Detailed explanation of Java IO file reading and writing examples

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Simple examples of file reading and writing in Java IO

1. File reading

public class ReadFromFile {
  /**
   * 以字节为单位读取文件，常用于读二进制文件，如图片、声音、影像等文件。
   */
  public static void readFileByBytes(String fileName) {
    File file = new File(fileName);
    InputStream in = null;
    try {
      System.out.println("以字节为单位读取文件内容，一次读一个字节：");
      // 一次读一个字节
      in = new FileInputStream(file);
      int tempbyte;
      while ((tempbyte = in.read()) != -1) {
        System.out.write(tempbyte);
      }
      in.close();
    } catch (IOException e) {
      e.printStackTrace();
      return;
    }
    try {
      System.out.println("以字节为单位读取文件内容，一次读多个字节：");
      // 一次读多个字节
      byte[] tempbytes = new byte[100];
      int byteread = 0;
      in = new FileInputStream(fileName);
      ReadFromFile.showAvailableBytes(in);
      // 读入多个字节到字节数组中，byteread为一次读入的字节数
      while ((byteread = in.read(tempbytes)) != -1) {
        System.out.write(tempbytes, 0, byteread);
      }
    } catch (Exception e1) {
      e1.printStackTrace();
    } finally {
      if (in != null) {
        try {
          in.close();
        } catch (IOException e1) {
        }
      }
    }
  }

  /**
   * 以字符为单位读取文件，常用于读文本，数字等类型的文件
   */
  public static void readFileByChars(String fileName) {
    File file = new File(fileName);
    Reader reader = null;
    try {
      System.out.println("以字符为单位读取文件内容，一次读一个字节：");
      // 一次读一个字符
      reader = new InputStreamReader(new FileInputStream(file));
      int tempchar;
      while ((tempchar = reader.read()) != -1) {
        // 对于windows下，\r\n这两个字符在一起时，表示一个换行。
        // 但如果这两个字符分开显示时，会换两次行。
        // 因此，屏蔽掉\r，或者屏蔽\n。否则，将会多出很多空行。
        if (((char) tempchar) != '\r') {
          System.out.print((char) tempchar);
        }
      }
      reader.close();
    } catch (Exception e) {
      e.printStackTrace();
    }
    try {
      System.out.println("以字符为单位读取文件内容，一次读多个字节：");
      // 一次读多个字符
      char[] tempchars = new char[30];
      int charread = 0;
      reader = new InputStreamReader(new FileInputStream(fileName));
      // 读入多个字符到字符数组中，charread为一次读取字符数
      while ((charread = reader.read(tempchars)) != -1) {
        // 同样屏蔽掉\r不显示
        if ((charread == tempchars.length)
            && (tempchars[tempchars.length - 1] != '\r')) {
          System.out.print(tempchars);
        } else {
          for (int i = 0; i < charread; i++) {
            if (tempchars[i] == &#39;\r&#39;) {
              continue;
            } else {
              System.out.print(tempchars[i]);
            }
          }
        }
      }

    } catch (Exception e1) {
      e1.printStackTrace();
    } finally {
      if (reader != null) {
        try {
          reader.close();
        } catch (IOException e1) {
        }
      }
    }
  }

  /**
   * 以行为单位读取文件，常用于读面向行的格式化文件
   */
  public static void readFileByLines(String fileName) {
    File file = new File(fileName);
    BufferedReader reader = null;
    try {
      System.out.println("以行为单位读取文件内容，一次读一整行：");
      reader = new BufferedReader(new FileReader(file));
      String tempString = null;
      int line = 1;
      // 一次读入一行，直到读入null为文件结束
      while ((tempString = reader.readLine()) != null) {
        // 显示行号
        System.out.println("line " + line + ": " + tempString);
        line++;
      }
      reader.close();
    } catch (IOException e) {
      e.printStackTrace();
    } finally {
      if (reader != null) {
        try {
          reader.close();
        } catch (IOException e1) {
        }
      }
    }
  }

  /**
   * 随机读取文件内容
   */
  public static void readFileByRandomAccess(String fileName) {
    RandomAccessFile randomFile = null;
    try {
      System.out.println("随机读取一段文件内容：");
      // 打开一个随机访问文件流，按只读方式
      randomFile = new RandomAccessFile(fileName, "r");
      // 文件长度，字节数
      long fileLength = randomFile.length();
      // 读文件的起始位置
      int beginIndex = (fileLength > 4) ? 4 : 0;
      // 将读文件的开始位置移到beginIndex位置。
      randomFile.seek(beginIndex);
      byte[] bytes = new byte[10];
      int byteread = 0;
      // 一次读10个字节，如果文件内容不足10个字节，则读剩下的字节。
      // 将一次读取的字节数赋给byteread
      while ((byteread = randomFile.read(bytes)) != -1) {
        System.out.write(bytes, 0, byteread);
      }
    } catch (IOException e) {
      e.printStackTrace();
    } finally {
      if (randomFile != null) {
        try {
          randomFile.close();
        } catch (IOException e1) {
        }
      }
    }
  }

  /**
   * 显示输入流中还剩的字节数
   */
  private static void showAvailableBytes(InputStream in) {
    try {
      System.out.println("当前字节输入流中的字节数为:" + in.available());
    } catch (IOException e) {
      e.printStackTrace();
    }
  }

  public static void main(String[] args) {
    String fileName = "C:/temp/newTemp.txt";
    ReadFromFile.readFileByBytes(fileName);
    ReadFromFile.readFileByChars(fileName);
    ReadFromFile.readFileByLines(fileName);
    ReadFromFile.readFileByRandomAccess(fileName);
  }
}

2. File writing

public class AppendToFile {
  /**
   * A方法追加文件：使用RandomAccessFile
   */
  public static void appendMethodA(String fileName, String content) {
    try {
      // 打开一个随机访问文件流，按读写方式
      RandomAccessFile randomFile = new RandomAccessFile(fileName, "rw");
      // 文件长度，字节数
      long fileLength = randomFile.length();
      //将写文件指针移到文件尾。
      randomFile.seek(fileLength);
      randomFile.writeBytes(content);
      randomFile.close();
    } catch (IOException e) {
      e.printStackTrace();
    }
  }

  /**
   * B方法追加文件：使用FileWriter
   */
  public static void appendMethodB(String fileName, String content) {
    try {
      //打开一个写文件器，构造函数中的第二个参数true表示以追加形式写文件
      FileWriter writer = new FileWriter(fileName, true);
      writer.write(content);
      writer.close();
    } catch (IOException e) {
      e.printStackTrace();
    }
  }

  public static void main(String[] args) {
    String fileName = "C:/temp/newTemp.txt";
    String content = "new append!";
    //按方法A追加文件
    AppendToFile.appendMethodA(fileName, content);
    AppendToFile.appendMethodA(fileName, "append end. \n");
    //显示文件内容
    ReadFromFile.readFileByLines(fileName);
    //按方法B追加文件
    AppendToFile.appendMethodB(fileName, content);
    AppendToFile.appendMethodB(fileName, "append end. \n");
    //显示文件内容
    ReadFromFile.readFileByLines(fileName);
  }
}

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