Fixed most reported issues by SonarCloud.

combinator/pom.xml

@@ -39,6 +39,12 @@
       <artifactId>junit</artifactId>
       <scope>test</scope>
     </dependency>
+
+    <dependency>
+      <groupId>org.junit.jupiter</groupId>
+      <artifactId>junit-jupiter-engine</artifactId>
+      <scope>test</scope>
+    </dependency>
   </dependencies>
 
 </project>

combinator/src/test/java/com/iluwatar/combinator/CombinatorAppTest.java

@@ -25,12 +25,19 @@ package com.iluwatar.combinator;
 
 import org.junit.Test;
 
-import static org.junit.Assert.*;
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
 
 public class CombinatorAppTest {
 
+  /**
+   * Issue: Add at least one assertion to this test case.
+   *
+   * Solution: Inserted assertion to check whether the execution of the main method in {@link CombinatorApp#main(String[])}
+   * throws an exception.
+   */
+
   @Test
-  public void main() {
-    CombinatorApp.main(new String[]{});
+  public void shouldExecuteApplicationWithoutException() {
+    assertDoesNotThrow(() -> CombinatorApp.main(new String[]{}));
   }
 }