Enjoy implementing calculator with stack and recursive call

This is a picture of unique shoji :)

1. Introduction

I enjoyed implementing a calculator. It can calculate as follows :)

1 + 2                       Answer: 3
1 + 2 x (10 + 1)        Answer: 23
(1 + 1) x (1 + 2)       Answer: 6

2. Program

2.1. Input

As for a program input, I supposed like the following letter string.

[Input]
1+2
1+1*(10+0)
(1+1)*(1+2)

2.2. Output

As for a program output, it's simply its calculation result.
In the above cases, the output is as follows.

[Output]
3
11
6

2.3. Implementation

I can simply implement by using a stack and a recursive call as follows.
Thanks to the recursive call, we can calculate the text within parentheses :)

Main.java

package org.inut;

import java.util.*;

class Main { 
    public void pushCalcStack(int currNum, char operator, Deque<Integer> calcStack) {
        if (operator == '+') {
            calcStack.push(currNum);
        } else if (operator == '-') {
            calcStack.push(-currNum);
        } else if (operator == '*') {
        int prevNum = calcStack.pop();
            calcStack.push(prevNum * currNum);
        } else if (operator == '/') {
        int prevNum = calcStack.pop();
        calcStack.push(prevNum / currNum);
        }
    }
 
    public int simpleCalc(String str) {
        Deque<Integer> calcStack = new ArrayDeque();
        int calcResult = 0;
        char operator = '+';
        int currNum = 0;
        int currIndex = 0;
        boolean isParentheses = false;
        int startIndex = 0;
     
        for (char letter : str.toCharArray()) {
            if (Character.isDigit(letter)) {
                currNum = 10 * currNum + letter - '0';
            } else {
                if (letter == '(') {
                    isParentheses = true;
                    startIndex = currIndex + 1;
                }
                if (letter == ')') {
                    isParentheses = false;
                    currNum = simpleCalc(s.substring(startIndex, currIndex));
                }
                if (!isParentheses) {
                    pushCalcStack(currNum, operator, calcStack);
                    operator = letter;
                    currNum = 0;
                }
            }
            currIndex++;
        }
     
        if (currNum != 0)
        pushCalcStack(currNum, operator, calcStack);
     
        for (int num : calcStack)
            calcResult += num;
     
        return  calcResult;
    }
 
    public static void main(String[] args) {
        String input = "2*(1+2)/2-1*(5*3)";
     
        int calcResult = new Main().simpleCalc(input);
        System.out.println(calcResult);
    }
}

3. Extension

If we want to use other parentheses such as '{' and '[', we can do that by using a recursive call and modifying some conditions :)