parsing - Java - Decimal Format.parse to return double value with specified number of decimal places

Question

I want to be able to convert a string to a Double given a number of decimal places in a format string. So "###,##0.000" should give me a Double to 3 decimal places.

Edit - added more info to what happens

The user enters the value in the UI - which is input into a String. The rule is this value is limited to 3 decimal places. The underlying code stores the value in the database which is then used in a calculation. Therefore the trailing decimal places will cause the calculations to be out slightly to what would be expected.

I have the following code:

try {
        // output current locale we are running under (this happens to be "nl_BE")
        System.out.println( "Current Locale is " + Locale.getDefault().toString() );

        // number in Central European Format with a format string specified in UK format
        String numberCE = "1,234567"; // 1.234567
        String formatUK = "###,##0.000";

        // do the format
        DecimalFormat formatterUK = new DecimalFormat( formatUK );
        Double valCEWithUKFormat = formatterUK.parse( numberCE ).doubleValue();

        // I want the number to DPs in the format string!!!
        System.out.println( "CE Value     " + numberCE + " in UK format (" + formatUK + ") is "
        + valCEWithUKFormat );

    } catch( ParseException ex ) {
        System.out.println("Cannot parse number");
    }
}

The DecimalFormat seems to ignore the format string and gives me the complete string as a Double of 1.234567.

Can DecimalFormat be forced to use the format string when parsing? Am I missing something?

Cheers,

Andez

Answer 1

DecimalFormat is used for two distinct purposes: parsing input and formatting output. If you want to do both, you'll have to use the format object twice.

If you want to take that value and format the output, restricting the number of significant digits, you need to use the format object again. This time it uses your formatting rules to create an output string from a numeric value:

String output = formatterUK.format(valCEWithUKFormat.doubleValue() );

This will give you the output of 1,235

It seems you want this numeric value to be presented in the 1.235 format. To do this, you should format the output using a specific locale (if yours uses a different format).

HOWEVER, I would recommend approaching this problem differently:

String text = "1,234567";
NumberFormat nf_in = NumberFormat.getNumberInstance(Locale.GERMANY);
double val = nf_in.parse(text).doubleValue();

NumberFormat nf_out = NumberFormat.getNumberInstance(Locale.UK);
nf_out.setMaximumFractionDigits(3);
String output = nf_out.format(val);

A few notes:

• Input parsing should be kept separate from output formatting. Especially when you start throwing in multiple Locales.
• Allow the standard library to do the heavy lifting for determining what a valid input value is for a given Locale. You just need to select an appropriate Locale (I chose GERMANY, but this would obviously work with others). Always use Locales when possible. Don't try to recreate formatting strings.
• Always store your input value SEPARATE from any output formatting. IE, if you want to show only three digits in the output, that's fine, but store the whole double value anyway.