squid:S2974 - Classes without public constructors should be final

null-object/src/main/java/com/iluwatar/nullobject/NullNode.java

@@ -29,7 +29,7 @@ package com.iluwatar.nullobject;
  * Implemented as Singleton, since all the NullNodes are the same.
  *
  */
-public class NullNode implements Node {
+public final class NullNode implements Node {
 
   private static NullNode instance = new NullNode();