isDigit验证

Question

我正在尝试验证输入1或2.但是，使用此代码，如果输入字母，则会导致程序崩溃。

如何解决这个问题？

System.out.print("Choice: ");
userSelection  = keyboard.nextInt();

while (flag == 0)
{
    if (userSelection == 1 || userSelection == 2)
    {
        flag = 1;
    }
    if(Character.isDigit(userSelection))
    {
        flag = 1;
    }
    else 
    {
        flag = 0;
    }
    if (flag == 0)
    {
        //this is a system clear screen to clear the console
        System.out.print("\033[H\033[2J");  
        System.out.flush(); 

        //display a warning messege that the input was invalid
        System.out.println("Invalid Input! Try again, and please type in selection 1 or selection 2 then hit enter");
        System.out.print("Choice: ");
        userSelection  = keyboard.nextInt();
    }
}

Answer 1

nextInt期望用户键入一个整数。否则就会失败。

您可能想要使用nextLine构建一些内容（它会读取您可以检查的输入行，reading single characters as they are being typed is tricky in Java）。

Answer 2

试试这段代码：

try (Scanner scanner = new Scanner(System.in)) {
   int choice;
   while (true) {
       System.out.print("Choice: ");
       if (!scanner.hasNextInt()) {
            scanner.nextLine();//read new line character
            System.err.println("Not a number !");
            continue; // read again
       }
       choice = scanner.nextInt(); //got numeric value
       if (1 != choice && 2 != choice) {
             System.err.println("Invalid choice! Type 1 or 2 and press ENTER key.");
             continue; //read again
       }
       break;//got desired value
   }
   System.out.printf("User Choice: %d%n", choice);
}