java - 我需要这个计算器来重定向用户输入，以防他输入错误

Question

在第三个输入中，它要求输入一个数字来进行特定的操作。但是当输入例如“5”时，它不会自动将其发送到默认部分以显示消息“该值不存在”。除此之外，我还需要它再次显示输入提示并要求输入从 1 到 4 的数字。谢谢

import java.io.BufferedReader;
import java.io.InputStreamReader;


public class Specialized {

    public static void main(String[] args) {
        String s1 = getInput("Enter a number: ");
        String s2 = getInput("Enter a number: ");
        String op = getInput("Enter: 1 = Add, 2 = Subtract, 3 = Multiply, 4    = Divide ");

         int opInt = Integer.parseInt(op);
         double result= 0;

        switch (opInt) {
        case 1:
            result = addValues(s1,s2);
            break;
        case 2:
            result = subtractValues(s1,s2);
            break;
        case 3:
            result = multiplyValues(s1,s2);
            break;
        case 4:
            result = divideValue(s1,s2);
            break;
        default:
            System.out.println("the value doesn't exist.");
            break;
        }
        System.out.println(result);
    }

    private static String getInput(String prompt) {
        BufferedReader stdin = new BufferedReader(
                new InputStreamReader(System.in));
        System.out.print(prompt);
        System.out.flush();

        try {
            return stdin.readLine();
        } catch (Exception e) {
            return "Error: " + e.getMessage();
        }
    }

    private static double addValues(String s1, String s2) 
         throws NumberFormatException {
        double d1 = Double.parseDouble(s1);
        double d2 = Double.parseDouble(s2);
        double result = d1 + d2;
        return result;
    }

    private static double subtractValues(String s1, String s2) {
        double d1 = Double.parseDouble(s1);
        double d2 = Double.parseDouble(s2);
        double result = d1 - d2;
        return result;
    }

    private static double multiplyValues(String s1, String s2) {
        double d1 = Double.parseDouble(s1);
        double d2 = Double.parseDouble(s2);
        double result = d1 * d2;
        return result;
    }

    private static double divideValue(String s1, String s2) {
        double d1 = Double.parseDouble(s1);
        double d2 = Double.parseDouble(s2);
        double result = d1 / d2;
        return result;
    }

}

Answer 1

通常，您用 while 循环包围“从用户那里获取输入”，直到获得所需的输入。像这样。

public static void main(String[] args) {
        String s1 = getInput("Enter a number: ");
        String s2 = getInput("Enter a number: ");

        //notice I declare this outside now so I can compute for new value inside loop
        int opInt = 0;
        double result = 0;

        //you can change the condition later if you add more options
        while(opInt < 1 || opInt > 4) {
            String op = getInput("Enter: 1 = Add, 2 = Subtract, 3 = Multiply, 4    = Divide ");
            //recalculate the choice
            opInt = Integer.parseInt(op);

            switch (opInt) {
                case 1:
                    result = addValues(s1, s2);
                    break;
                case 2:
                    result = subtractValues(s1, s2);
                    break;
                case 3:
                    result = multiplyValues(s1, s2);
                    break;
                case 4:
                    result = divideValue(s1, s2);
                    break;
                default:
                    System.out.println("the value doesn't exist.");
                    break;
            }
        }
        System.out.println(result);
    }