如何在不使用 try 和 catch 的情况下捕获 Java 中的错误“InputMismatchException”并打印个性化消息？

Question

    Scanner sc = new Scanner (System.in);
    int source_radix = sc.nextInt();
    String sr = source_radix+"";
    if (!Pattern.matches("[0-9]+",sr) || source_radix <1 || source_radix >36)
    {
        System.out.print("error");
        System.exit(0);
    }
   

I want to print "error" when the input is not an integer.

Answer 1

The simplest way to prevent that exception being thrown from nextInt() is to call hasNextInt() first. In your specific example, we could do this:

Scanner sc = new Scanner (System.in);
int radix = 0;
if (sc.hasNextInt()) {
    radix = sc.nextInt();
}
if (radix < 1 || radix > 36) {
    System.out.print("error");
    System.exit(0);
}

You could modify that to print different error messages to distinguish the cases where the user didn't enter a valid integer, and where they entered a integer that was outside of the required range.

In some situations, it may be necessary to call sc.nextLine() to discard something that isn't a valid number, or anything on the line after the number. (But it isn't necessary if the app is going to terminate on getting bad input.)

Finally, it is often a bad idea to call System.exit(0) . If this is in your main method, return will work just as well. If it is not in main , then it probably should not be this code's responsibility to "pull the plug" on the application. But... it is not clear cut.

Answer 2

You can check if input is number of range from 1 to 36 by only using Regex:

Scanner sc = new Scanner(System.in);
String sr  = sc.nextLine();

if (!Pattern.matches("[1-9]|[12][0-9]|3[0-6]", sr)) {
    System.out.print("error");
    System.exit(0);
}

Answer 3

You may try this Java code:

   Scanner sc = new Scanner (System.in);
   String sr = sc.next(); // read a string

   // check if we have all digits in input
   boolean match = Pattern.matches("[0-9]+", sr);

   // if yes then convert to int and do comparison
   if (match) {
       int source_radix = Integer.parseInt(sr);
       if (source_radix <1 || source_radix >36) match = false;
   }

   // handle error if match is false
   if (!match) {
       System.out.print("error");
       System.exit(0);
   }

Answer 4

You can do something this:

Scanner sc = new Scanner(file);
while(sc.hasNext()) {
    String str =sc.nextLine();
    if (!Pattern.matches("[0-9]+",str) ||( Pattern.matches("[0-9]+",str) &&     Integer.parseInt(str)) <1 )|| ( Pattern.matches("[0-9]+",str) && Integer.parseInt(str)) <1 ) >36) {
        System.out.print("error");
        System.exit(0);
    }
}