Java Scanner类的一些经典坑拆解

public class InputHungryDemo {
   public static void main(String\[] args) {
       try (Scanner scanner = new Scanner(System.in)) {
           System.out.print("请输入您的年龄：");
           int age = scanner.nextInt(); // 读取整数用户输入
           System.out.print("请输入您的用户名：");
          String username = scanner.nextLine(); // 读取字符串用户输入
           System.out.println("年龄：" + age + "，用户名：" + username);
      }
   }
}

public class FixInputHungryDemo1 {
   public static void main(String\[] args) {
       try (Scanner scanner = new Scanner(System.in)) {
           System.out.print("请输入您的年龄：");
          int age = scanner.nextInt(); // 读取整数，光标停留在换行符之前
           // 【修复关键】额外调用一次nextLine()，吃掉残留的换行符，将光标移动到换行符之后
           scanner.nextLine();
           System.out.print("请输入您的用户名：");
           String username = scanner.nextLine(); // 正常读取用户名
           System.out.println("年龄：" + age + "，用户名：" + username);
       }
   }
}

public class FixInputHungryDemo2 {
   public static void main(String\[] args) {
      try (Scanner scanner = new Scanner(System.in)) {
           System.out.print("请输入您的年龄：");
           // 用nextLine()读取整行字符串，再手动转换为整数，搭配正则校验格式
           int age = Integer.parseInt(scanner.nextLine());
           System.out.print("请输入您的用户名：");
           String username = scanner.nextLine(); // 正常读取用户名
           System.out.println("年龄：" + age + "，用户名：" + username);
       }
   }
}

public class NextLineEmptyDemo {
   public static void main(String\[] args) {
       try (Scanner scanner = new Scanner(System.in)) {
           System.out.print("请输入一个指令：");
           String cmd = scanner.next(); // 读取一个令牌，比如输入：save，按下回车
           System.out.print("请输入备注信息：");
           String remark = scanner.nextLine(); // 直接读取到空字符串
           System.out.println("指令：" + cmd + "，备注：" + remark);
       }
   }
}

public class InputMismatchDemo {
   public static void main(String\[] args) {
       try (Scanner scanner = new Scanner(System.in)) {
           System.out.print("请输入您的年龄：");
           int age = scanner.nextInt(); // 如果输入的不是整数，比如"abc"，直接抛出异常
           System.out.println("年龄：" + age);
       }
   }
}

public class FixInputMismatchDemo {
   public static void main(String\[] args) {
       try (Scanner scanner = new Scanner(System.in)) {
           System.out.print("请输入您的年龄：");
           // 【修复关键】先校验下一个令牌是否为合法整数，循环重试，直到输入合法
           while (!scanner.hasNextInt()) {
               // 输入不匹配，读取并丢弃当前不合法的令牌，打印友好错误提示
               String invalidInput = scanner.next();
               System.out.printf("输入的「%s」不是合法整数，请重新输入：%n", invalidInput);
           }
           // 校验通过，再读取合法整数
           int age = scanner.nextInt();
           System.out.println("年龄：" + age);
       }
   }
}

public class NoSuchElementDemo {
  public static void main(String\[] args) {
       Scanner scanner = new Scanner(System.in);
       System.out.print("请输入用户名：");
       String username = scanner.nextLine();
       // 不当的资源关闭顺序：提前关闭了Scanner流
       scanner.close();
       System.out.print("请输入年龄：");
       int age = scanner.nextInt(); // 抛出异常，流已被关闭
   }
}

public class CharsetMismatchDemo {
   public static void main(String\[] args) throws FileNotFoundException {
       // 读取GBK编码的文件，中文会乱码
       File file = new File("test\_gbk.txt");
       try (Scanner scanner = new Scanner(file)) {
           while (scanner.hasNextLine()) {
               String line = scanner.nextLine();
               System.out.println(line); // 中文乱码
           }
       }
   }
}

public class FixCharsetMismatchDemo {
   public static void main(String\[] args) throws FileNotFoundException {
       File file = new File("test\_gbk.txt");
       // 【修复关键1】手动指定与文件一致的GBK字符集编码
       try (Scanner scanner = new Scanner(file, "GBK")) {
           while (scanner.hasNextLine()) {
               String line = scanner.nextLine();
               System.out.println(line); // 中文正常显示
           }
       }
       // 【修复关键2】读取控制台输入时，指定全局标准编码为UTF-8
       try (Scanner scanner = new Scanner(System.in, StandardCharsets.UTF\_8.name())) {
           System.out.print("请输入包含中文的用户名：");
           String username = scanner.nextLine();
           System.out.println("用户名：" + username);
       }
   }
}