RandomAccessFile oSavedFile = new RandomAccessFile("down.txt","rw");
long nPos = 10;
//定位文件指针到nPos位置
oSavedFile.seek(nPos);
byte[] b = {'a','b'};
//从输入流中读入字节流
oSavedFile.write(b,0,2);
oSavedFile.seek(20);
oSavedFile.write(b,0,2);
oSavedFile .close();