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

@@ -492,10 +492,11 @@
      *             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();
 

@@ -512,10 +513,11 @@
 
     /**
      * 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();
 

@@ -554,7 +556,8 @@
      */
     public Spliterator<E> spliterator() {
         return TreeMap.keySpliteratorFor(m);
     }
 
+    @java.io.Serial
     private static final long serialVersionUID = -2479143000061671589L;
 }