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

@@ -410,11 +410,12 @@
 
     @Override
     @HotSpotIntrinsicCandidate
     public String toString() {
         // Create a copy, don't share the array
-        return new String(value, 0, count);
+        return isLatin1() ? StringLatin1.newString(value, 0, count)
+                          : StringUTF16.newStringSB(value, 0, count);
     }
 
     /**
      * Save the state of the {@code StringBuilder} instance to a stream
      * (that is, serialize it).

@@ -428,20 +429,27 @@
      */
     private void writeObject(java.io.ObjectOutputStream s)
         throws java.io.IOException {
         s.defaultWriteObject();
         s.writeInt(count);
-        s.writeObject(value);
+        char[] val = new char[capacity()];
+        if (isLatin1()) {
+            StringLatin1.getChars(value, 0, count, val, 0);
+        } else {
+            StringUTF16.getChars(value, 0, count, val, 0);
+        }
+        s.writeObject(val);
     }
 
     /**
      * readObject is called to restore the state of the StringBuffer from
      * a stream.
      */
     private void readObject(java.io.ObjectInputStream s)
         throws java.io.IOException, ClassNotFoundException {
         s.defaultReadObject();
         count = s.readInt();
-        value = (char[]) s.readObject();
+        char[] val = (char[]) s.readObject();
+        initBytes(val, 0, val.length);
     }
 
 }