java请求一个整数

Question

我试图解决下一个任务： 1.要求用户输入正整数。 2.如果用户添加负数或实数或两者，控制台上应显示下一条错误消息：“输入错误”

这就是我到目前为止所做的

Scanner sc = new Scanner(System.in);
System.out.print("Please, add a positive integer! ");
int num = sc.nextInt();
if (num < 0) {
    System.out.println("wrong input");
}

一切正常，但如果用户输入的不是整数而是实数，我无法确保用户收到错误消息。在这种情况下，程序出错

我将非常感谢你的帮助

Answer 1

有时，我发现读入字符串然后尝试解析它更容易。这假定您希望重复提示，直到获得有效的数字

Scanner sc = new Scanner(System.in);
int num = -1;
while (num < 0) {
    System.out.print("Please, add a positive integer! ");

    String str = sc.nextLine();
    try {
        num = Integer.parseInt(str);
    } catch (NumberFormatException e) {
         System.out.println("Only integers are accepted.");
         continue;
    }
    if (num < 0) {
        System.out.println("Input is < 0.");
    }
}

在这里阅读关于NumberFormatException的文章

Answer 2

import java.util.Scanner;
import java.util.InputMismatchException;

public class ScanTest
{
  public static void main(String[] args)
  {
    Scanner sc = new Scanner(System.in);
    boolean badInput = true;
    int num;

    // Keep asking for input in a loop until valid input is received

    while(badInput)
    {
        System.out.print("Please, add a positive integer! ");

        try {
            num = Integer.parseInt(sc.nextLine());
            // the try catch means that nothing below this line will run if the exception is encountered
            // control flow will move immediately to the catch block
            if (num < 0) {
                System.out.println("Please input a positive value.");
            } else {
                // The input is good, so we can set a flag that allows us to exit the loop
                badInput = false;
            }
        }
        catch(InputMismatchException e) {
            System.out.println("Please input an integer.");
        }
        catch(NumberFormatException e) {
          System.out.println("Please input an integer.");
        }
      }
    }
}

Answer 3

使用Scanner.nextInt()时，输入应为整数。输入任何其他内容，包括实数，都会抛出InputMismatchException。请确保您的程序不尝试处理无效的输入：

int num;
try {
    num = sc.nextInt();
    // Continue doing things with num
} catch (InputMismatchException e) {
    // Tell the user the error occured
}