java io文件操作从文件读取数据的六种方法

@Test
void testReadFile1() throws IOException {
   //文件内容：Hello World|Hello Zimug
   String fileName = "D:\\data\\test\\newFile4.txt";
   try (Scanner sc = new Scanner(new FileReader(fileName))) {
      while (sc.hasNextLine()) {  //按行读取字符串
         String line = sc.nextLine();
         System.out.println(line);
      }
   }
   try (Scanner sc = new Scanner(new FileReader(fileName))) {
      sc.useDelimiter("\\|");  //分隔符
      while (sc.hasNext()) {   //按分隔符读取字符串
         String str = sc.next();
         System.out.println(str);
      }
   }
   //sc.hasNextInt() 、hasNextFloat() 、基础数据类型等等等等。
   //文件内容：1|2
   fileName = "D:\\data\\test\\newFile5.txt";
   try (Scanner sc = new Scanner(new FileReader(fileName))) {
      sc.useDelimiter("\\|");  //分隔符
      while (sc.hasNextInt()) {   //按分隔符读取Int
          int intValue = sc.nextInt();
         System.out.println(intValue);
      }
   }
}

@Test
void testReadFile2() throws IOException {
   String fileName = "D:\\data\\test\\newFile.txt";
   // 读取文件内容到Stream流中，按行读取
   Stream<String> lines = Files.lines(Paths.get(fileName));
   // 随机行顺序进行数据处理
   lines.forEach(ele -> {
      System.out.println(ele);
   });
}

// 按文件行顺序进行处理
lines.forEachOrdered(System.out::println);

// 按文件行顺序进行处理
lines.parallel().forEachOrdered(System.out::println);

// 转换成List<String>, 要注意java.lang.OutOfMemoryError: Java heap space
List<String> collect = lines.collect(Collectors.toList());

@Test
void testReadFile3() throws IOException {
   String fileName = "D:\\data\\test\\newFile3.txt";
   // 转换成List<String>, 要注意java.lang.OutOfMemoryError: Java heap space
   List<String> lines = Files.readAllLines(Paths.get(fileName),
               StandardCharsets.UTF_8);
   lines.forEach(System.out::println);

}

@Test
void testReadFile4() throws IOException {
   String fileName = "D:\\data\\test\\newFile3.txt";
   // java 11 开始提供的方法，读取文件不能超过2G，与你的内存息息相关
   //String s = Files.readString(Paths.get(fileName));
}

@Test
void testReadFile5() throws IOException {
   String fileName = "D:\\data\\test\\newFile3.txt";
   //如果是JDK11用上面的方法，如果不是用这个方法也很容易
   byte[] bytes = Files.readAllBytes(Paths.get(fileName));
   String content = new String(bytes, StandardCharsets.UTF_8);
   System.out.println(content);
}

@Test
void testReadFile6() throws IOException {
   String fileName = "D:\\data\\test\\newFile3.txt";
   // 带缓冲的流读取，默认缓冲区8k
   try (BufferedReader br = new BufferedReader(new FileReader(fileName))){
      String line;
      while ((line = br.readLine()) != null) {
         System.out.println(line);
      }
   }
   //java 8中这样写也可以
   try (BufferedReader br = Files.newBufferedReader(Paths.get(fileName))){
      String line;
      while ((line = br.readLine()) != null) {
         System.out.println(line);
      }
   }

}

try (FileInputStream fis = new FileInputStream(fileName);
     ObjectInputStream ois = new ObjectInputStream(fis)){
   ois.readObject();
}