如何处理反向抛光符号计算器中的数字？

Question

import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.Scanner;
import java.util.Stack;


public class PostFixEval {

    static Scanner input = new Scanner(System.in);
    static String expression;
    static Stack<Integer> calcStack = new Stack<Integer>();
    static List<String> expressionList = new ArrayList<String>();
    static boolean flag = false;

    public static void main(String[] args) {
        System.out.print("Enter Your Expression: ");
        while (flag == false){
            expression = input.next();
            System.out.println("");
            expression = whiteSpaceRemover(expression);

            if (expression.substring(expression.length() - 1) == ";"){
                expressionList.add(expression);
            } else if (expression.substring(expression.length() - 1) == "@"){
                expressionList.add(expression);
                flag = true;
            } else {
                System.out.println("Not a valid expression (must end in a ; or a @)");
            }
        }

        calc(expressionList);               
    }

    public static void calc(List<String> list){
        for (int i=0;i<list.size();i++){

            String currentExpression = list.get(i);

            for (char ch : currentExpression.toCharArray()){
                if (ch == '+'){
                    calcStack.push(calcStack.pop()+calcStack.pop());

                    System.out.println("Pop R1");
                    System.out.println("Pop R2");
                    System.out.println("ADD R1, R2");
                    System.out.println("PUSH R1");
                } 
                else if (ch == '-')
                {
                    int temp = calcStack.pop();
                    calcStack.push(calcStack.pop() - temp);

                    System.out.println("Pop R2");
                    System.out.println("Pop R1");
                    System.out.println("SUB R1, R2");
                    System.out.println("PUSH R1");
                } 
                else if (ch == '*')
                {
                    calcStack.push(calcStack.pop() * calcStack.pop());

                    System.out.println("Pop R1");
                    System.out.println("Pop R2");
                    System.out.println("MULT R1, R2");
                    System.out.println("PUSH R1");
                } 
                else if (ch == '/')
                {
                    int temp = calcStack.pop();

                    if(temp != 0){
                        calcStack.push(calcStack.pop() / temp);
                        System.out.println("Pop R1");
                        System.out.println("Pop R2");
                        System.out.println("DIV R1, R2");
                        System.out.println("PUSH R1");
                    }else{
                        System.out.println("CANNOT DIVIDE BY ZERO");
                        break;
                    }
                }
                else if
                {
                    int num = Integer.parseInt(ch);
                }


            }
        }

    }

    public static String whiteSpaceRemover(String express){
        return express.replaceAll(" ","");
    }

}

我一直在研究反向波兰表示法计算器，但我遇到了一个问题。我已经意识到我无法将数字添加到堆栈中，因为我的代码分别遍历每个表达式的每个字符。我已经考虑过将每个数字添加到一个数组中，然后将其转换为int，但我不确定我是怎么做的。任何帮助将不胜感激。

感谢。