src/java.base/share/classes/java/time/YearMonth.java

*** 132,141 ****
--- 132,142 ----
          implements Temporal, TemporalAdjuster, Comparable<YearMonth>, Serializable {
  
      /**
       * Serialization version.
       */
+     @java.io.Serial
      private static final long serialVersionUID = 4183400860270640070L;
      /**
       * Parser.
       */
      private static final DateTimeFormatter PARSER = new DateTimeFormatterBuilder()
*** 1220,1239 ****
--- 1221,1242 ----
       *  out.writeByte(month);
       * </pre>
       *
       * @return the instance of {@code Ser}, not null
       */
+     @java.io.Serial
      private Object writeReplace() {
          return new Ser(Ser.YEAR_MONTH_TYPE, this);
      }
  
      /**
       * Defend against malicious streams.
       *
       * @param s the stream to read
       * @throws InvalidObjectException always
       */
+     @java.io.Serial
      private void readObject(ObjectInputStream s) throws InvalidObjectException {
          throw new InvalidObjectException("Deserialization via serialization delegate");
      }
  
      void writeExternal(DataOutput out) throws IOException {