diff --git a/java-numbers-3/pom.xml b/java-numbers-3/pom.xml
index bf5fe9b0e7..495618885a 100644
--- a/java-numbers-3/pom.xml
+++ b/java-numbers-3/pom.xml
@@ -12,15 +12,32 @@
         <version>0.0.1-SNAPSHOT</version>
         <relativePath>../parent-java</relativePath>
     </parent>
-    
+
     <dependencies>
-    	<dependency>
+        <dependency>
             <groupId>it.unimi.dsi</groupId>
             <artifactId>dsiutils</artifactId>
             <version>${dsiutils.version}</version>
         </dependency>
+        <dependency>
+            <groupId>io.vavr</groupId>
+            <artifactId>vavr</artifactId>
+            <version>${vavr.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>org.apache.commons</groupId>
+            <artifactId>commons-lang3</artifactId>
+            <version>${commons.version}</version>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>org.assertj</groupId>
+            <artifactId>assertj-core</artifactId>
+            <version>${assertj.version}</version>
+            <scope>test</scope>
+        </dependency>
     </dependencies>
-   
+
     <build>
         <finalName>java-numbers-3</finalName>
         <resources>
@@ -33,6 +50,9 @@
 
     <properties>
         <dsiutils.version>2.6.0</dsiutils.version>
+        <vavr.version>0.10.2</vavr.version>
+        <commons.version>3.9</commons.version>
+        <assertj.version>3.6.1</assertj.version>
     </properties>
 
 </project>
diff --git a/java-numbers-3/src/test/java/com/baeldung/parsedouble/StringToDoubleParserUnitTest.java b/java-numbers-3/src/test/java/com/baeldung/parsedouble/StringToDoubleParserUnitTest.java
new file mode 100644
index 0000000000..b3dec3d14f
--- /dev/null
+++ b/java-numbers-3/src/test/java/com/baeldung/parsedouble/StringToDoubleParserUnitTest.java
@@ -0,0 +1,149 @@
+package com.baeldung.parsedouble;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.util.Optional;
+
+import org.apache.commons.lang3.math.NumberUtils;
+import org.junit.Test;
+
+import com.google.common.base.MoreObjects;
+import com.google.common.primitives.Doubles;
+
+import io.vavr.control.Try;
+
+public class StringToDoubleParserUnitTest {
+
+    @Test
+    public void givenNullValue_whenParseStringToDouble_thenDefaultNaNValueIsReturned() {
+        assertThat(parseStringToDouble(null)).isNaN();
+    }
+
+    @Test
+    public void givenEmptyStringValue_whenParseStringToDouble_thenDefaultNaNValueIsReturned() {
+        assertThat(parseStringToDouble("")).isNaN();
+    }
+
+    @Test
+    public void givenStringValue_whenParseStringToDouble_thenDoubleValueIsReturned() {
+        assertThat(parseStringToDouble("1")).isEqualTo(1.0d);
+    }
+
+    @Test
+    public void givenStringValue_whenParseStringToDoubleWithDefault_thenDoubleValueIsReturned() {
+        assertThat(parseStringToDouble("1", 2.0d)).isEqualTo(1.0d);
+    }
+
+    @Test
+    public void givenEmptyStringValue_whenParseStringToDoubleWithDefault_thenDefaultValueIsReturned() {
+        assertThat(parseStringToDouble("", 1.0d)).isEqualTo(1.0d);
+    }
+
+    @Test
+    public void givenNullValue_whenParseStringToDoubleWithDefault_thenDefaultValueIsReturned() {
+        assertThat(parseStringToDouble(null, 1.0d)).isEqualTo(1.0d);
+    }
+
+    @Test
+    public void givenStringValue_whenParseStringToOptionalDouble_thenOptionalValueIsReturned() {
+        assertThat(parseStringToOptionalDouble("1")).isEqualTo(Optional.of(1.0d));
+    }
+
+    @Test
+    public void givenNullValue_whenParseStringToOptionalDouble_thenOptionalValueIsEmpty() {
+        assertThat(parseStringToOptionalDouble(null)).isEqualTo(Optional.empty());
+    }
+
+    @Test
+    public void givenEmptyStringValue_whenParseStringToOptionalDouble_thenOptionalValueIsEmpty() {
+        assertThat(parseStringToOptionalDouble("")).isEqualTo(Optional.empty());
+    }
+
+    @Test
+    public void givenEmptyStringValue_whenParseStringToOptionalDouble_thenDefaulOptionalValueIsReturned() {
+        assertThat(parseStringToOptionalDouble("").orElse(1.0d)).isEqualTo(1.0d);
+    }
+
+    @Test
+    public void givenNullValue_whenParseStringToOptionalDouble_thenDefaulOptionalValueIsReturned() {
+        assertThat(parseStringToOptionalDouble(null).orElse(1.0d)).isEqualTo(1.0d);
+    }
+
+    @Test
+    public void givenStringValue_whenTryStringToDouble_thenDoubleValueIsReturned() {
+        assertThat(tryStringToDouble("1", 2.0d)).isEqualTo(1.0d);
+    }
+
+    @Test
+    public void givenNullValue_whenTryStringToDoubleWithDefault_thenDoubleValueIsReturned() {
+        assertThat(tryStringToDouble(null, 2.0d)).isEqualTo(2.0d);
+    }
+
+    @Test
+    public void givenEmptyStringValue_whenTryStringToDoubleWithDefault_thenDoubleValueIsReturned() {
+        assertThat(tryStringToDouble("", 2.0d)).isEqualTo(2.0d);
+    }
+
+    @Test
+    public void givenTwoStringValues_whenTryParseFirstNonNull_thenDoubleValueIsReturned() {
+        assertThat(Doubles.tryParse(MoreObjects.firstNonNull("1.0", "2.0"))).isEqualTo(1.0d);
+    }
+
+    @Test
+    public void givenNullStringValue_whenTryParseFirstNonNull_thenSecondDoubleValueIsReturned() {
+        assertThat(Doubles.tryParse(MoreObjects.firstNonNull(null, "2.0"))).isEqualTo(2.0d);
+    }
+
+    @Test
+    public void givenEmptyStringValue_whenTryParseFirstNonNull_thenNullIsReturned() {
+        assertThat(Doubles.tryParse(MoreObjects.firstNonNull("", "2.0"))).isEqualTo(null);
+    }
+
+    @Test
+    public void givenStringValue_whenToDouble_thenDoubleValueIsReturned() {
+        assertThat(NumberUtils.toDouble("1.0")).isEqualTo(1.0d);
+    }
+
+    @Test
+    public void givenNullValue_whenToDouble_thenLibraryDefaultDoubleValueIsReturned() {
+        String nullString = null;
+        assertThat(NumberUtils.toDouble(nullString)).isEqualTo(0.0d);
+    }
+
+    @Test
+    public void givenEmptyStringValue_whenToDouble_thenLibraryDefaultDoubleValueIsReturned() {
+        assertThat(NumberUtils.toDouble("")).isEqualTo(0.0d);
+    }
+
+    @Test
+    public void givenEmptyStringValue_whenToDoubleWithDefault_thenDoubleValueIsReturned() {
+        assertThat(NumberUtils.toDouble("", 2.0d)).isEqualTo(2.0d);
+    }
+
+    @Test
+    public void givenNullValue_whenToDoubleWithDefault_thenDoubleValueIsReturned() {
+        String nullString = null;
+        assertThat(NumberUtils.toDouble(nullString, 2.0d)).isEqualTo(2.0d);
+    }
+
+    @Test
+    public void givenStringValue_whenToDoubleWithDefault_thenDoubleValueIsReturned() {
+        assertThat(NumberUtils.toDouble("1.0", 2.0d)).isEqualTo(1.0d);
+    }
+
+    private static Optional<Double> parseStringToOptionalDouble(String value) {
+        return value == null || value.isEmpty() ? Optional.empty() : Optional.of(Double.valueOf(value));
+    }
+
+    private static double parseStringToDouble(String value) {
+        return value == null || value.isEmpty() ? Double.NaN : Double.parseDouble(value);
+    }
+
+    private static double parseStringToDouble(String value, double defaultValue) {
+        return value == null || value.isEmpty() ? defaultValue : Double.parseDouble(value);
+    }
+
+    private static double tryStringToDouble(String value, double defaultValue) {
+        return Try.of(() -> Double.parseDouble(value)).getOrElse(defaultValue);
+    }
+}