继续提示，直到满足条件

Question

我正在练习异常处理，或多或少地掌握了基本概念。

我一直在搜索如何通过提示用户输入满足特定标准的值来继续执行，尽管使用循环捕捉到了异常

我有这个特定的代码，它请求用户输入一个范围之间的值，并在输入字符串时进行异常处理。但是，程序在打印出异常处理后停止执行。

有什么想法吗？我如何实现一个循环或任何其他方法，可以在异常处理后继续执行程序？

Scanner scanner = new Scanner(System.in);
int num = 0;
try
{
    System.out.print("Please enter a number between 1 to 50 : ");
    num = scanner.nextInt();
} 
catch (InputMismatchException e) {
    System.out.println("Not a number");
    return;
}
while (num > 50 || num < 1) {
    System.out.print("Out of range. Enter a number between 1 to 50 : ");
    num = scanner.nextInt();
}
System.out.println("The number is : " + num);

Answer 1

异常应该处理异常情况，即您无法提前预料的情况。由于您可以肯定地预料到用户可能会输入无效输入，因此您可以在没有任何异常处理的情况下处理该无效输入：

    Scanner scanner = new Scanner(System.in);
    int num = 0;
    while (num > 50 || num < 1) {
        System.out.print("nPlease enter a number between 1 to 50 : ");
        while (!scanner.hasNextInt()) {
            scanner.next(); // discard non-integer inputs
            System.out.print("nPlease enter a number between 1 to 50 : ");
        } 
        num = scanner.nextInt();
    }
    System.out.println("You entered " + num);

样本输出：

Please enter a number between 1 to 50 : -1
Please enter a number between 1 to 50 : 53
Please enter a number between 1 to 50 : ff
Please enter a number between 1 to 50 : rr rr ff
Please enter a number between 1 to 50 : 
Please enter a number between 1 to 50 : 
Please enter a number between 1 to 50 : 13
You entered 13

请注意，此代码比使用异常处理的版本要短得多。

Answer 2

使用此：

Scanner scanner = new Scanner(System.in);
boolean isInputValid = false; // input flag, valid = true / invalid = false
int num = 0;
while(!isInputValid) {
    try
    {
        System.out.print("Please enter a number between 1 to 50 : ");
        num = scanner.nextInt();
        // Input is a valid integer
        if (!(num > 0 && num < 51)) { // input out of range
            System.out.print("Out of range.");
        }
        else
            isInputValid = true; // input valid, proceed & break loop
    }
    catch (InputMismatchException ex) { // input not an integer
        System.out.println("Not a number");
        scanner.next();
    }
    catch (Exception ex) {
        ex.printStackTrace();
    }
}
System.out.println("The number is : " + num);