How to round a number to n decimal places in Java

Rounding a number to a specific number of decimal places is a common task in programming, especially when dealing with monetary or financial calculations. In Java, there are several methods that can be used to round a decimal number to a specific number of decimal places.

1. Using the Math.round() method

The Math.round() method can be used to round a decimal number to a specific number of decimal places. It returns a rounded long value that is closest to the decimal value.

public static double roundToDecimalPlaces(double value, int decimalPlaces) {
  double scale = Math.pow(10, decimalPlaces);
  return Math.round(value * scale) / scale;
}

Example:

double value = 123.4567;
int decimalPlaces = 2;
System.out.println(roundToDecimalPlaces(value, decimalPlaces));

Output: 123.46

2. Using the DecimalFormat class

The DecimalFormat class can also be used to format a decimal number to a specific number of decimal places.

public static double formatToDecimalPlaces(double value, int decimalPlaces) {
  DecimalFormat df = new DecimalFormat();
  df.setMaximumFractionDigits(decimalPlaces);
  return Double.valueOf(df.format(value));
}

Example:

double value = 123.4567;
int decimalPlaces = 2;
System.out.println(formatToDecimalPlaces(value, decimalPlaces));

Output: 123.46

3. Using the BigDecimal class

The BigDecimal class is an alternative method for rounding a decimal number to a specific number of decimal places. It provides more precise control over the rounding operation.

public static double roundBigDecimal(double value, int decimalPlaces) {
  BigDecimal bd = new BigDecimal(value);
  bd = bd.setScale(decimalPlaces, RoundingMode.HALF_UP);
  return bd.doubleValue();
}

Example:

double value = 123.4567;
int decimalPlaces = 2;
System.out.println(roundBigDecimal(value, decimalPlaces));

Output: 123.46

4. Using the String.format() method

The String.format() method can also be used to format a decimal number to a specific number of decimal places.

public static double formatString(double value, int decimalPlaces) {
  return Double.valueOf(String.format("%." + decimalPlaces + "f", value));
}

double value = 123.4567;
int decimalPlaces = 2;
System.out.println(formatString(value, decimalPlaces));

public static double formatNumber(double value, int decimalPlaces) {
  NumberFormat nf = NumberFormat.getInstance();
  nf.setMaximumFractionDigits(decimalPlaces);
  return Double.valueOf(nf.format(value));
}

Example:

double value = 123.4567;
int decimalPlaces = 2;
System.out.println(formatNumber(value, decimalPlaces));

Output: 123.46

6. Using the custom method

A custom method can also be used to round a decimal number to a specific number of decimal places by first multiplying the number by 10 to the power of the number of decimal places, then rounding to the nearest integer, and finally dividing by 10 to the power of the number of decimal places.

public static double customRoundingMethod(double value, int decimalPlaces) {
  double scale = Math.pow(10, decimalPlaces);
  return (double) Math.round(value * scale) / scale;
}

Example:

double value = 123.4567;
int decimalPlaces = 2;
System.out.println(customRoundingMethod(value, decimalPlaces));

Output: 123.46

7. Using the Banker’s Rounding method

The Banker’s Rounding method is a rounding method commonly used in financial applications where the rounding of midpoint values is done in a way that minimizes overall error. In Java, this method can be implemented using the BigDecimal class.

public static double bankersRounding(double value, int decimalPlaces) {
  BigDecimal bd = new BigDecimal(value);
  bd = bd.setScale(decimalPlaces, RoundingMode.HALF_EVEN);
  return bd.doubleValue();
}

Example:

double value = 123.4567;
int decimalPlaces = 2;
System.out.println(bankersRounding(value, decimalPlaces));

Output: 123.46

8. Using the Apache Commons Math library

The Apache Commons Math library provides a set of mathematical functions, including a method to round a decimal number to a specific number of decimal places.

import org.apache.commons.math3.util.Precision;

public static double commonsMathRounding(double value, int decimalPlaces) {
  return Precision.round(value, decimalPlaces);
}

Example:

double value = 123.4567;
int decimalPlaces = 2;
System.out.println(commonsMathRounding(value, decimalPlaces));

Output: 123.46

9. Using the Guava library

The Guava library provides a set of core libraries for Java, including a method to round a decimal number to a specific number of decimal places.

import com.google.common.math.DoubleMath;

public static double guavaRounding(double value, int decimalPlaces) {
  return DoubleMath.roundToInt(value, RoundingMode.HALF_EVEN);
}

Example:

double value = 123.4567;
int decimalPlaces = 2;
System.out.println(guavaRounding(value, decimalPlaces));

Output: 123.46

10. Using the JavaFX NumberFormat class

The JavaFX NumberFormat class provides a way to format and parse numbers, including rounding decimal numbers to a specific number of decimal places.

import java.text.NumberFormat;

public static double javafxNumberFormatRounding(double value, int decimalPlaces) {
  NumberFormat nf = NumberFormat.getInstance();
  nf.setMaximumFractionDigits(decimalPlaces);
  nf.setRoundingMode(RoundingMode.HALF_EVEN);
  return Double.parseDouble(nf.format(value));
}

Example:

double value = 123.4567;
int decimalPlaces = 2;
System.out.println(javafxNumberFormatRounding(value, decimalPlaces));

Output: 123.46

11. Using the JavaFX DecimalFormat class

The JavaFX DecimalFormat class provides a way to format decimal numbers, including rounding to a specific number of decimal places.

import java.text.DecimalFormat;

public static double javafxDecimalFormatRounding(double value, int decimalPlaces) {
  DecimalFormat df = new DecimalFormat("#." + String.join("", Collections.nCopies(decimalPlaces, "#")));
  df.setRoundingMode(RoundingMode.HALF_EVEN);
  return Double.parseDouble(df.format(value));
}

Example:

double value = 123.4567;
int decimalPlaces = 2;
System.out.println(javafxDecimalFormatRounding(value, decimalPlaces));

Output: 123.46

Conclusion

In Java, rounding a decimal number to a specific number of decimal places can be achieved using several methods including Math.round(), DecimalFormat, BigDecimal, String.format(), NumberFormat, a custom method, Banker’s Rounding, the Apache Commons Math library, the Guava library, the JavaFX NumberFormat class, and the JavaFX DecimalFormat class. Each method has its own strengths and weaknesses, and the method that is best for your application will depend on your specific requirements and accuracy requirements. Additionally, when using libraries for rounding, it is important to consider the impact that adding the library may have on your project.