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/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;
+    }
+
+}