How to round a number to n decimal places in Java
What I would like is a method to convert a double to a string which rounds using the half-up method - ie if the decimal to be rounded is 5, it always rounds up to the previous number. This is the standard method of rounding most people expect in most situations.
I also would like only significant digits to be displayed - ie there should not be any trailing zeroes.
I know one method of doing this is to use the String.format method:
String.format("%.5g%n", 0.912385);
returns:
0.91239
which is great, however it always displays numbers with 5 decimal places even if they are not significant:
String.format("%.5g%n", 0.912300);
returns:
0.91230
Another method is to use the DecimalFormatter :
DecimalFormat df = new DecimalFormat("#.#####");
df.format(0.912385);
returns:
0.91238
However as you can see this uses half-even rounding. That is it will round down if the previous digit is even. What I'd like is this:
0.912385 -> 0.91239
0.912300 -> 0.9123
What is the best way to achieve this in Java?
Use setRoundingMode , set the RoundingMode explicitly to handle your issue with the half-even round, then use the format pattern for your required output.
Example:
DecimalFormat df = new DecimalFormat("#.####");
df.setRoundingMode(RoundingMode.CEILING);
for (Number n : Arrays.asList(12, 123.12345, 0.23, 0.1, 2341234.212431324)) {
Double d = n.doubleValue();
System.out.println(df.format(d));
}
gives the output:
12
123.1235
0.23
0.1
2341234.2125
Assuming value is a double , you can do:
(double)Math.round(value * 100000d) / 100000d
That's for 5 digits precision. The number of zeros indicate the number of decimals.
new BigDecimal(String.valueOf(double)).setScale(yourScale, BigDecimal.ROUND_HALF_UP);
will get you a BigDecimal . To get the string out of it, just call that BigDecimal 's toString method, or the toPlainString method for Java 5+ for a plain format string.
Sample program:
package trials;
import java.math.BigDecimal;
public class Trials {
public static void main(String[] args) {
int yourScale = 10;
System.out.println(BigDecimal.valueOf(0.42344534534553453453-0.42324534524553453453).setScale(yourScale, BigDecimal.ROUND_HALF_UP));
}
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