Java 为什么我的try-catch在用于捕获错误时打印出错误和我的catch块？

我目前正在学习try-catch代码块，我正在编写一段代码，接收用户的输入并检查它是否为整数。我的代码是

int classSize = 0;
boolean error = false;
    do {
        try{
            System.out.println("Hello! Please enter how many Students you have in your class: ");
            classSize = in.nextInt();
            error = false;
        }
        catch(InputMismatchException e){
            System.out.println("Invalid input. please make sure you enter numbers ONLY! No symbols or letters.");
            classSize = in.nextInt();
            error = true;
        }
    }while(error);

当我试图通过输入字母来测试它时，我会得到错误消息和try block消息

Hello! Please enter how many Students you have in your class: 
asdfsda

Invalid input. please make sure you enter numbers ONLY! No symbols or 
letters.

Exception in thread "main" java.util.InputMismatchException
at java.base/java.util.Scanner.throwFor(Scanner.java:939)
at java.base/java.util.Scanner.next(Scanner.java:1594)
at java.base/java.util.Scanner.nextInt(Scanner.java:2258)
at java.base/java.util.Scanner.nextInt(Scanner.java:2212)
at ExcerciseOne.main(ExcerciseOne.java:65)

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为什么它会显示catch块和错误消息？我是否正确使用了try-catch块

您正在捕获try子句中抛出的异常，但随后再次从catch中的扫描器读取，从而抛出另一个异常，这次是未捕获的异常

classSize = in.nextInt();

存在于两个子句中，两次都会引发异常，但仅在从try子句内部引发时才会被捕获。

catch块中的in.nextInt调用读取try块中第一次调用in.nextInt留下的坏流

您可以做的是删除catch块中的.nextInt调用：

或者更好：

由于InputMismatchException是RuntimeException的一个子类，引发此错误的错误类型可以通过if语句过滤掉。因为您只需要一个适当的整数作为输入，所以可以使用正则表达式验证输入

boolean hasError = true;
String integerRegex = "[\\d]+"; // it means 1-to-many digits
while (hasError) {
    System.out.println("Hello! Please enter how many Students you have in your class: ");
    String input = in.nextLine();
    if (input.matches(integerRegex)) {
        classSize = Integer.parseInt(input);
        hasError = false;
    } else {
        System.out.println("Invalid input. please make sure you enter numbers ONLY! No symbols or letters.");
        // variable hasError remains true
    }
}

---

宪兵队的答案很清楚，我为您添加了一个正确的演示：

  public static void main(String[] args) {
      int studentsCount = 0;
      boolean hasError;
      do {
        try {
          System.out.println("Hello! Please enter how many Students you have in your class: ");
          Scanner in = new Scanner(System.in);
          studentsCount = in.nextInt();
          hasError = false;
        } catch (InputMismatchException e) {
          System.out.println("--> Invalid input. please make sure you enter numbers ONLY! No symbols or letters.");
          hasError = true;
        }
      } while (hasError);
      System.out.println("There are " + studentsCount + " students in your class.");
  }

---

查看引发异常的行。我打赌它不在试球区，而是在接球区。好的。我明白问题所在，但如何解决呢？我基本上希望用户在使用catch块重新输入数字时使用它。我必须在试捕中进行试捕吗？这就是循环的目的，对吗？如果失败，它将返回try子句。我认为在catch子句中没有理由要求输入。在不同类型的异常中有很好的相关性。

  public static void main(String[] args) {
      int studentsCount = 0;
      boolean hasError;
      do {
        try {
          System.out.println("Hello! Please enter how many Students you have in your class: ");
          Scanner in = new Scanner(System.in);
          studentsCount = in.nextInt();
          hasError = false;
        } catch (InputMismatchException e) {
          System.out.println("--> Invalid input. please make sure you enter numbers ONLY! No symbols or letters.");
          hasError = true;
        }
      } while (hasError);
      System.out.println("There are " + studentsCount + " students in your class.");
  }