First Midterm.8 Mathematical Expression

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.PriorityQueue;
import java.util.Stack;

public class ExpressionEvaluator {

    public static int evaluate(int a, char sign, int b) {
        if (sign == '+') return a + b;
        else if (sign == '*') return a * b;
        return 0;
    }

    public static int evaluateExpression(String expression) {
        char symbols[] = expression.toCharArray();

        Stack<Character> charStack = new Stack<>();
        Stack<Integer> intStack = new Stack<>();

        for (int i = 0; i < symbols.length; i++) {
            if (symbols[i] == '+' || symbols[i] == '*') {
                charStack.push(symbols[i]);
            } else {
                intStack.push(symbols[i] - '0');
            }
        }
        /*System.out.println(intStack.toString());
        System.out.println(charStack.toString());

        System.out.println();*/

        charStack = new Stack<>();
        intStack = new Stack<>();
        StringBuilder sb = new StringBuilder();
        for (int i=0 ; i<expression.length(); ++i) {

            char curr = expression.charAt(i);
            if ( curr == '+' || curr =='*') {
                intStack.push(Integer.parseInt(sb.toString()));
                sb = new StringBuilder();
                charStack.push(curr);
            }

            else {
                sb.append(curr);
            }

        }
        intStack.push(Integer.parseInt(sb.toString()));

        /*System.out.println(intStack.toString());
        System.out.println(charStack.toString());*/

        while (!charStack.isEmpty()) {
            int a = intStack.pop();
            int b = intStack.pop();
            char sign = charStack.pop();

            if (sign == '*') {
                intStack.push(a * b);
            } else {
                if (charStack.isEmpty() || charStack.peek() == '+') {
                    intStack.push(a + b);
                } else {
                    /* a & b want to add, but b must multiply first */
                    int temp = intStack.pop(); /* get next int */
                    charStack.pop(); /* another multiplication, just get rid of it */
                    intStack.push(b * temp); /* second * third */
                    intStack.push(a); /* add a on top after b is done */
                    charStack.push(sign); /* preserve the + we got at the start */
                }
            }
        }
        return intStack.pop();
    }

    public static void main(String[] args) throws IOException {
        BufferedReader input = new BufferedReader(new InputStreamReader(System.in));
        System.out.println(evaluateExpression(input.readLine()));
    }

}