while((b=in.read())!=-1)
我前几个月已经知道了
Reads the next byte of data from this input stream. The value * byte is returned as an <code>int</code> in the range * <code>0</code> to <code>255</code>. If no byte is available * because the end of the stream has been reached, the value * <code>-1</code> is returned. This method blocks until input data * is available, the end of the stream is detected, or an exception * is thrown.
read返回字节序列长度,序列下标都是从0开始,0代表第一个,所以-1代表第0个即序列为空