JDK源码阅读--StringBuffer

public final class StringBuffer
   extends AbstractStringBuilder
   implements java.io.Serializable, CharSequence

 /**
      * 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

 
StringBuffer是被final修饰的，表示不能被其它类继承，；继承了AbstractStringBuilder抽象类，实现了Serializable、CharSequence接口
 
StringBuffer中的方法被public修饰的都使用了synchronized，保证线程安全。所以StringBuffer是线程安全的类。

 /**
      * 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);
     }

StringBuffer的容量默认是16，当容量不够的时候，容量会继续增加16，依次类推。