Java round double value to 2 decimal places Example

Mary ZhengApril 25th, 2019Last Updated: April 22nd, 2019

0 82 10 minutes read

1. Introduction

A double variable is used to hold a floating point value, such as 12.578. A decimal place is the position of a digit to the right of a decimal point. The 12.578 with 2 decimal places is represented as 12.57. Java has provided PrintStream, String.format, Formatter, NumberFormat, DecimalFormat, and BigDecimal to format and round a number based on the precision and RoundingMode since version 1.5.

In this example, I will create several junit test classes to demonstrate how to format and round a double value with 2 decimal places.

2. Technologies Used

The example code in this article was built and run using:

Java 1.8.101
Eclipse Oxygen
Junit
Maven 3.3.9

3. Maven Project

3.1 Dependency

Add Junit to the pom.xml.

pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
	<modelVersion>4.0.0</modelVersion>
	<groupId>java-round-double</groupId>
	<artifactId>java-round-double</artifactId>
	<version>0.0.1-SNAPSHOT</version>

	<properties>
		<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
	</properties>

	<build>
		<sourceDirectory>src</sourceDirectory>
		<plugins>
			<plugin>
				<artifactId>maven-compiler-plugin</artifactId>
				<version>3.8.0</version>
				<configuration>
					<release>11</release>
				</configuration>
			</plugin>
		</plugins>
	</build>

	<dependencies>
		<dependency>
			<groupId>junit</groupId>
			<artifactId>junit</artifactId>
			<version>4.12</version>
			<scope>test</scope>
		</dependency>
	</dependencies>

</project>

3.2 Constants

In this step, I will create a TEST_DOUBLES variable to hold a double[] array and a TWO_DECIMAL_PLACES_STRING_FORMAT variable to hold a String for 2 decimal places format which will be used in PrintStream, String, and Formatter classes.

TestConstants.java

package org.jcg.zheng;

public class TestConstants {

    public static double[] TEST_DOUBLES = { -123.345, 0.4, 0.446, 124, 124.56679 };

    public static final String TWO_DECIMAL_PLACES_STRING_FORMAT = "%.2f";

}

3.3 PrintStream

PrintStream provides printf and format methods. Here are the methods’ signatures:

PrintStream	printf(Locale l, String format, Object... args)
A convenience method to write a formatted string to this output stream using the specified format string and arguments.
PrintStream	printf(String format, Object... args)
A convenience method to write a formatted string to this output stream using the specified format string and arguments.
PrintStream	format(Locale l, String format, Object... args)
Writes a formatted string to this output stream using the specified format string and arguments.
PrintStream	format(String format, Object... args)
Writes a formatted string to this output stream using the specified format string and arguments.

In this step, I will create a junit test class to format a double value with 2 decimal places. The String format syntax for 2 decimal places is "%.2f" which is defined in the TestConstants class.

PrintStreamTest.java

package org.jcg.zheng;

import org.junit.Test;

public class PrintSteamTest {

    @Test
    public void test_printf() {
        for (double testDouble : TestConstants.TEST_DOUBLES) {
            System.out.print("Original double:" + testDouble);
            System.out.printf(", Rounded=" + TestConstants.TWO_DECIMAL_PLACES_STRING_FORMAT + "\n",
                    testDouble);
        }
    }

    @Test
    public void test_format() {
        for (double testDouble : TestConstants.TEST_DOUBLES) {
            System.out.format("Rounded=" + TestConstants.TWO_DECIMAL_PLACES_STRING_FORMAT + "\n",
                    testDouble);
        }
    }

}

3.4 String.format

The String class provides two static format methods. Here are the methods’ signatures:

static String	format(Locale l, String format, Object... args)
Returns a formatted string using the specified locale, format string, and arguments.
static String	format(String format, Object... args)
Returns a formatted string using the specified format string and arguments.

In this step, I will create a junit test class to format a double value with 2 decimal places and make sure the original double value is equal to the rounded value within 2 decimal places.

StringFormatTest.java

package org.jcg.zheng;

import static org.junit.Assert.assertEquals;

import org.junit.Test;

public class StringFormatTest {

    @Test
    public void test_print_String_format() {
        for (double testDouble : TestConstants.TEST_DOUBLES) {
            String doubleTwoDecimal = String.format(TestConstants.TWO_DECIMAL_PLACES_STRING_FORMAT,
                    testDouble);

            double roundedValue = Double.valueOf(doubleTwoDecimal);
            assertEquals(testDouble, roundedValue, 0.01);

            System.out.println("Rounded=" + doubleTwoDecimal);
        }
    }

}

3.5 Formatter

The Formatter class provides the format method. Here are the methods’ signatures:

Formatter	format(Locale l, String format, Object... args)
Writes a formatted string to this object's destination using the specified locale, format string, and arguments.
Formatter	format(String format, Object... args)
Writes a formatted string to this object's destination using the specified format string and arguments.

In this step, I will create a junit test class to format a double value with 2 decimal places.

FormatterTest.java

package org.jcg.zheng;

import static org.junit.Assert.assertEquals;

import java.util.Formatter;

import org.junit.Test;

public class FormaterTest {

    @Test
    public void test_Formatter() {
        for (double testDouble : TestConstants.TEST_DOUBLES) {
            Formatter fmt = new Formatter();
            fmt.format(TestConstants.TWO_DECIMAL_PLACES_STRING_FORMAT, testDouble);
            double roundedValue = Double.valueOf(fmt.toString());

            assertEquals(testDouble, roundedValue, 0.01);
            System.out.println("Rounded=" + roundedValue);
            fmt.close();
        }
    }

}

3.6 NumberFormat

The NumberFormat class provides the format method. Here are the methods’ signatures:

String	format(double number)
Specialization of format.
abstract StringBuffer	format(double number, StringBuffer toAppendTo, FieldPosition pos)
Specialization of format.

In this step, I will create a junit test class to format a double value with 2 decimal places with Java 8 steam API.

NumberFormatTest.java

package org.jcg.zheng;

import static org.junit.Assert.assertEquals;

import java.math.RoundingMode;
import java.text.NumberFormat;
import java.util.stream.DoubleStream;
import java.util.stream.Stream;

import org.junit.Test;

public class NumberFormatTest {

    private DoubleStream doubleStream = DoubleStream.of(-123.345, 0.4, 0.446, 124, 124.56679);

    @Test
    public void test_NumberFormat_format() {
        NumberFormat nf = NumberFormat.getInstance();
        nf.setMaximumFractionDigits(2);
        nf.setMinimumFractionDigits(2);

        doubleStream.forEach(testDouble -> {
            Stream.of(RoundingMode.values()).forEach(roundingMode -> {
                nf.setRoundingMode(roundingMode);
                System.out.print("Original double:" + testDouble);
                try {
                    double roundedDouble = Double.valueOf(nf.format(testDouble));

                    assertEquals(testDouble, roundedDouble, 0.01);

                    System.out.println(
                            " " + roundingMode.name() + ", formatted double:" + roundedDouble);
                } catch (Exception e) {
                    System.out.println(
                            " " + roundingMode.name() + "Caught exception" + e.getMessage());
                }

            });
        });

    }

}

3.7 DecimalFormat

The DecimalFormat class provides a format method. Here is the method’s signature:

StringBuffer	format(double number, StringBuffer result, FieldPosition fieldPosition)
Formats a double to produce a string.

In this step, I will create a junit test class to format a double value with 2 decimal places.

Note:

0 – prints a digit if provided, otherwise prints 0.
# – prints a digit if provided, otherwise prints nothing.

DecimalFormatTest.java

package org.jcg.zheng;

import static org.junit.Assert.assertEquals;

import java.math.RoundingMode;
import java.text.DecimalFormat;

import org.junit.Test;

public class DecimalFormatTest {

    private final String TWO_DECIMAL_FORMAT_PAD_ZERO_AFTER = "#.00";
    private final String TWO_DECIMAL_FORMAT_PAD_ZERO_BEFORE = "0.##";
    private final String TWO_DECIMAL_FORMAT_NOT_PAD = "#.##";
    private final String TWO_DECIMAL_FORMAT_PAD_ZERO = "0.00";

    public String[] TWO_PLACES_FORMATS = { TWO_DECIMAL_FORMAT_PAD_ZERO_AFTER,
        TWO_DECIMAL_FORMAT_PAD_ZERO_BEFORE, TWO_DECIMAL_FORMAT_NOT_PAD,
        TWO_DECIMAL_FORMAT_PAD_ZERO };

    @Test
    public void test_DecimalFormat() {
        for (double testDouble : TestConstants.TEST_DOUBLES) {
            for (String decimalFormat : TWO_PLACES_FORMATS) {
                DecimalFormat df = new DecimalFormat(decimalFormat);
                for (RoundingMode rm : RoundingMode.values()) {
                    if (!RoundingMode.UNNECESSARY.equals(rm)) {
                        df.setRoundingMode(rm);
                        double roundedValue = Double.valueOf(df.format(testDouble));
                        assertEquals(testDouble, roundedValue, 0.01);
                        printDouble(testDouble, df.format(testDouble), rm, decimalFormat);
                    }
                }
            }
        }
    }

    private void printDouble(double testDouble, String rounded, RoundingMode rm,
            String decimalFormat) {
        System.out.println("Original=" + testDouble + " Rounded with \"" + decimalFormat + "\" "
                + rm.name() + " =" + rounded);
    }

}

3.8 BigDecimal.round

The BigDecimal class provides a round method to round a double value based on a MathContext. Here is the method’s signature:

BigDecimal	round(MathContext mc)
Returns a BigDecimal rounded according to the MathContext settings.

In this step, I will create a junit test class to format a double value with 2 decimal places.

DecimalRoundTest.java

package org.jcg.zheng;

import static org.junit.Assert.assertEquals;

import java.math.BigDecimal;
import java.math.RoundingMode;

import org.junit.Test;

public class BigDecimalRoundTest {

    private static final double TEST_DOUBLE = 0.56679;

    @Test
    public void test_round_2() {
        BigDecimal bigDec = BigDecimal.valueOf(TEST_DOUBLE);

        System.out.println("original double: " + TEST_DOUBLE);

        for (RoundingMode rm : RoundingMode.values()) {
            System.out.print("\tRoundingMode=" + rm.name());
            try {
                double roundValue = bigDec.setScale(2, rm).doubleValue();

                assertEquals(TEST_DOUBLE, roundValue, 0.01);

                System.out.println("\t With 2 decimal places: " + roundValue);
            } catch (ArithmeticException ae) {
                System.out.println("Caught exception for " + rm.name());
            }
        }
    }

    @Test
    public void test_round_2and3() {
        BigDecimal bigDec = BigDecimal.valueOf(TEST_DOUBLE);

        double round2 = bigDec.setScale(2, BigDecimal.ROUND_CEILING).doubleValue();

        double round3 = bigDec.setScale(3, BigDecimal.ROUND_CEILING).doubleValue();

        assertEquals(0.57, round2, 0.01);
        assertEquals(0.567, round3, 0.001);
    }

}