src/java.base/share/classes/java/lang/StringBuilder.java

*** 86,95 ****
--- 86,96 ----
      extends AbstractStringBuilder
      implements java.io.Serializable, Comparable<StringBuilder>, CharSequence
  {
  
      /** use serialVersionUID for interoperability */
+     @java.io.Serial
      static final long serialVersionUID = 4383685877147921099L;
  
      /**
       * Constructs a string builder with no characters in it and an
       * initial capacity of 16 characters.
*** 456,465 ****
--- 457,467 ----
       *             string builder ({@code char[]}).   The length of the
       *             {@code char} array may be greater than the number of
       *             characters currently stored in the string builder, in which
       *             case extra characters are ignored.
       */
+     @java.io.Serial
      private void writeObject(java.io.ObjectOutputStream s)
          throws java.io.IOException {
          s.defaultWriteObject();
          s.writeInt(count);
          char[] val = new char[capacity()];
*** 473,482 ****
--- 475,485 ----
  
      /**
       * readObject is called to restore the state of the StringBuffer from
       * a stream.
       */
+     @java.io.Serial
      private void readObject(java.io.ObjectInputStream s)
          throws java.io.IOException, ClassNotFoundException {
          s.defaultReadObject();
          count = s.readInt();
          char[] val = (char[]) s.readObject();