Use DecimalFormat to get varying amount of decimal places

Question

So I want to use the Decimal Format class to round numbers:

double value = 10.555;

DecimalFormat fmt = new DecimalFormat ("0.##");

System.out.println(fmt.format(value));

Here, the variable value would be rounded to 2 decimal places, because there are two #s. However, I want to round value to an unknown amount of decimal places, indicated by a separate integer called numPlaces. Is there a way I could accomplish this by using the Decimal Formatter?

e.g. If numPlaces = 3 and value = 10.555, value needs to be rounded to 3 decimal places

Answer 1

Create a method to generate a certain number of # to a string, like so:

public static String generateNumberSigns(int n) {

    String s = "";
    for (int i = 0; i < n; i++) {
        s += "#";
    }
    return s;
}

And then use that method to generate a string to pass to the DecimalFormat class:

double value = 1234.567890;
int numPlaces = 5;

String numberSigns = generateNumberSigns(numPlaces);
DecimalFormat fmt = new DecimalFormat ("0." + numberSigns);

System.out.println(fmt.format(value));

---

OR simply do it all at once without a method:

double value = 1234.567890;
int numPlaces = 5;

String numberSigns = "";
for (int i = 0; i < numPlaces; i++) {
    numberSigns += "#";
}

DecimalFormat fmt = new DecimalFormat ("0." + numberSigns);

System.out.println(fmt.format(value));

Answer 2

If you don't need the DecimalFormat for any other purpose, a simpler solution is to use String.format or PrintStream.format and generate the format string in a similar manner to Mike Yaworski's solution.

int precision = 4; // example
String formatString = "%." + precision + "f";
double value = 7.45834975; // example
System.out.format(formatString, value); // output = 7.4583

Answer 3

How about this?

double value = 10.5555123412341;
int numPlaces = 5;
String format = "0.";

for (int i = 0; i < numPlaces; i++){
    format+="#";
}
DecimalFormat fmt = new DecimalFormat(format);

System.out.println(fmt.format(value));

Answer 4

If you're not absolutely bound to use DecimalFormat, you could use BigDecimal.round() for this in conjunction with MathContext of the precision that you want, then just BigDecimal.toString().