How do you format a fractional percentage with java.text.MessageFormat

Question

My percentages get truncated by the default java.text.MessageFormat function, how do you format a percentage without losing precision?

Example:

String expectedResult = "12.5%";
double fraction = 0.125;

String actualResult = MessageFormat.format("{0,number,percent}", fraction);
assert expectedResult.equals(actualResult) : actualResult +" should be formatted as "+expectedResult;

Answer 1

How about

DecimalFormat f = new DecimalFormat( "###.#" );
System.out.println( f.format( 12.5 ) );

The format char '#' does not print a 0 as absent. So 12.5 ->"12.5", 12.0 -> "12", not "12.0". You could of course set up your formatter with e.g. "###,###.##", the hundreths place will only show up if you need the precision.

Answer 2

You do realize that "expectedResult == actualResult" will always be false, right?

Anyway, the best solution I can find is to set the formatter explicitly. This is the code I tested:

String expectedResult = "12.5%";
double fraction = 0.125;
MessageFormat fmt = new MessageFormat("{0,number,percent}");
NumberFormat nbFmt = NumberFormat.getPercentInstance();
nbFmt.setMaximumFractionDigits(1); // or 2, or however many you need
fmt.setFormatByArgumentIndex(0, nbFmt);
String actualResult = fmt.format(new Object[] {fraction});
assert expectedResult.equals(actualResult) : actualResult +" is getting rounded off";

Answer 3

Looks like this:

String actualResult = MessageFormat.format("{0,number,#.##%}", fraction);

... is working.

EDIT: To see how are the #'s and %'s interpreted, see the javadoc of java.text.DecimalFormat.

EDIT 2: And, yes, it is safe for internationalization. The dot in format string is interpreted as a decimal separator, not as a hardcoded dot. :-)

Answer 4

I think the proper way to do it is the following:

NumberFormat percentFormat = NumberFormat.getPercentInstance();
percentFormat.setMaximumFractionDigits(1);
String result = percentFormat.format(0.125);

It also takes internalization into account. For example on my machine with hungarian locale I got "12,5%" as expected. Initializing percentFormat as NumberFormat.getPercentInstance(Locale.US) gives "12.5%" of course.