捕获/尝试在扫描仪的输入不匹配异常中不起作用？

Question

Alright so I'm taking a CS security class that has some basic java programing in it and our first assignment is to play with BigInteger . However, we also have to "bullet proof" our program. And while my method isn't the most ideal, it works EXCEPT for the first input. Aka if I type in an invalid input for any of the input.new---(); my program will prompt the user to try again with valid numbers. But the first input.nextInt(); will still take invalid input and crash, displaying the "java.util.InputMismatchException" and then rambles on about scanner. Any ideas as to why this is happening? ps The program must not display an error log under any circumstances.

import java.io.*;
import java.util.*;
import java.math.*;

class BigNumber{
   public static void main(String args[]){

     while(true){ 
      try{
         Scanner input = new Scanner(System.in);

         System.out.print("if you wish to exit the program type in '0,' to continue running the program type in any other value: ");
         double esc= input.nextDouble();
         if(esc == 0){ break;}
         else{System.out.println("Ok, program running...");}
         input.nextLine();

         System.out.print("Enter number a: ");
         String aValue = input.nextLine();

         System.out.print("Enter number b: ");
         String bValue = input.nextLine();

         System.out.print("Enter a number 'n': ");
         String nValue = input.nextLine();
         while (Integer.parseInt(nValue) < 1)
         {
            if (Integer.parseInt(nValue) < 1)
            {
               System.out.print("Please enter a valid number (n>1): ");
               nValue = input.nextLine();
            }
         }

         BigInteger a= new BigInteger(aValue);
         BigInteger b= new BigInteger(bValue);
         BigInteger n= new BigInteger(nValue);

         System.out.println("---------------------------");
         System.out.println("1) a xor b: " + a.xor(b));
         System.out.println("2) b xor b: " + b.xor(b));
         System.out.println("3) a xor b xor a: " + a.xor(b).xor(a));
         System.out.println(" ");
         System.out.println("4) ab mod n: " + a.modPow(b,n));
         System.out.println(" ");
         System.out.println("5) a shifted to the right by 6: " + a.shiftRight(6));
         System.out.println("6) b shifted to the left by 3: " + b.shiftLeft(3));
         System.out.println("---------------------------");

      }
      catch (NumberFormatException e){
         System.out.println("-----> Please try entering a valid number(s) <-----");
      }

    } 
  }
}

Answer 1

Your catch block only handles NumberFormatException . If you're trying to handle all Exception (s) then I suggest you change this

catch (NumberFormatException e){

to something like

catch (Exception e){

Answer 2

nextDouble() will read double value. If parseDouble fails, a InputMismatchException is thrown.

Here the code of the class:

public double nextDouble() {
    // Check cached result
    if ((typeCache != null) && (typeCache instanceof Double)) {
        double val = ((Double)typeCache).doubleValue();
        useTypeCache();
        return val;
    }
    setRadix(10);
    clearCaches();
    // Search for next float
    try {
        return Double.parseDouble(processFloatToken(next(floatPattern())));
    } catch (NumberFormatException nfe) {
        position = matcher.start(); // don't skip bad token
        throw new InputMismatchException(nfe.getMessage());
    }
}

Answer 3

Oracle says

public class InputMismatchException extends NoSuchElementException Thrown by a Scanner to indicate that the token retrieved does not match the pattern for the expected type, or that the token is out of range for the expected type.

public class NumberFormatException extends IllegalArgumentException Thrown to indicate that the application has attempted to convert a string to one of the numeric types, but that the string does not have the appropriate format.

you used only NumberFormatException in the catch block so it only catch the NumberFormatException . To catch other exception you have to add other catch block

catch (NumberFormatException e){
         System.out.println("-----> Please try entering a valid number(s) <-----");
catch (InputMismatchException e){
         System.out.println("-----> Please try entering a valid number(s) <-----");

OR if you used base Exception class it will catch any kind of Exception

catch (Exception e){
         System.out.println("-----> Please try entering a valid number(s) <-----");

Answer 4

InputMismatchException and NumberFormatException are different classes. there is no relationship between them,your program throws InputMismatchException and you are trying to catch NumberFormatException . You will need one more catch block for InputMismatchException exception.