5