通过练习对前几天知识巩固

例如：文件的读写

 
public static byte[] readFile(String filename) throws IOException ...{

    File file =new File(filename);
    if(filename==null || filename.equals(""))
    ...{
      throw new NullPointerException("无效的文件路径");
    }
    long len = file.length();
    byte[] bytes = new byte[(int)len];

    BufferedInputStream bufferedInputStream=new BufferedInputStream(new FileInputStream(file));
    int r = bufferedInputStream.read( bytes );
    if (r != len)
      throw new IOException("读取文件不正确");
    bufferedInputStream.close();

    return bytes;

}

/** *//**
   * 将数据写入文件
 
   */
public static void writeFile(byte[] data,String filename) throws IOException ...{
    File file =new File(filename);
    file.getParentFile().mkdirs();
    BufferedOutputStream bufferedOutputStream=new BufferedOutputStream(new FileOutputStream(file));
    bufferedOutputStream.write(data);
    bufferedOutputStream.close();

}

/** *//**
   * 从jar文件里读取class
 
public byte[] readFileJar(String filename) throws IOException ...{
    BufferedInputStream bufferedInputStream=new BufferedInputStream(getClass().getResource(filename).openStream());
    int len=bufferedInputStream.available();
    byte[] bytes=new byte[len];
    int r=bufferedInputStream.read(bytes);
    if(len!=r)
    ...{
      bytes=null;
      throw new IOException("读取文件不正确");
    }
    bufferedInputStream.close();
    return bytes;
}