BAEL-5461 code for the Unit Test Private Methods in Java article (#12240)

Co-authored-by: thibault.faure <thibault.faure@mimacom.com>

core-java-modules/core-java-reflection-2/pom.xml

@@ -47,6 +47,25 @@
                     <compilerArgument>-parameters</compilerArgument>
                 </configuration>
             </plugin>
+            <plugin>
+                <groupId>org.jacoco</groupId>
+                <artifactId>jacoco-maven-plugin</artifactId>
+                <version>0.8.8</version>
+                <executions>
+                    <execution>
+                        <goals>
+                            <goal>prepare-agent</goal>
+                        </goals>
+                    </execution>
+                    <execution>
+                        <id>report</id>
+                        <phase>prepare-package</phase>
+                        <goals>
+                            <goal>report</goal>
+                        </goals>
+                    </execution>
+                </executions>
+            </plugin>
         </plugins>
     </build>
 

core-java-modules/core-java-reflection-2/src/main/java/com/baeldung/reflection/privatemethods/Utils.java

@@ -0,0 +1,19 @@
+package com.baeldung.reflection.privatemethods;
+
+public class Utils {
+
+    public static Integer validateAndDouble(Integer input) {
+        if (input == null) {
+            throw new IllegalArgumentException("input should not be null");
+        }
+        return doubleInteger(input);
+    }
+
+    private static Integer doubleInteger(Integer input) {
+        if (input == null) {
+            return null;
+        }
+        return 2 * input;
+    }
+
+}

core-java-modules/core-java-reflection-2/src/test/java/com/baeldung/reflection/privatemethods/UtilsUnitTest.java

@@ -0,0 +1,42 @@
+package com.baeldung.reflection.privatemethods;
+
+import static com.baeldung.reflection.privatemethods.Utils.validateAndDouble;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import java.lang.reflect.InvocationTargetException;
+import java.lang.reflect.Method;
+
+import org.junit.jupiter.api.Test;
+
+public class UtilsUnitTest {
+
+    // Let's start with the tests of the public API
+    @Test
+    void givenNull_WhenValidateAndDouble_ThenThrows() {
+        assertThrows(IllegalArgumentException.class, () -> validateAndDouble(null));
+    }
+
+    @Test
+    void givenANonNullInteger_WhenValidateAndDouble_ThenDoublesIt() {
+        assertEquals(4, validateAndDouble(2));
+    }
+
+    // Further on, let's test the private method
+    @Test
+    void givenNull_WhenDoubleInteger_ThenNull() throws InvocationTargetException, IllegalAccessException, NoSuchMethodException {
+        assertEquals(null, getDoubleIntegerMethod().invoke(null, new Integer[] { null }));
+    }
+
+    @Test
+    void givenANonNullInteger_WhenDoubleInteger_ThenDoubleIt() throws NoSuchMethodException, InvocationTargetException, IllegalAccessException {
+        assertEquals(74, getDoubleIntegerMethod().invoke(null, 37));
+    }
+
+    private Method getDoubleIntegerMethod() throws NoSuchMethodException {
+        Method method = Utils.class.getDeclaredMethod("doubleInteger", Integer.class);
+        method.setAccessible(true);
+        return method;
+    }
+
+}