BAEL-821 - How to round a number to n decimal places in Java (#1953)

* Code for Dependency Injection Article.

* Added Java based configuration. Downloaded formatter.xml and reformatted
all changed files. Manually changed tab into 4 spaces in XML
configuration files.

* BAEL-434 - Spring Roo project files generated by Spring Roo. No
formatting applied. Added POM, java and resources folders.

* Moved project from roo to spring-roo folder.

* BAEL-838 Initial code showing how to remove last char - helper class and tests.

* BAEL-838 Corrected Helper class and associated empty string test case. Added StringUtils.substing tests.

* BAEL-838 Refromatted code using formatter.xml. Added Assert.assertEquals import. Renamed test to follow convention. Reordered tests.

* BAEL-838 - Added regex method and updated tests.

* BAEL-838 Added new line examples.

* BAEL-838 Renamed RemoveLastChar class to StringHelper and added Java8 examples. Refactord code.

* BAEL-838 Changed method names

* BAEL-838 Tiny change to keep code consistant. Return null or empty.

* BAEL-838 Removed unresolved conflict.

* BAEL-821 New class that shows different rounding techniques. Updated POM.

* BAEL-821 - Added unit test for different round methods.

* BAEL-821 Changed test method name to follow the convention

core-java/pom.xml

@@ -45,6 +45,12 @@
 			<artifactId>commons-math3</artifactId>
 			<version>${commons-math3.version}</version>
 		</dependency>
+		
+		<dependency>
+    		<groupId>org.decimal4j</groupId>
+    		<artifactId>decimal4j</artifactId>
+    		<version>${decimal4j.version}</version>
+		</dependency>
 
 		<dependency>
 			<groupId>org.bouncycastle</groupId>
@@ -369,6 +375,7 @@
 		<bouncycastle.version>1.55</bouncycastle.version>
 		<commons-codec.version>1.10</commons-codec.version>
 		<commons-math3.version>3.6.1</commons-math3.version>
+		<decimal4j.version>1.0.3</decimal4j.version>
 		<commons-io.version>2.5</commons-io.version>
 		<commons-collections4.version>4.1</commons-collections4.version>
 		<collections-generic.version>4.01</collections-generic.version>

core-java/src/main/java/com/baeldung/maths/Round.java

@@ -0,0 +1,38 @@
+package com.baeldung.maths;
+
+import java.math.BigDecimal;
+import java.math.RoundingMode;
+import java.text.DecimalFormat;
+
+import org.apache.commons.math3.util.Precision;
+import org.decimal4j.util.DoubleRounder;
+
+public class Round {
+    private static final double PI = 3.1415d;
+    
+    public static void main (String args[]) {
+        System.out.println("PI: " + PI);
+        System.out.printf("Value with 3 digits after decimal point %.3f %n", PI);
+        // OUTPUTS: Value with 3 digits after decimal point 3.142
+        DecimalFormat df = new DecimalFormat("###.###");
+        System.out.println(df.format(PI));
+        System.out.println(round(PI, 3));
+        System.out.println(roundOptional(PI, 3));
+        System.out.println(Precision.round(PI, 3));
+        System.out.println(DoubleRounder.round(PI, 3));
+    }
+    
+    public static double round(double value, int places) {
+        if (places < 0) throw new IllegalArgumentException();
+        
+        BigDecimal bd = new BigDecimal(value);
+        bd = bd.setScale(places, RoundingMode.HALF_UP);
+        return bd.doubleValue();
+    }
+    
+    public static double roundOptional(double value, int places) {
+        double scale = Math.pow(10, places);
+        double rounded = Math.round(value * scale) / scale;
+        return rounded;
+    }
+}

core-java/src/test/java/com/baeldung/maths/RoundTest.java

@@ -0,0 +1,29 @@
+package com.baeldung.maths;
+
+import org.apache.commons.math3.util.Precision;
+import org.decimal4j.util.DoubleRounder;
+import org.junit.Assert;
+import org.junit.Test;
+
+public class RoundTest {
+    private double value = 2.03456d;
+    private int places = 2;
+    private double delta = 0.0d;
+    private double expected = 2.03d;
+
+    @Test
+    public void givenDecimalNumber_whenRoundToNDecimalPlaces_thenGetExpectedResult() {   
+        Assert.assertEquals(expected, Round.round(value, places), delta);
+        Assert.assertEquals(expected, Round.roundOptional(value, places), delta);
+        Assert.assertEquals(expected, Precision.round(value, places), delta);
+        Assert.assertEquals(expected, DoubleRounder.round(value, places), delta);
+        
+        places = 3;
+        expected = 2.035d;
+        
+        Assert.assertEquals(expected, Round.round(value, places), delta);
+        Assert.assertEquals(expected, Round.roundOptional(value, places), delta);
+        Assert.assertEquals(expected, Precision.round(value, places), delta);
+        Assert.assertEquals(expected, DoubleRounder.round(value, places), delta);
+    }
+}