仅检查用户输入的正值

Question

我正在阻止用户输入字符串，但我也想检查用户输入的是零还是负数。我应该如何在同一个 do-while 循环中执行它？

这是我的代码：

import java.util.Scanner;
class Check{
public static void main(String[] args){
     Scanner input = new Scanner(System.in);
     int a = 0;
     boolean invalidInput;
      do {
            invalidInput = false;
            try {
                System.out.println("\nEnter the positive integer");
                a = input.nextInt();
            } catch (Exception e) {
                System.out.println("Please enter a valid positive integer.\n");  
                invalidInput = true;
                input.nextLine();
            } 
        } while (invalidInput);
   }
}

Answer 1

试试这个：

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        int a = 0;
        boolean invalidInput;
        do {
            invalidInput = false;
            try {
                System.out.println("\nEnter the positive integer");
                a = input.nextInt();
                if(a<0){
                    throw new Exception("Negative number entered");
                } else {
                    System.out.println("It is a positive integer. Exiting.");
                }
            } catch (Exception e) {
                System.out.println("Please enter a valid positive integer.\n");  
                invalidInput = true;
                input.nextLine();
            } 
        } while (invalidInput);

    }
}

输出：

Enter the positive integer                                                                                                                                  
10                                                                                                                                                          
It is a positive integer. Exiting.

再次运行否定检查：

Enter the positive integer                                                                                                                                  
-10                                                                                                                                                         
Please enter a valid positive integer.                                                                                                                      
                                                                                                                                                            
                                                                                                                                                            
Enter the positive integer

Answer 2

您可以创建一个特定的类来询问用户输入，并使用必要的逻辑来过滤无效答案。

类似：

class Console {

    private Scanner reader = new Scanner(System.in);

    private String readLine() {
        if (!reader.hasNextLine()) return ""; // Control exit here
        return reader.nextLine();
    }

    public int answerInt(String message) {
        System.out.println(message);

        String provided = readLine();
        if (isInteger(provided)) return Integer.parseInt(provided);

        System.err.println("You have entered an invalid value. Please provide a number.");
        return answerInt(message);
    }

    private boolean isInteger(String string) { // This should be a number util, not a console class responsability.
        try {
            Integer.parseInt(string);
            return true;
        } catch (Exception er) {
            return false;
        }
    }

}

然后，例如，只需添加一个新方法来处理允许的数字范围。

public int answerNumberBetween(int min, int max, String message) {
    int num = answerInt(message);

    // Validate if the number is between the provided range.
    if (num >= min && num <= max) return num;

    System.err.println("You have entered an invalid value. Please provide a number between '%d' and '%d'.", min, max);
    return answerNumberBetween(min, max, message);
}

Answer 3

希望对您有所帮助：

 do {
            invalidInput = false;
            try {
                System.out.println("\nEnter the positive integer");
                String num = input.next();
                int a = Integer.parseInt(num); // <-- this will throw NumberformatException if user inputs Alphabets
                if(a <= 0) { //<-- added 
                    throw new NumberFormatException();
                }
            } catch (NumberFormatException e) {
                System.out.println("Please enter a valid positive integer.\n");  
                invalidInput = true;
                input.nextLine();
            } 
        } while (invalidInput);

输出：

Enter the positive integer
0
Please enter a valid positive integer.

Enter the positive integer
-1
Please enter a valid positive integer.