如何验证以便用户只输入整数

Question

Iv'e been trying to get my code to work so that the user can only input integers. However, its keeps crashing when I enter something with characters like "awsd". I've tried using a bool to help, but it only catches negative inputs. Also, the input method must start as an integer, so I cant switch it to a string. Please help.

import java.util.Scanner;   // Needed for Scanner class
import java.io.*;           // Needed for File I/O classes

public class Reverse {
    public static void main(String[] args) throws IOException {
        Scanner keyboard = new Scanner(System.in);
        String Continue = "yes";
        int num;

        //creates the file name
        File fileWR = new File("outDataFile.txt");
        //creates the file object
        fileWR.createNewFile();
        //file scanner
        BufferedWriter output = new BufferedWriter(new FileWriter(fileWR, true));

        while (Continue.equals("yes")) {
            System.out.print("Enter an integer number greater than 0 :");
            num = keyboard.nextInt();
            keyboard.nextLine();
                if (fileWR.exists())
                {
                    validate(num, output);
                }
                else
                {
                    fileWR.createNewFile();
                }

                //option if the user wants to continue
                System.out.println("Do you wish to continue?(yes or no): ");
                Continue = keyboard.nextLine();
            }
        output.close();
    }


    public static void validate(int num, BufferedWriter output) throws IOException {
        Scanner keyboard = new Scanner(System.in);
        while(!checkNum(num))
        {
            System.out.print("That is not an integer greater than 0, please try again: ");
            num = keyboard.nextInt();
            keyboard.nextLine();
        }
        System.out.print("The original numbers are " + num +"\n");
        output.write("\r\nThe original numbers are " + num +"\r\n");
        reverse (num, output);
        even (num, output);
        odd(num, output);

    }// end of public static void validate

    public static void reverse(int num, BufferedWriter output) throws IOException {                                                                                                         
        String input = String.valueOf(num);         //must output result within the void method for it to count as a void method
        String result = "";                         //otherwise, you cannot output it in the main method.

        for (int i = (input.length() - 1); i >= 0; i--) 
           {
               result = result + input.charAt(i)+' ';
           }
           result = "the number reversed "+ result +"\r\n";
           System.out.print(result);
           output.write(result);
    }// end of public static void reverse

    public static void even(int num, BufferedWriter output) throws IOException {
        String input = String.valueOf(num);
        String result = "";

           for (int i = 0; (i < input.length()); i++) 
           {
               if (Character.getNumericValue(input.charAt(i)) % 2 == 0) 
                   result = result + input.charAt(i) + ' ';
           }
           if (result == "") {
               result = "There are no even digits" + "\r\n";
           } else {
               result = "the even digits are "+ result +"\r\n";
           }
           System.out.print(result);
           output.write(result);
    }// end of public static void even

    public static void odd(int num, BufferedWriter output) throws IOException {
        String input = String.valueOf(num);
        String result = "";

           for (int i = 0; (i < input.length()); i++) 
           {
               if (Character.getNumericValue(input.charAt(i)) % 2 == 1)
               {
                   result = result + input.charAt(i) + ' ';
               }
           }
           if (result == "") {
               result = "There are no odd digits" + "\r\n";
           } else {
               result = "the even odd digits are "+ result +"\r\n";
           }
           System.out.print(result);
           output.write(result);
           System.out.print("------------------------\n");
           output.write("------------------------\n");

    }// end of public static void odd
    public static boolean checkNum(int num)
    {
        if(num > 0) {
            return true;
        }
        else {
            return false;
        }

    }
}

Answer 1

You should first get the String, and then try to cast it:

try {
    String numStr = keyboard.nextLine();
    num = Integer.parseInt(numStr);
    //Complete with your code
} catch (NumberFormatException ex) {
    System.out.print("That is not an integer");
}