Java: Format number in millions

问题:

Is there a way to use DecimalFormat (or some other standard formatter) to format numbers like this:

1,000,000 => 1.00M

1,234,567 => 1.23M

1,234,567,890 => 1234.57M

Basically dividing some number by 1 million, keeping 2 decimal places, and slapping an 'M' on the end. I've thought about creating a new subclass of NumberFormat but it looks trickier than I imagined.

I'm writing an API that has a format method that looks like this:

public String format(double value, Unit unit); // Unit is an enum

Internally, I'm mapping Unit objects to NumberFormatters. The implementation is something like this:

public String format(double value, Unit unit)
{
    NumberFormatter formatter = formatters.get(unit);
    return formatter.format(value);
}

Note that because of this, I can't expect the client to divide by 1 million, and I can't just use String.format() without wrapping it in a NumberFormatter.

回答1:

String.format("%.2fM", theNumber/ 1000000.0);

For more information see the String.format javadocs.

回答2:

Note that if you have a BigDecimal, you can use the movePointLeft method:

new DecimalFormat("#.00").format(value.movePointLeft(6));

回答3:

Here's a subclass of NumberFormat that I whipped up. It looks like it does the job but I'm not entirely sure it's the best way:

private static final NumberFormat MILLIONS = new NumberFormat()
{
    private NumberFormat LOCAL_REAL = new DecimalFormat("#,##0.00M");

    public StringBuffer format(double number, StringBuffer toAppendTo, FieldPosition pos)
    {
        double millions = number / 1000000D;
        if(millions > 0.1) LOCAL_REAL.format(millions, toAppendTo, pos);

        return toAppendTo;
    }

    public StringBuffer format(long number, StringBuffer toAppendTo, FieldPosition pos)
    {
        return format((double) number, toAppendTo, pos);
    }

    public Number parse(String source, ParsePosition parsePosition)
    {
        throw new UnsupportedOperationException("Not implemented...");
    }
};

回答4:

Why not simply?

DecimalFormat df = new DecimalFormat("0.00M");
System.out.println(df.format(n / 1000000));

回答5:

Take a look at ChoiseFormat.

A more simplistic way would be to use a wrapper that auto divided by 1m for you.