java - 我正在使用 Java 构建计算器，但在执行运算符时遇到问题

Question

我对 Java 还很陌生，以前从未编程过。当尝试为每个运算符运行代码时，我收到以下错误。我希望它以字符串形式接收用户输入，但返回 double 值作为结果。我知道可能会有几个错误，但我们将不胜感激。

线程“main”java.lang.Error中出现异常: Unresolved 编译问题: 类型不匹配:无法从 String 转换为 double 对于参数类型 java.lang.String、java.lang.String ，运算符 - 未定义 对于参数类型 java.lang.String、java.lang.String ，运算符 * 未定义 对于参数类型 java.lang.String、java.lang.String ，运算符/未定义 对于参数类型 java.lang.String、java.lang.String 未定义运算符 % 无法对非静态字段 Math 进行静态引用 无法对非静态字段 Math 进行静态引用 命令无法解析为类型 对于参数类型 Class、int ，运算符 >= 未定义 标记“.”上的语法错误，此标记后应有类 对于参数类型 String, int ，运算符 <= 未定义

at Calculator.main(Calculator.java:72)

import java.util.Scanner;
import java.lang.Math;

/*@public class for calculator*/
public class Calculator {
    String ADD = "+";
    String SUB = "-";
    String DIV = "/";
    String MUL = "*";
    String MOD = "%";
    String SQRT = "sqrt";
    String POW = "^";
    String Math.PI = "pi";

    public static void main(String[] args) {


        /*@Scanner to read user input */
        Scanner myScanner = new Scanner(System.in);
        boolean continueLoop = true;

        /*@try statement to begin program */
        try {



            String x,y,command;
            double result = 0;
            double myDouble = result;

            /*@print line welcoming user to calculator */
            System.out.print("Welcome to my Calculator! Lets Begin!" + "\n");
            /*@print line asking user to either type HELP or START to begin */
            System.out.print("Type HELP or START to begin: ");
            /*@string to read user input for HELP or START command */
            String firstCommand = myScanner.nextLine();
            if (firstCommand.equals("HELP")) {
                System.out.println("Welcome to my calculator. Make sure to enter only numbers into the first two fields. \nDo NOT divide by 0. \nBe sure to enter the number matching your desired operation.");
                System.out.print("Type HELP or START to begin: ");
                command = myScanner.nextLine();
            }




            Double firstNumber = Double.parseDouble(myScanner.nextLine());
            /*@print statement asking user for first number input */
            System.out.println("Please Enter your First number: ");
            /*@variable x to be read by scanner */

            x = myScanner.nextLine();

            Double secondNumber = Double.parseDouble(myScanner.nextLine());
            /*@print statement asking user for second number input *        
            System.out.println("Please Enter your Second Number: ");

            /*@variable y to be read by scanner */
            y = myScanner.nextLine();

            /*@param converts string into double*/
            Double converted = Double.parseDouble(myScanner.nextLine());    
            /*Print statements with instructions on what commands to put in and where to put in.*/
            System.out.print("\n1: Add. \n2: Sub. \n3: Mult. \n4: Divide. \n5: Modulo. \n6: Pi \n7: Pow.");
            System.out.print("\nEnter your operator: ");

            command = myScanner.nextLine();

            switch(command) {
            /*@returns addition of firstNumber and secondNumber */
            case "1":
                result = (x+y); break;
            /*@returns subtraction of firstNumber and secondNumber */
            case "2":
                result = (x-y); break;
            /*@returns multiplication of firstNumber and secondNumber */
            case "3":
                result = (x*y); break;
            /*@returns division of firstNumber and secondNumber */
            case "4":
                result = (x/y); break;
            /*@returns modulo of firstNumber and secondNumber */
            case "5":
                result = (x%y); break;
            /*@returns Math.PI */
            case "6":
                result = (Math.PI);
            /*@returns power of firstNumber to the second number secondNumber*/
            case "7":
                result = (Math.pow(x, y));
            }
            if(command. >= 1 && command <= 5)
                System.out.println("Your answer is: " + result);

            /*@if statements for all mathematical operators */
            // cannot get sqrt, pow, pie to work


        /*@throws exception if anything other than a number is typed in by user */  
        /*catch exception if anything other than number is keyed in*/
        } catch (InputMismatchException inputMismatchException) {
            /*@error statement printed*/
            System.err.printf("%nException: %s%n", inputMismatchException);
            /*@statement for scanner to read the next line*/
            myScanner.nextLine(); // discard input, so user can try again
            /*@print statement letting user know to try again*/
            System.out.printf("You must enter integers. Please try again %n%n");

        }
            /*@throws exception if zero is keyed in as denominator
            /*exception if zero is keyed in as the denominator in division since you cannot divide by zero*/
        catch (ArithmeticException arithmeticException) {
            /*error statement printed*/
            System.err.printf("%nException: %s%n", arithmeticException);
            /*print statement letting user that 0 is not valid and to please try again
            not able to get the loop to go back and restart*/
            System.out.printf("Zero is an invalid denominator.Please try again. %n %n");
        }
        /*@param finally statement closes scanner*/
        finally {
            myScanner.close();
        } while(continueLoop);

    } 



}

Answer 1

以下是我第一眼看到的一些问题。

1. 使用 try 时缺少 catch 或 finally block 。
2. 比较字符串与整数(比较之前需要进行一些类型转换)

3. myScanner.toString() 不读取用户输入。要读取用户输入，您可以使用 myScanner.next()。

4. 里面的if条件if (command.equals("HELP"))...，这里你还没有从用户输入中读取命令，看来你想使用if (firstCommand.equals("HELP")).

5.代码中从未使用过名为“firstNumber”和“secondNumber”的变量。

在提示用户提供输入之前使用 myScanner.nextLine()。

case语句中使用的变量是String类型。但是，应将其视为整数。正确的方法应该是这样case "1"://命令的数据类型是字符串，所以应该用引号 ""括起来。