BAEL-4384: Invoke a private method using reflection (#10470)

* - initial commit of sample code

* BAEL-4209:  Moving the localized exception module into core-java-exceptions-3.

* BAEL-4209: Removed the old files for localizing exception messages.

* Added test case to explain invoking a private method.

* Made the code example more useful so we don't look like we're hacking another framework.

* Forgot to make the test class and method package-private.

* Incorporated Spring ReflectionTestUtils into the code sample.

* Updated the Spring Test version to match the article.

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

@@ -14,6 +14,15 @@
         <relativePath>../</relativePath>
     </parent>
     
+    <dependencies>
+        <dependency>
+            <groupId>org.springframework</groupId>
+            <artifactId>spring-test</artifactId>
+            <version>${spring.version}</version>
+            <scope>test</scope>
+        </dependency>
+    </dependencies>
+    
     <build>
         <finalName>core-java-reflection-2</finalName>
         <resources>
@@ -40,5 +49,6 @@
         <maven-compiler-plugin.version>3.8.0</maven-compiler-plugin.version>
         <source.version>1.8</source.version>
         <target.version>1.8</target.version>
+        <spring.version>5.3.4</spring.version>
     </properties>
 </project>

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

@@ -0,0 +1,18 @@
+package com.baeldung.reflection.access.privatemethods;
+
+public class LongArrayUtil {
+
+    public static int indexOf(long[] array, long target) {
+        return indexOf(array, target, 0, array.length);
+    }
+
+    private static int indexOf(long[] array, long target, int start, int end) {
+        for (int i = start; i < end; i++) {
+            if (array[i] == target) {
+                return i;
+            }
+        }
+        return -1;
+    }
+
+}

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

@@ -0,0 +1,28 @@
+package com.baeldung.reflection.access.privatemethods;
+
+import org.junit.jupiter.api.Test;
+import org.springframework.test.util.ReflectionTestUtils;
+
+import java.lang.reflect.Method;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+class InvokePrivateMethodsUnitTest {
+
+    private final long[] someLongArray = new long[] { 1L, 2L, 1L, 4L, 2L };
+
+    @Test
+    void whenSearchingForLongValueInSubsequenceUsingReflection_thenTheCorrectIndexOfTheValueIsReturned() throws Exception {
+        Method indexOfMethod = LongArrayUtil.class.getDeclaredMethod("indexOf", long[].class, long.class, int.class, int.class);
+        indexOfMethod.setAccessible(true);
+
+        assertEquals(2, indexOfMethod.invoke(LongArrayUtil.class, someLongArray, 1L, 1, someLongArray.length), "The index should be 2.");
+    }
+
+    @Test
+    void whenSearchingForLongValueInSubsequenceUsingSpring_thenTheCorrectIndexOfTheValueIsReturned() throws Exception {
+        int indexOfSearchTarget = ReflectionTestUtils.invokeMethod(LongArrayUtil.class, "indexOf", someLongArray, 1L, 1, someLongArray.length);
+        assertEquals(2, indexOfSearchTarget, "The index should be 2.");
+    }
+
+}