Java 整数形式的字符串方程答案

我在一个低级java类中，需要使用java和JavaScript制作一个应用程序。我决定创建一个游戏，使用加法、减法、乘法和除法创建一个随机方程。我遇到的问题是，我的方程式是以字符串的形式创建的，我需要能够计算出答案，并将其与用户的答案进行比较。我以为我有一个下降的解决方案，但我不断得到一些问题。你能帮忙吗？我的代码如下：

import java.util.*;
public class GameClient {

    private static Stack<String> operations;
    private static Stack<Integer> numbers;
    private static String[] tokens;
    private static int i;
    private static Stack<String> subAdd;
    private static Stack<Integer> restOfNumbers;

    public static void main(String[] args) {
        Random variable = new Random();
        int numberOfVariables = variable.nextInt(5)+2;      //determines length of equation
        String equation = "";
        equation = equationGenerator(equation, numberOfVariables, variable);
        System.out.println(equation);
        System.out.print(calculateAnswer(equation));

    }
    public static String equationGenerator(String equation, int numberOfVariables, Random variable){
        int operation;
        int var;

        var = variable.nextInt(10)+1;

        equation += var;
        operation = variable.nextInt(4)+1;

        if(numberOfVariables == 1){
            equation += " = ";
            return equation;
        }

        if(operation == 1)
        {
            equation += " + ";
        }
        else if(operation == 2)
        {
            equation += " - ";
        }
        else if(operation == 3)
        {
            equation += " / ";
        }
        else if(operation == 4)
        {
            equation += " * ";
        }

        return equationGenerator(equation, numberOfVariables-1, variable);
    }

    public static int calculateAnswer(String equation){
        String delims = "[ ]+";
        tokens = equation.split(delims);
        for(i=0; i< tokens.length; i++)                     //does all multiplication and division first leaving just addition and subtraction
            switch(tokens[i]){
            case "0": case "1":case "2":case "3":case "4":case "5":case "6":case "7":case "8":case "9":
                int number = Integer.parseInt(tokens[i]);
                popOrPush(number);
                break;
            case "*": case "/": case "+": case "-":
                popOrPush(tokens[i], tokens);
            }

        while(!numbers.empty()){                    //flips number and operation stacks to do addition and subtraction in correct order
            restOfNumbers.push(numbers.pop());
        }
        while(!operations.empty()){
            subAdd.push(operations.pop());
        }
        while(!subAdd.empty()){
            switch(subAdd.pop()){
            case "+":
                restOfNumbers.push(restOfNumbers.pop() + restOfNumbers.pop());
            case "-":
                restOfNumbers.push(restOfNumbers.pop() - restOfNumbers.pop());
            }
        }
        return restOfNumbers.pop();
    }

    public static void popOrPush(int number){
        numbers.push(number);
    }
    public static void popOrPush(String operation, String[] tokens){
        switch(operation){
        case "*": 
            int multipliedValue = numbers.pop();
            i++;
            multipliedValue = multipliedValue * Integer.parseInt(tokens[i]);
            numbers.push(multipliedValue);
        case "/": 
            int dividedValue = numbers.pop();
            i++;
            dividedValue = dividedValue / Integer.parseInt(tokens[i]);
            numbers.push(dividedValue);
        case "+": case "-":
            operations.push(operation);
    }
    }
}

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您没有初始化堆栈，因此在尝试使用numbers变量时可能会得到NPE

确保初始化所有变量，尤其是堆栈对象。例如：

Stack<Integer> numbers = new Stack<>();

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那么，它是java还是javascript？因为我有消息要告诉你，不可能两者都有。当你不知道标签的含义时，为什么还要添加标签？如果是Java，请看下面的答案。这是Java而不是Javascript。好的，酷，是它的Java。但这不是问题所在。对不起，我不知道我到底在说什么，所以我在这个网站上。有人知道为什么我在尝试运行此程序时会出现NullPointerException吗？它必须在calculateAnswer方法或popOrPush方法中，因为它将创建没有问题的方程。我一直得到一些问题是一个完全无用的问题描述。如果您有错误，发布堆栈跟踪，描述您为解决该错误所做的工作，并解释具体的问题所在。