diff a/src/java.base/share/classes/java/lang/StringBuilder.java b/src/java.base/share/classes/java/lang/StringBuilder.java
--- a/src/java.base/share/classes/java/lang/StringBuilder.java
+++ b/src/java.base/share/classes/java/lang/StringBuilder.java
@@ -1,7 +1,7 @@
 /*
- * Copyright (c) 2003, 2019, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2003, 2020, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
  * under the terms of the GNU General Public License version 2 only, as
  * published by the Free Software Foundation.  Oracle designates this
@@ -25,10 +25,12 @@
 
 package java.lang;
 
 import jdk.internal.HotSpotIntrinsicCandidate;
 
+import java.io.IOException;
+
 /**
  * A mutable sequence of characters.  This class provides an API compatible
  * with {@code StringBuffer}, but with no guarantee of synchronization.
  * This class is designed for use as a drop-in replacement for
  * {@code StringBuffer} in places where the string buffer was being
@@ -456,10 +458,13 @@
      *             builder ({@code int}), followed by the characters in the
      *             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.
+     *
+     * @param  s the {@code ObjectOutputStream} to which data is written
+     * @throws IOException if an I/O error occurs
      */
     @java.io.Serial
     private void writeObject(java.io.ObjectOutputStream s)
         throws java.io.IOException {
         s.defaultWriteObject();
@@ -474,14 +479,18 @@
     }
 
     /**
      * readObject is called to restore the state of the StringBuffer from
      * a stream.
+     *
+     * @param  s the {@code ObjectInputStream} from which data is read
+     * @throws IOException if an I/O error occurs
+     * @throws ClassNotFoundException if a serialized class cannot be loaded
      */
     @java.io.Serial
     private void readObject(java.io.ObjectInputStream s)
-        throws java.io.IOException, ClassNotFoundException {
+        throws IOException, ClassNotFoundException {
         s.defaultReadObject();
         count = s.readInt();
         char[] val = (char[]) s.readObject();
         initBytes(val, 0, val.length);
     }
