Java 文件基本技术（三）：文本文件和字符流

1. 将字符串 “hello, 123，老隋” 写到文件 hello.txt 中，编码格式为 GB-2312

Writer writer = new OutputStreamWriter(new FileOutputStream("hello.txt"), "GB2312);
try {
    String str = "hello, 123, 老隋";
    writer.write(str);
} finally {
    writer.close();
}

2. 接上题，将上面写入的文件读进来

//假定一次 read() 调用就读到了所有内容，且假定长度不超过 1024。为了确保读到所有内容，可以借助 CharArrayWriter 或 StringWriter
Reader reader = new InputStreamReader(new FileInputStream("hello.txt"), "GB2312");
try {
    char[] cbuf = new char[1024];
    int charsRead = reader.read(cbuf);
    System.out.println(new String(cbuf, 0, charsRead));
} finally {
    reader.close();
}

3. 使用 CharArrayWriter，改进上面的读写代码，确保将所有文件内容读入

Reader reader = new InputStreamReader(new FileInputStream("hello.txt"), "GB2312");
try {
    CharArrayWriter writer = new CharArrayWriter();
    char[] cbuf = new char[1024];
    int charsRead = 0;
    while((charsRead=reader.read(cbuf)) != -1) {
        writer.write(cbuf, 0, charsRead);
    }
    System.out.println(writer.toString());
} finally {
    reader.close();
}

4. 使用可读的文本进行保存，一行保存一条学生信息，学生字段之间用逗号分隔

• 使用缓冲类，按行读写

  public static void writeStudents(List<Student> students) throws IOException {
      BufferedWriter writer = null;
      try {
          writer = new BufferedWriter(new FileWriter("students.txt));
          for(Student s : students) {
              writer.write(s.getName() + ", " + s.getAge() + ", " + s.getScore());
              writer.newLine();
          }
      } finally {
          if(writer != null) {
              writer.close();
          }
      }
  }

• 保存后的文件内容显示为

  张三，18，80.9
  李四，17，67.5

• 从文件中读取

  public static List<Student> readStudents() throws IOException {
      BufferedReader reader = null;
      try {
          reader = new BufferedReader(new FileReader("students.txt"));
          List<Student> students = new ArrayList<> ();
          //字符 \r 或 \n 或 \r\n 被视为换行符，readLine() 返回一行内容，但不会包含换行符
          String line = reader.readLine();
          while(line != null) {
              String[] fields = line.split(", ");
              Student s = new Student();
              s.setName(fields[0]);
              s.setAge(Integer.parseInt(fields[1]));
              s.setScore(Double.parseDouble(fields[2]));
              students.add(s);
              line = reader.readLine();
          }
          return students;
      } finally {
          if(reader != null) {
              reader.close();
          }
      }
  }

• 使用 PrintWriter 保存学生列表

  public static void writeStudents(List<Student> students) throws IOException {
      PrintWriter writer = new PrintWriter("students.txt);
      try {
          for(Student s : students) {
              writer.println(s.getName() + ", " + s.getAge() + ", " + s.getScore());
          }
      } finally {
          if(writer != null) {
              writer.close();    
          }
      }
  }

• 使用 Scanner 解析学生记录

  public static List<Student> readStudents() throws IOException {
      BufferedReader reader = new BufferedReader(new Filereader("students.txt"));
      try {
          List<Student> students = new ArrayList<Student> ();
          String line = reader.readLine();
          while(line != null) {
              Student s = new Student();
              Scanner scanner = new Scanner(line).useDelimiter(", ");
              s.setName(scanner.next());
              s.setAge(scanner.nextInt());
              s.setScore(scanner.nextDouble());
              students.add(s);
              line = reader.readLine();
          }
          return students;
      } finallly {
          reader.close();
      }
  }

5. 从键盘接受一个整数并输出

Scanner in = new Scanner(System.in);
int num = in.nextInt();
System.out.println(num);

6. 复制 Reader 到 Writer

public static void copy(final Reader input, final Writer output) throws IOException {
    char[] buf = new char[4096];
    int charsRead = 0;
    while((charsRead = input.read(buf)) != -1) {
        output.write(buf, 0, charsRead);
    }
}

7. 将文件全部内容读入到一个字符串，参数为文件名和编码类型

public static String readFileToString(final String fileName, final String encoding) throws IOException {
    BufferedReader reader = null;
    try {
        reader = new BufferedReader(new InputStreamReader(new FileInputStream(fileName), encoding));
        StringWriter writer = new StringWriter();
        copy(reader, writer);
        return writer.toString();
    } finally {
        if(reader != null) {
            reader.close();
        }
    }
}

8. 将字符串写到文件，参数为文件名、字符串内容和编码类型

public static void writeStringToFile(final String fileName, final String data, final String encoding) throws IOException {
    Writer writer = null;
    try {
        writer = new OutputStreamWriter(new FileOutputStream(fileName), encoding);
        writer.write(data);
    } finally {
        if(writer != null) {
            writer.close();
        }
    }
}

9. 按行将多行数据写到文件，参数为文件名、编码类型、行的集合

public static void writeLines(final String fileName, final String encoding, final Collection<?> lines) throws IOException {
    PrintWriter writer = null;
    try {
        writer = new PrintWriter(fileName, encoding);
        for(Object line : lines) {
            writer.println(line);
        }
    } finally {
        if(writer != null) {
            writer.close();
        }
    }
}

10. 按行将文件内容读到一个列表中，参数为文件名、编码类型

public static List<String> readLines(final String fileName, final String encoding) throws IOException {
    BufferedReader reader = null;
    try {
        reader = new BufferedReader(new InputStreamReader(new FileInputStream(fileName), encoding));
        List<String> list = new ArrayList<> ();
        String line = reader.readLine();
        while(line != null) {
            list.add(line);
            line = reader.readLine();
        }
        return list;
    } finally {
        if(reader != null) {
            reader.close();
        }
    }
}