为什么我在while前加了一句bis.read(b);就写不进去了，拿掉就可以，这是为什么？

import java.io.BufferedInputStream;

import java.io.BufferedOutputStream;

import java.io.FileInputStream;

import java.io.FileNotFoundException;

import java.io.FileOutputStream;

import java.io.IOException;

public class CopyTxt {

//使用Bufferded复制文件信息

public static void main(String[] args) {

try {

BufferedInputStream bis=new BufferedInputStream(new FileInputStream("E:\\one.txt"));

BufferedOutputStream bos=new BufferedOutputStream(new FileOutputStream("E:\\oneCopy.txt"));

int n;

byte[] b = new byte[1024];

bis.read(b);

while((n=bis.read(b))!=-1) {

bos.write(b,0,n);

}

bos.flush();

bis.close();

bos.close();

} catch (FileNotFoundException e) {

// TODO Auto-generated catch block

e.printStackTrace();

}catch(IOException e) {

e.printStackTrace();

}

}

}