Fixed most reported issues by SonarCloud.

subclass-sandbox/pom.xml

@@ -44,6 +44,11 @@
             <groupId>com.github.stefanbirkner</groupId>
             <artifactId>system-rules</artifactId>
         </dependency>
+        <dependency>
+            <groupId>org.junit.jupiter</groupId>
+            <artifactId>junit-jupiter-engine</artifactId>
+            <scope>test</scope>
+        </dependency>
     </dependencies>
 
 </project>

subclass-sandbox/src/test/java/com/iluwatar/subclasssandbox/AppTest.java

@@ -25,13 +25,15 @@ package com.iluwatar.subclasssandbox;
 
 import org.junit.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * App unit tests.
  */
 public class AppTest {
 
   @Test
-  public void testMain() {
-    App.main(new String[]{});
+  public void shouldExecuteWithoutException() {
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }