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

*** 32,41 ****
--- 32,42 ----
   * @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.
       */