Add Format Number implementation and tests

java-numbers/src/main/java/com/baeldung/formatNumber/FormatNumber.java

@@ -0,0 +1,63 @@
+package com.baeldung.formatNumber;
+
+import java.math.BigDecimal;
+import java.math.RoundingMode;
+import java.text.DecimalFormat;
+import java.text.NumberFormat;
+import java.util.Locale;
+
+public class FormatNumber {
+        private static final double D = 4.2352989244d;
+        private static final double F = 8.6994540927d;
+
+        public static void main(String[] args) {
+                System.out.println(D + " with Big Decimal (2 places) is " + withBigDecimal(D, 2));
+                System.out.println(D + " with Big Decimal (3 places) is " + withBigDecimal(D, 3));
+                System.out.println(F + " with Big Decimal (2 places) is " + withBigDecimal(F, 2));
+                System.out.println(F + " with Big Decimal (3 places) is " + withBigDecimal(F, 3));
+                System.out.println(D + " with Math.round is (2 places) " + withMathRound(D, 2));
+                System.out.println(D + " with Math.round is (3 places) " + withMathRound(D, 3));
+                System.out.println(F + " with Math.round is (2 places) " + withMathRound(F, 2));
+                System.out.println(F + " with Math.round is (3 places) " + withMathRound(F, 3));
+                System.out.println(D + " with String Format is (2 places) " + withStringFormat(D, 2));
+                System.out.println(D + " with String Format is (3 places) " + withStringFormat(D, 3));
+                System.out.println(F + " with String Format is (2 places) " + withStringFormat(F, 2));
+                System.out.println(F + " with String Format is (3 places) " + withStringFormat(F, 3));
+                System.out.println(D + " with Decimal Format (local) is " + withDecimalFormat(D, 0));
+                System.out.println(D + " with Decimal Format (2 places) is " + withDecimalFormat(D, 2));
+                System.out.println(D + " with Decimal Format (3 places) is " + withDecimalFormat(D, 3));
+                System.out.println(F + " with Decimal Format is (local) " + withDecimalFormat(F, 0));
+                System.out.println(F + " with Decimal Format is (2 places) " + withDecimalFormat(F, 2));
+                System.out.println(F + " with Decimal Format is (3 places) " + withDecimalFormat(F, 3));
+        }
+
+        public static double withBigDecimal(double value, int places) {
+                if (places < 0)
+                        throw new IllegalArgumentException();
+
+                BigDecimal bigDecimal = new BigDecimal(value);
+                bigDecimal = bigDecimal.setScale(places, RoundingMode.HALF_UP);
+                return bigDecimal.doubleValue();
+        }
+
+        public static double withMathRound(double value, int places) {
+                double scale = Math.pow(10, places);
+                return Math.round(value * scale) / scale;
+        }
+
+        public static double withDecimalFormat(double value, int places) {
+                DecimalFormat df = (DecimalFormat) NumberFormat.getNumberInstance(Locale.getDefault());
+                DecimalFormat df2 = new DecimalFormat("#,###,###,##0.00");
+                DecimalFormat df3 = new DecimalFormat("#,###,###,##0.000");
+                if (places == 2)
+                        return new Double(df2.format(value));
+                else if (places == 3)
+                        return new Double(df3.format(value));
+                else
+                        return new Double(df.format(value));
+        }
+
+        public static String withStringFormat(double value, int places) {
+                return String.format("%." + places + "f", value);
+        }
+}

java-numbers/src/test/java/com/baeldung/formatNumber/FormatNumberTest.java

@@ -0,0 +1,42 @@
+package com.baeldung.formatNumber;
+
+import org.apache.commons.math3.util.Precision;
+import org.decimal4j.util.DoubleRounder;
+import org.junit.Assert;
+import org.junit.Test;
+
+public class FormatNumberTest {
+        private double value = 2.03456d;
+        private int places = 2;
+        private double expected = 2.03d;
+
+        @Test public void givenDecimalNumber_whenFormatNumberToNDecimalPlaces_thenGetExpectedResult() {
+                double delta = 0.0d;
+                Assert.assertEquals(expected, FormatNumber.withBigDecimal(value, places), delta);
+                Assert.assertEquals(expected, FormatNumber.withDecimalFormat(value, places), delta);
+                Assert.assertEquals(expected, FormatNumber.withMathRound(value, places), delta);
+
+                places = 3;
+                expected = 2.035d;
+
+                Assert.assertEquals(expected, FormatNumber.withBigDecimal(value, places), delta);
+                Assert.assertEquals(expected, FormatNumber.withDecimalFormat(value, places), delta);
+                Assert.assertEquals(expected, FormatNumber.withMathRound(value, places), delta);
+
+                value = 256.024d;
+                places = 2;
+                expected = 256.02d;
+
+                Assert.assertEquals(expected, FormatNumber.withBigDecimal(value, places), delta);
+                Assert.assertEquals(expected, FormatNumber.withDecimalFormat(value, places), delta);
+                Assert.assertEquals(expected, FormatNumber.withMathRound(value, places), delta);
+
+                value = 260.773d;
+                places = 2;
+                expected = 260.77d;
+
+                Assert.assertEquals(expected, FormatNumber.withBigDecimal(value, places), delta);
+                Assert.assertEquals(expected, FormatNumber.withDecimalFormat(value, places), delta);
+                Assert.assertEquals(expected, FormatNumber.withMathRound(value, places), delta);
+        }
+}