StringWriter 调用flush后为什么缓存区还有内容？？

如题：

             StringWriter sw=new StringWriter();
             sw.write("aa");
             System.out.println("1:"+sw);
             sw.flush();
             System.out.println("2:"+sw);

两处都有结果，flush后缓存没有清空吗？

找到了，

     public void flush() {
    }

Flush is suppose to ensure that all characters written to the Writer are
fully processed. For example, if the Writer were outputting to a
terminal, calling flush would ensure that all characters appeared on the
screen. Since StringWriter doesn't do any buffering (that is, it
updates its internal buffer on every write), flushing it is a no-op.

这竟然是个空方法……………………