**1. Overview**

In this short tutorial, we'll learn how to round a number to *n* decimal places in Java.

## Further reading:

## Number of Digits in an Integer in Java

## Check If a Number Is Prime in Java

## Check If a String Is Numeric 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 ***Double*s With *BigDecimal*

*Double*s With

*BigDecimal*

To round *double*s 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*

*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.