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1. Overview

In this short article, we’re going to look at how to round a number to n decimal places in Java.

2. Decimal Numbers in Java

Java provides two primitive types that can be used for storing decimal numbers: float and doubleDouble is the type used by default:

double PI = 3.1415;

However, both types should never be used 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 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 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 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 the library is added 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, for example:

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 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

And so, this method is listed here for learning purposes only.

7. Conclusion

In this quick tutorial, 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’ve also covered a few libraries that deal with this problem.

The code used during the discussion can be found over on GitHub.

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Slava Semushin
Guest

There is a wrong character in the code example: if (places &lt; 0)

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