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

@@ -136,10 +136,11 @@
  * @since   1.2
  */
 public class HashMap<K,V> extends AbstractMap<K,V>
     implements Map<K,V>, Cloneable, Serializable {
 
+    @java.io.Serial
     private static final long serialVersionUID = 362498820763181265L;
 
     /*
      * Implementation notes.
      *

@@ -1487,10 +1488,11 @@
      *             <i>size</i> (an int, the number of key-value
      *             mappings), followed by the key (Object) and value (Object)
      *             for each key-value mapping.  The key-value mappings are
      *             emitted in no particular order.
      */
+    @java.io.Serial
     private void writeObject(java.io.ObjectOutputStream s)
         throws IOException {
         int buckets = capacity();
         // Write out the threshold, loadfactor, and any hidden stuff
         s.defaultWriteObject();

@@ -1504,10 +1506,11 @@
      * @param s the stream
      * @throws ClassNotFoundException if the class of a serialized object
      *         could not be found
      * @throws IOException if an I/O error occurs
      */
+    @java.io.Serial
     private void readObject(java.io.ObjectInputStream s)
         throws IOException, ClassNotFoundException {
         // Read in the threshold (ignored), loadfactor, and any hidden stuff
         s.defaultReadObject();
         reinitialize();