postfix using stack

import java.util.Stack;

public class postfix {
    public static void main(String[] args) {
        String str = "9-(5+3)*4/6";
        Stack<String> val = new Stack<>();
        Stack<Character> op = new Stack<>();
        for (int i = 0; i < str.length(); i++) {
            char ch = str.charAt(i);
            if (Character.isDigit(ch)) {
                StringBuilder num = new StringBuilder();
                num.append(ch);
                while (i + 1 < str.length() && Character.isDigit(str.charAt(i + 1))) {
                    num.append(str.charAt(++i));
                }
                val.push(num.toString());
            } else if (ch == '(') {
                op.push(ch);
            } else if (ch == ')') {
                while (!op.isEmpty() && op.peek() != '(') {
                    String v2 = val.pop();
                    String v1 = val.pop();
                    char o = op.pop();
                    String t =   v1 + v2+o;
                    val.push(t);
                }
                op.pop(); // Remove '('
            } else {
                while (!op.isEmpty() && precedence(ch) <= precedence(op.peek())) {
                    String v2 = val.pop();
                    String v1 = val.pop();
                    char o = op.pop();
                    String t =  v1 + v2 + o;
                    val.push(t);
                }
                op.push(ch);
            }
        }
        while (!op.isEmpty()) {
            String v2 = val.pop();
            String v1 = val.pop();
            char o = op.pop();
            String t = v1 + v2 + o;
            val.push(t);
        }
        String pre = val.pop();
        System.out.println(pre);
    }

    private static int precedence(char op) {
        switch (op) {
            case '+':
            case '-':
                return 1;
            case '*':
            case '/':
                return 2;
            default:
                return 0;
        }
    }
}