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

*** 492,501 ****
--- 492,502 ----
       *             elements it contains) (int), followed by all of its
       *             elements (each an Object) in order (as determined by the
       *             set's Comparator, or by the elements' natural ordering if
       *             the set has no Comparator).
       */
+     @java.io.Serial
      private void writeObject(java.io.ObjectOutputStream s)
          throws java.io.IOException {
          // Write out any hidden stuff
          s.defaultWriteObject();
  
*** 512,521 ****
--- 513,523 ----
  
      /**
       * Reconstitute the {@code TreeSet} instance from a stream (that is,
       * deserialize it).
       */
+     @java.io.Serial
      private void readObject(java.io.ObjectInputStream s)
          throws java.io.IOException, ClassNotFoundException {
          // Read in any hidden stuff
          s.defaultReadObject();
  
*** 554,560 ****
--- 556,563 ----
       */
      public Spliterator<E> spliterator() {
          return TreeMap.keySpliteratorFor(m);
      }
  
+     @java.io.Serial
      private static final long serialVersionUID = -2479143000061671589L;
  }