Java String.format() with HALF_EVEN rounding

Question

I'd like to use String.format() to format some BigDecimals as part of a string:

// Example:
String getPrice( String pattern )
{
  BigDecimal price = basePrice.multiply( BigDecimal.ONE.add( vatRate ) );
  BigDecimal priceInPence = price.multiply( new BigDecimal( "100" ) );
  BigDecimal annualPrice = price.multiply( new BigDecimal( "365" ) );

  return String.format( pattern, priceInPence, annualPrice );
}

String myPrice1 = getPrice( "Your price is %1$.3fp/day (£2$.2f/year) including VAT" );
// --> "Your price is 32.100p/day (£117.16/year) including VAT"

String myPrice2 = getPrice( "Around £%2$.0f annualy" );
// --> "Around £117 annually"

However the docs for String.format() say that any rounding of BigDecimals will be done with HALF_UP rounding, whereas I need HALF_EVEN.

I know how to manually set the scale of BigDecimals (Set specific precision of a BigDecimal) - but in this case I want to be able to use an arbitrary pattern string (including non-numeric pattern elements), so I won't know in advance what scale to use.

My question is therefore:

can I set the rounding mode used by String.format()? OR
is there another formatter or library that would format the numbers as in my example?

Take a look at java.math.MathContext. You can construct a MathContext using a java.math.RoundingMode (HALF_EVEN), and then pass that to the constructor of BigDecimal or to one of the multiply methods: multiply(BigDecimal multiplicand, MathContext mc) — mttdbrd
– mttdbrd, Commented Oct 30, 2014 at 4:06
@mttdbrd - sorry, I should have made clear I need arbitrary precision, based on the pattern (see my edit above). — Richard Inglis
– Richard Inglis, Commented Oct 30, 2014 at 4:21
stackoverflow.com/questions/9482889/… and stackoverflow.com/questions/15352229/… — ComputerFellow
– ComputerFellow, Commented Oct 30, 2014 at 4:56
@ComputerFellow - sorry, those are answers about BigDecimals, my question is about changing the rounding mode used by String.Format() — Richard Inglis
– Richard Inglis, Commented Oct 30, 2014 at 7:39

bestsss · Accepted Answer · 2014-11-05 22:38:28Z

3

can I set the rounding mode used by String.format()?
Short answer: no.

is there another formatter or library that would format the numbers as in my example?
The BigDecimal is converted internally via new MathContext(compPrec) or plain HALF_UP. You can take the code of java.util.Formatter of the latest (or your preferred) version Java and modify the creation of the MathContext to use HALF_EVEN. It should be 10-15minutes work. But then you need a custom method to mimic String.format:

public static String format(String format, Object... args) {
    return new FormatterHALF_EVEN().format(format, args).toString();
}

answered Nov 5, 2014 at 22:38

bestsss

12.1k3 gold badges56 silver badges65 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Community · Accepted Answer · 2020-06-20 09:12:55Z

1

+100

Really "solid" advice to add 5000 lines of dead-weight code to your project! From what I see, the Formatter will not set scale unless it is already set to what is needed. So help it out, parse the format string and set your scale:

public static String getPrice(String pattern) {
    BigDecimal basePrice = new BigDecimal("23");
    BigDecimal vatRate = new BigDecimal("0.5");
    BigDecimal price = basePrice.multiply(BigDecimal.ONE.add(vatRate));
    BigDecimal priceInPence = price.multiply(new BigDecimal("100"));
    BigDecimal annualPrice = price.multiply(new BigDecimal("365"));

    Matcher matcher = Pattern.compile("%(\\d+)\\$.(\\d+)f").matcher(pattern);

    while (matcher.find()) {
        String index = matcher.group(1);

        int scale = Integer.parseInt(matcher.group(2));

        if (index.equals("1"))
            priceInPence = priceInPence.setScale(scale, RoundingMode.HALF_EVEN);
        else if (index.equals("2"))
            annualPrice = annualPrice.setScale(scale, RoundingMode.HALF_EVEN);
    }

    return String.format(pattern, priceInPence, annualPrice);
}

with these numbers I get this output:

Your price is 3450.000p/day (£12592.50/year) including VAT

Around £12592 annualy

So it applies correct rounding.

edited Jun 20, 2020 at 9:12

CommunityBot

11 silver badge

answered Nov 9, 2014 at 15:10

Denis Tulskiy

19.2k7 gold badges54 silver badges69 bronze badges

1 Comment

Richard Inglis Over a year ago

I like this approach. It could be even better if it was extended a bit to handle the case where a token is used more than once with different scales: getPrice( "Around £%2$.0f annualy (£%2.2f to be exact)" );

Kevin Krumwiede · Accepted Answer · 2014-10-30 08:18:19Z

0

Set the scale with the rounding mode you like, and include the values in the format string as strings, using BigDecimal#toString().

answered Oct 30, 2014 at 8:18

Kevin Krumwiede

1

1 Comment

Richard Inglis Over a year ago

The problem is, I don't know what scale to use until runtime: the precision information is in the pattern String.

Collectives™ on Stack Overflow

Java String.format() with HALF_EVEN rounding

3 Answers 3

Comments

1 Comment

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

1 Comment

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related