Java: BigDecimal round up for million number

Question

I have a method that calculates 17780.00 x 115.00, the result should be 2044700.00, however, the method calculated it as 2045000.00 (seems to be rounded up for the whole number). This method will also handle numbers with decimals, eg. 0.97 x 0.5. The code looks like this:

  public Double multiplyNumber(Double d1, Double d2) {

     return new BigDecimal(d1).multiply(
           new BigDecimal(d2), 
           new MathContext(4, RoundingMode.HALF_UP)).doubleValue(); 
  }

Please advise how to make the code calculate the correct result. Thanks!

Look at the Javadoc for `MathContext`. What do you think the `4` means? — tgdavies, Oct 26 '21 at 23:14
Does this answer your question? [How to convert BigDecimal to Double in Java?](https://stackoverflow.com/questions/19650917/how-to-convert-bigdecimal-to-double-in-java) — op_, Oct 26 '21 at 23:16
@tgdavies yeah, I thought it meant for the decimal places. how do I make it work for both the decimal places and the whole numbers. — jlp, Oct 26 '21 at 23:19
I'm not sure what you want to achieve. In what way does `d1 * d2` not do what you want? — tgdavies, Oct 26 '21 at 23:26
@tgdavies isn't it possible that only doing d1*d2 would produce a result like 9.99999999999 ? — jlp, Oct 26 '21 at 23:30

Henry Xiloj Herrera · Accepted Answer · 2021-10-26T23:30:20.723

2

You try run this.

public class Test {

    public static void main(String... strings)  {

        System.out.println("result "+multiplyNumber(17780.00, 115.00) );
    }

    public static Double multiplyNumber(Double d1, Double d2) {

         return new BigDecimal(d1).multiply(
               new BigDecimal(d2)).setScale(4, RoundingMode.HALF_UP).doubleValue(); 
    }
    
}

edited Oct 26 '21 at 23:30

answered Oct 26 '21 at 23:18

Henry Xiloj Herrera

124
6

This actually works! so if a calculation result is 9.123456, will it become 9.1234 because of the `setScale` to 4? – jlp Oct 26 '21 at 23:35
That right, but you want more decimal you change the number – Henry Xiloj Herrera Oct 26 '21 at 23:43
yeah, when a result for a multiply is 9.123456, i want only 4 decimals, 9.1234. Thank you very much! – jlp Oct 26 '21 at 23:44

Bohemian · Answer 2 · 2021-10-26T23:26:28.720

Rounding only deals with digits after the decimal place. To round at places to the left, divide, round, multiply back:

public Double multiplyNumber(Double d1, Double d2) {
    return new BigDecimal(d1)
            .multiply(new BigDecimal(d2))
            .divide(BigDecimal.TEN.pow(3), new MathContext(0, RoundingMode.HALF_UP))
            .multiply(BigDecimal.TEN.pow(3))
            .doubleValue();
}

BTW, this also produces the value you want:

public Double multiplyNumber(Double d1, Double d2) {
    return d1 * d2;
}

score 0 · Answer 3 · answered Oct 26 '21 at 23:33

0

Use below return statement instead:

return new BigDecimal(d1).multiply(new BigDecimal(d2)).round(new MathContext(4, RoundingMode.HALF_UP)).doubleValue();

answered Oct 26 '21 at 23:33

Coder

1
2

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Java: BigDecimal round up for million number

3 Answers3