不能多次捕获异常

Question

when running the code, if I input any character other than number, the exeception "java.util.InputMismatchException" gets catched, but next time if I input any character other than number, the program gets terminated with error "Exception in thread "main" java.util.InputMismatchException". how to make it able to catch more than one simultaneous exceptions untill a valid input is given.

/*
Twenty students were asked to rate on a scale of 1 to 5 the quality of the food in the
student cafeteria, with 1 being “awful” and 5 being “excellent.” Place the 20 responses
in an integer array and determine the frequency of each rating.
 */
import java.util.Scanner;

public class StudentPoll 
{
    static Scanner input=new Scanner(System.in);
    public static void main(String[] args)
    {
        System.out.println("Rate the quality of food from 1 to 5." +
                "\n1 being “awful” and 5 being “excellent”.\n");
        int[] array=new int[10];
        int num =0;
        for(int i=0;i<array.length;i++)
        {

            do{
                System.out.println("Student "+(i+1)+" Enter Your Response:");
                try
                {
                    num=input.nextInt();
                }
                catch(java.util.InputMismatchException e)
                {
                    System.out.println("Enter numbers only.");
                    input.nextLine();
                    num=input.nextInt();
                }   
                if(num<=0 || num>5)

                {
                    System.out.println("Enter 1 to 5 only.");
                }
            }while(num<=0 || num>5);
            array[i]=num;

        }

        int[] frequency=new int[6];

        for ( int i = 0; i < array.length; i++ )
        {
            frequency[array[i]]=frequency[array[i]]+1;      
        }

        System.out.printf("*    :%d (awful)\n",frequency[1]);
        System.out.printf("**   :%d\n",frequency[2]);
        System.out.printf("***  :%d\n",frequency[3]);
        System.out.printf("**** :%d\n",frequency[4]);
        System.out.printf("*****:%d (excellent)\n",frequency[5]);
    }
}`

Answer 1

Because you do not have another try-catch statement in your catch statement, you can just re-run the loop if you want to catch the exception again. So you would replace:

catch(java.util.InputMismatchException e)
{
    System.out.println("Enter numbers only.");
    input.nextLine();
    num = input.nextInt();
} 

with:

catch(java.util.InputMismatchException e)
{
    System.out.println("Enter numbers only.");
    input.nextLine();
    continue;
}

Answer 2

Because when first exception occur in the try block that is caught by the catch block. And if the user again give a invalid input then again the exception thrown. But there is no mechanism in your code to catch that exception. Therefore the main thread stopped. Catch block will not responsible to catch exception that is throw inside that block.

import java.util.Scanner;

public class StudentPoll
{
static Scanner input=new Scanner(System.in);
public static void main(String[] args)
{
    System.out.println("Rate the quality of food from 1 to 5." +
            "\n1 being “awful” and 5 being “excellent”.\n");
    int[] array=new int[10];
    int num =0;
    for(int i=0;i<array.length;i++)
    {

        do{
            System.out.println("Student "+(i+1)+" Enter Your Response:");
            try
            {
                num=input.nextInt();
            }
            catch(java.util.InputMismatchException e)
            {
                input.next(); // consume the leftover new line
                System.out.println("Enter numbers only.");
            }
            if(num<=0 || num>5)

            {
                System.out.println("Enter 1 to 5 only.");
            }

        }while(num<1 || num>5); // change in the logic
        array[i]=num;

    }

    int[] frequency=new int[6];

    for ( int i = 0; i < array.length; i++ )
    {
        frequency[array[i]]=frequency[array[i]]+1;
    }

    System.out.printf("*    :%d (awful)\n",frequency[1]);
    System.out.printf("**   :%d\n",frequency[2]);
    System.out.printf("***  :%d\n",frequency[3]);
    System.out.printf("**** :%d\n",frequency[4]);
    System.out.printf("*****:%d (excellent)\n",frequency[5]);
}
}  

Answer 3

Your Problem statements is after the wrong input the program doesn't wait for next user input You can achieve this my accepting the input via java.util.Scanner & making for loop within do while.

try the below code it should work as per your need.

/*
Twenty students were asked to rate on a scale of 1 to 5 the quality of the food in the
student cafeteria, with 1 being “awful” and 5 being “excellent.” Place the 20 responses
in an integer array and determine the frequency of each rating.
 */
import java.util.Scanner;

public class StudentPoll  {
    static Scanner input = new Scanner(System.in);

    public static void main(String[] args) {
        System.out.println("Rate the quality of food from 1 to 5."
                + "\n1 being “awful” and 5 being “excellent”.\n");
        int[] array = new int[10];
        int num = 0;

        do {
            for (int i = 0; i < array.length; i++) {
                System.out.println("Student " + (i + 1)
                        + " Enter Your Response:");
                Scanner sc = new Scanner(System.in);
                try {
                    num = sc.nextInt();
                } catch (java.util.InputMismatchException e) {
                    System.out.println("Enter numbers only.");
                    num = 0;
                    --i;
                    continue;
                }
                if (num <= 0 || num > 5) {
                    System.out.println("Enter 1 to 5 only.");
                    num = 0;
                    --i;
                    continue;
                }
                array[i] = num;
            }
        } while (num <= 0 || num > 5);


        int[] frequency = new int[6];

        for (int i = 0; i < array.length; i++) {
            frequency[array[i]] = frequency[array[i]] + 1;
        }

        System.out.printf("*    :%d (awful)\n", frequency[1]);
        System.out.printf("**   :%d\n", frequency[2]);
        System.out.printf("***  :%d\n", frequency[3]);
        System.out.printf("**** :%d\n", frequency[4]);
        System.out.printf("*****:%d (excellent)\n", frequency[5]);
    }
}