Refactor Code and add Tests (#5251)

core-java/src/main/java/com/baeldung/string/DoubleToString.java

@@ -0,0 +1,41 @@
+package com.baeldung.string;
+
+import java.math.RoundingMode;
+import java.text.DecimalFormat;
+import java.text.NumberFormat;
+
+public class DoubleToString {
+
+    public static String truncateByCast(double d) {
+        return String.valueOf((int) d);
+    }
+
+    public static String roundWithStringFormat(double d) {
+        return String.format("%.0f", d);
+    }
+
+    public static String truncateWithNumberFormat(double d) {
+        NumberFormat nf = NumberFormat.getInstance();
+        nf.setMaximumFractionDigits(0);
+        nf.setRoundingMode(RoundingMode.FLOOR);
+        return nf.format(d);
+    }
+
+    public static String roundWithNumberFormat(double d) {
+        NumberFormat nf = NumberFormat.getInstance();
+        nf.setMaximumFractionDigits(0);
+        return nf.format(d);
+    }
+
+    public static String truncateWithDecimalFormat(double d) {
+        DecimalFormat df = new DecimalFormat("#,###");
+        df.setRoundingMode(RoundingMode.FLOOR);
+        return df.format(d);
+    }
+
+    public static String roundWithDecimalFormat(double d) {
+        DecimalFormat df = new DecimalFormat("#,###");
+        return df.format(d);
+    }
+
+}

core-java/src/test/java/com/baeldung/string/DoubleToStringUnitTest.java

@@ -0,0 +1,45 @@
+package com.baeldung.string;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import org.junit.Test;
+
+public class DoubleToStringUnitTest {
+    
+    private static final double DOUBLE_VALUE = 3.56;
+    private static final String TRUNCATED_DOUBLE = "3";
+    private static final String ROUNDED_UP_DOUBLE = "4";
+    
+    
+    @Test
+    public void truncateByCastTest() {
+        assertThat(DoubleToString.truncateByCast(DOUBLE_VALUE)).isEqualTo(TRUNCATED_DOUBLE);
+    }
+    
+    @Test
+    public void roundingWithStringFormatTest() {
+        assertThat(DoubleToString.roundWithStringFormat(DOUBLE_VALUE)).isEqualTo(ROUNDED_UP_DOUBLE);
+    }
+
+    @Test
+    public void truncateWithNumberFormatTest() {
+        assertThat(DoubleToString.truncateWithNumberFormat(DOUBLE_VALUE)).isEqualTo(TRUNCATED_DOUBLE);
+    }
+    
+    @Test
+    public void roundWithNumberFormatTest() {
+        assertThat(DoubleToString.roundWithNumberFormat(DOUBLE_VALUE)).isEqualTo(ROUNDED_UP_DOUBLE);
+    }
+
+    @Test
+    public void truncateWithDecimalFormatTest() {
+        assertThat(DoubleToString.truncateWithDecimalFormat(DOUBLE_VALUE)).isEqualTo(TRUNCATED_DOUBLE);
+    }
+    
+    @Test
+    public void roundWithDecimalFormatTest() {
+        assertThat(DoubleToString.roundWithDecimalFormat(DOUBLE_VALUE)).isEqualTo(ROUNDED_UP_DOUBLE);
+    }
+
+
+}