扫描器和方法的Java异常问题

Question

我已经尝试过了，但没有找到任何可以理解的解决方案（我对java非常陌生，这对我来说很难），或者自己解决它。以下是完整的代码（我知道代码不是那么高效）：

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        int con = 1;
        System.out.println("Hey!,welcome to my games!");
        Scanner scanner = new Scanner(System.in);
        String game;
        while (con == 1) {
            System.out.println(
                    "Here Some games i have(Enter the text for the one you want):\nto stop=0\n1)calculator=1\n2)!Soon! Random numbers");
            game = scanner.nextLine();
            calculator();
            scanner.reset();
            System.out.println(game);
//          if(game.equals("0")) {
//              con=0;
//          }
//          else if(game.equals("1")) {

//              System.out.println("Welcome Back!");
//          }
//          else {
//              System.out.println("There is a mistake in your text");
//          }

        }
        scanner.close();
    }

    static void calculator() {
        int num1, num2, con = 1, stop = 1;
        String op, ad = "add", su = "sub", mul = "multi", di = "div";
        Scanner af = new Scanner(System.in);
        while (con == 1) {
            stop = 1;
            System.out.println("Write number 1");
            num1 = af.nextInt();
            System.out.println("Write number 2");
            num2 = af.nextInt();
            System.out.println(
                    "Write an operation (one of these):\nAddition=add\nSubtraction=sub\nMultiplication=multi\nDivision=div");
            op = af.next();
            op = op.toLowerCase();
            int minus = num1 - num2;
            int plus = num1 + num2;
            if (op.equals(ad)) {
                System.out.println("The Operation is:Addition\n" + num1 + "+" + num2 + "=" + plus);
            } else if (op.equals(su)) {
                System.out.println("The Operation is:Subtraction\n" + num1 + "-" + num2 + "=" + minus);
            } else if (op.equals(mul)) {
                System.out.println("The Operation is:Multiplication\n" + num1 + "*" + num2 + "=" + num1 * num2);
            } else if (op.equals(di)) {
                System.out.println("The Operation is:Division\n" + num1 + "/" + num2 + "=" + num1 / num2);
            } else {
                System.out.println("Um,Did you make a mistake in your text?\nDo you want the calculator again?");
                String yn = af.next();
                yn = yn.toLowerCase();
                if (yn.equals("yes") || yn.equals("yep")) {
                    stop = 0;
                }
            }
            if (stop == 1) {
                con = 0;
            }
        }
        af.close();
    }
}

        

正如您所看到的，我试图自己解决它，甚至在一些代码上添加了注释，但当它运行到方法并返回时，它失败了，因为扫描器认为在我写东西之前有东西要扫描。这里有个例外-

Exception in thread "main" java.util.NoSuchElementException
    at java.base/java.util.Scanner.throwFor(Scanner.java:937)
    at java.base/java.util.Scanner.next(Scanner.java:1478)
    at Main.main(Main.java:12)

Answer 1

1. 您需要一种机制，在输入无效的情况下进行循环（即要求用户再次输入数据）。最常见的方法之一是使用do-while循环，该循环保证至少执行一次其主体。查看本教程以了解有关它的更多信息。
2. 使用scanner::nextline而不是scanner::nextint、scanner::nextint等，因为它们不使用从输入点击Enter键创建的换行符。查看此讨论以了解更多信息。
3. 最后但并非最不重要的一点是，永远不要关闭system.in的扫描程序，因为它也会关闭system.in，而且如果不重新启动JVM就无法再次打开它。注意：如果使用扫描程序扫描文件，请确保关闭该文件以释放资源。

演示：

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        int option;
        boolean valid;
        Scanner input2 = new Scanner(System.in);
        do {
            valid = true;
            System.out.println("Choose an option:\n" + "1-Addition\n" + "2-Subtraction\n" + "3-Exit");
            try {
                option = Integer.parseInt(input2.nextLine());
                if (option < 1 || option > 3) {
                    throw new IllegalArgumentException();
                }
                // ...Place here the rest of code (which is based on the value of option)
            } catch (IllegalArgumentException e) {
                System.out.println("This is an invalid entry. Please try again.");
                valid = false;
            }
        } while (!valid);
    }
}

示例运行：

Choose an option:
1-Addition
2-Subtraction
3-Exit
x
This is an invalid entry. Please try again.
Choose an option:
1-Addition
2-Subtraction
3-Exit
10.5
This is an invalid entry. Please try again.
Choose an option:
1-Addition
2-Subtraction
3-Exit
1