/**

      * A cache of the last value returned by toString. Cleared

      * whenever the StringBuffer is modified.

      */

     private transient char[] toStringCache;//被transient修饰，表示这个字段不被序列化

 /**

      * Constructs a string buffer with no characters in it and an

      * initial capacity of 16 characters.

      */

     public StringBuffer() {

         super(16);

     }

     /**

      * Constructs a string buffer with no characters in it and

      * the specified initial capacity.

      *

      * @param      capacity  the initial capacity.

      * @exception  NegativeArraySizeException  if the {@code capacity}

      *               argument is less than {@code 0}.

      */

     public StringBuffer(int capacity) {

         super(capacity);

     }

     /**

      * Constructs a string buffer initialized to the contents of the

      * specified string. The initial capacity of the string buffer is

      * {@code 16} plus the length of the string argument.

      *

      * @param   str   the initial contents of the buffer.

      */

     public StringBuffer(String str) {

         super(str.length() + 16);

         append(str);

     }