Scanner(System.in)-无限循环

问题描述

为什么我在递归方法中遇到了无限循环，而没有机会输入任何符号来破坏它?

Why I'm getting infinite loop in recursion method, without a chance to input any symbol to break it?

class Test {
   int key=0;
   void meth(){
     System.out.println("Enter the number here: ");
     try(Scanner scan = new Scanner(System.in)) {
        key = scan.nextInt();
        System.out.println(key+1);
     } catch(Exception e) {
        System.out.println("Error");
        meth();
     }
   }
}

class Demo {
  main method {
    Test t = new Test();
    t.meth();
  }
}

如果您尝试创建错误(将字符串值放入键中，然后尝试向其添加数字)，则您将在控制台中获得无限的错误"文本，取而代之的是，在出现第一个错误之后，程序应再次询问数字，然后才决定要做什么.

If you try to create an error (putting string value in key and then try to add to it a number), you will get infinite "Error" text in console, instead of that, after first error, program should ask again the number and only then decide what to do.

推荐答案

如果nextInt()失败，它将引发异常，但不会使用无效数据.从文档:

If nextInt() fails, it throws an exception but doesn't consume the invalid data. From the documentation:

然后您再次递归调用meth()，这将尝试第二次使用相同的无效数据，再次失败(不使用它)，然后递归.

You're then recursively calling meth() again, which will try to consume the same invalid data a second time, fail again (without consuming it), and recurse.

首先，我不会在这里首先使用递归.最好选择一个简单的循环.接下来，如果输入无效，则应在再次尝试之前适当地使用它.最后，考虑使用hasNextInt而不是仅使用nextInt并捕获异常.

Firstly, I wouldn't use recursion here in the first place. Prefer a simple loop. Next, if you have invalid input you should consume it appropriately before trying again. Finally, consider using hasNextInt instead of just using nextInt and catching the exception.

所以也许是这样的:

import java.util.Scanner;

class Test {
   public static void main(String[] args){
       try (Scanner scanner = new Scanner(System.in)) {
           System.out.println("Enter the number here:");
           while (!scanner.hasNextInt() && scanner.hasNext()) {
               System.out.println("Error");
               // Skip the invalid token
               scanner.next();
           }
           if (scanner.hasNext()) {
               int value = scanner.nextInt();
               System.out.println("You entered: " + value);
           } else {
               System.out.println("You bailed out");
           }
       }
   }
}

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