Java 输入不匹配异常

Question

I have this code and I want to catch the letter exception but it keeps having these errors:

Exception in thread "main" java.util.InputMismatchException
    at java.util.Scanner.throwFor(Scanner.java:840)
    at java.util.Scanner.next(Scanner.java:1461)
    at java.util.Scanner.nextInt(Scanner.java:2091)
    at java.util.Scanner.nextInt(Scanner.java:2050)
    at exercise_one.Exercise.main(Exercise.java:17)

And here is my code:

 System.out.print("Enter the number of students: ");

 students = input.nextInt(); 

 while (students <= 0) {

     try {

        System.out.print("Enter the number of students: ");

        students = input.nextInt();

     }

     catch (InputMismatchException e) {

        System.out.print("Enter the number of students");

     }
 }    

Answer 1

You can use a do-while loop instead to eliminate the first input.nextInt() .

do {
    try {
        System.out.print("Enter the number of students: ");
        students = input.nextInt();
    } catch (InputMismatchException e) {
        System.out.print("Invalid number of students. ");
    }
    input.nextLine(); // clears the buffer
} while (students <= 0);

Therefore all InputMismatchException can be handled in one place.

Answer 2

from the doc

Scanner.nextInt Scans the next token of the input as an int. if the next token does not match the Integer regular expression, or is out of range

So it seems you are not entering any integer as input.

you can use

     while (students <= 0) {

         try {
            System.out.print("Enter the number of students: ");

            students = input1.nextInt();

         }

         catch (InputMismatchException e) {
             input1.nextLine();
         }
     } 

Answer 3

Reading data from Scanner and assigning it to Int type. Since you are supplying String this will throw exception. To handle this situation you must write your snippet inside Try- Catch block only.

Answer 4

If you wanna be sure about being integer for all input you can try this:

while(true) {
            try {
                System.out.print("Kolon sayısını giriniz: ");
                c = scan.nextInt();
                
            } catch (Exception e) {
                System.out.print("Geçersiz giriş.. ");
                scan.nextLine();
                continue;
            }
            break;
        }


// or this...
while(true) {
            System.out.print("Give me a number");
            try {
                input = scan.nextInt();
                break;
            } catch (Exception e) {
                System.out.print("There was mismatch");
                scan.nextLine();
            }
        }

Answer 5

John Stef,

The method nextInt() just Throws the following exceptions:

InputMismatchException - if the next token does not match the Integer regular expression, or is out of range NoSuchElementException - if input is exhausted

IllegalStateException - if this scanner is closed

If you need/want throw another one type of exception you got to specify your own exception. Use throw statement. Here's an example of a throw statement.

throw someThrowableObject;

For example:

try {
    System.out.print("Enter the number of students: ");
    students = input.nextInt();
    if(students <=0){
        throw new Exception("Null or Negative number of students is invalid.");
    }
    } catch (InputMismatchException e) {
        System.out.print("Invalid input. Please enter a number for student number.");
    } catch (Exception e) {
        System.out.print(e.getMessage());
    }
}

This will catch the both mismatch and negative exceptions.

Although the do... while posted by Siyu Song achieve the desired input from the user, it don't catch negative int exceptions as you wish.

You can use this try and do...while from Siyu Song to achieve what you wanting. The complete code looks like this:

do {
    try {
           System.out.print("Enter the number of students: ");
           students = input.nextInt();
           if(students <=0){
                throw new Exception("Negative number of students is invalid.");
           }
       } catch (InputMismatchException e) {
             System.out.print("Invalid input. Please enter a number for students number.");
       } catch (Exception e) {
              System.out.print(e.getMessage());
     }
     input.nextLine();
} while (students <=0);