1. Overview
In this short tutorial, we’ll learn how to round a number to n decimal places in Java.
2. Decimal Numbers in Java
Java provides two primitive types that we can use for storing decimal numbers: float and double. Double is the default type:
double PI = 3.1415;
However, we should never use either type for precise values, such as currencies. For that, and also for rounding, we can use the BigDecimal class.
3. Formatting a Decimal Number
If we just want to print a decimal number with n digits after the decimal point, we can simply format the output String:
System.out.printf("Value with 3 digits after decimal point %.3f %n", PI);
// OUTPUTS: Value with 3 digits after decimal point 3.142
Alternatively, we can format the value with the DecimalFormat class:
DecimalFormat df = new DecimalFormat("###.###");
System.out.println(df.format(PI));
DecimalFormat allows us to explicitly set rounding behavior, giving more control of the output than the String.format() used above.
*4. Rounding Doubles With BigDecimal*
To round doubles to n decimal places, we can write a helper method:
private static double round(double value, int places) {
if (places < 0) throw new IllegalArgumentException();
BigDecimal bd = new BigDecimal(Double.toString(value));
bd = bd.setScale(places, RoundingMode.HALF_UP);
return bd.doubleValue();
}
There is one important thing to notice in this solution; when constructing BigDecimal, we must always use BigDecimal(String) constructor. This prevents issues with representing inexact values.
We can achieve the same result by using the Apache Commons Math library:
<dependency>
<groupId>org.apache.commons</groupId>
<artifactId>commons-math3</artifactId>
<version>3.5</version>
</dependency>
The latest version can be found here.
Once we add the library to the project, we can use the Precision.round() method, which takes two arguments – value and scale:
Precision.round(PI, 3);
By default, it is using the same HALF_UP rounding method as our helper method; therefore, the results should be the same.
Note that we can change rounding behavior by passing the desired rounding method as a third parameter.
5. Rounding Doubles With DoubleRounder
DoubleRounder is a utility in the decimal4j library. It provides a fast and garbage-free method for rounding doubles from 0 to 18 decimal points.
We can get the library (the latest version can be found here) by adding the dependency to the pom.xml:
<dependency>
<groupId>org.decimal4j</groupId>
<artifactId>decimal4j</artifactId>
<version>1.0.3</version>
</dependency>
Now we can simply use:
DoubleRounder.round(PI, 3);
However, DoubleRounder fails in a few scenarios:
System.out.println(DoubleRounder.round(256.025d, 2));
// OUTPUTS: 256.02 instead of expected 256.03
6. Math.round() Method
Another way of rounding numbers is to use the Math.Round() Method.
In this case, we can control n number of decimal places by multiplying and dividing by 10^n:
public static double roundAvoid(double value, int places) {
double scale = Math.pow(10, places);
return Math.round(value * scale) / scale;
}
This method is not recommended as it’s truncating the value. In many cases values are rounded incorrectly:
System.out.println(roundAvoid(1000.0d, 17));
// OUTPUTS: 92.23372036854776 !!
System.out.println(roundAvoid(260.775d, 2));
// OUTPUTS: 260.77 instead of expected 260.78
As a result, this method is listed here for learning purposes only.
7. Conclusion
In this article, we covered different techniques for rounding numbers to n decimal places.
We can simply format the output without changing the value, or we can round the variable by using a helper method. We also discussed a few libraries that deal with this problem.
The code used in this article can be found over on GitHub.