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

1 /**2      * A cache of the last value returned by toString. Cleared3      * whenever the StringBuffer is modified.4      */5     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。

 

1 /** 2      * Constructs a string buffer with no characters in it and an 3      * initial capacity of 16 characters. 4      */ 5     public StringBuffer() { 6         super(16); 7     } 8  9     /**10      * Constructs a string buffer with no characters in it and11      * the specified initial capacity.12      *13      * @param      capacity  the initial capacity.14      * @exception  NegativeArraySizeException  if the {
    @code capacity}15      *               argument is less than {
    @code 0}.16      */17     public StringBuffer(int capacity) {18         super(capacity);19     }20 21     /**22      * Constructs a string buffer initialized to the contents of the23      * specified string. The initial capacity of the string buffer is24      * {
    @code 16} plus the length of the string argument.25      *26      * @param   str   the initial contents of the buffer.27      */28     public StringBuffer(String str) {29         super(str.length() + 16);30         append(str);31     }

 

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。