src/java.base/share/classes/java/util/RegularEnumSet.java

@@ -32,10 +32,11 @@
  * @author Josh Bloch
  * @since 1.5
  * @serial exclude
  */
 class RegularEnumSet<E extends Enum<E>> extends EnumSet<E> {
+    @java.io.Serial
     private static final long serialVersionUID = 3411599620347842686L;
     /**
      * Bit vector representation of this set.  The 2^k bit indicates the
      * presence of universe[k] in this set.
      */