一个java实现的简单的4则运算器

有些基础知识有欠缺，补一下，顺便练习一下java

import com.sun.deploy.util.ArrayUtil;

import java.util.*;

public class Main {

    public static int op_priority(char c) {
        switch (c) {
            case '(':
                return 18;
            case '+':
            case '-':
                return 1;
            case '*':
            case '/':
                return 4;
            default:
                return -1;
        }
    }

    private static Set<Character> operators = new HashSet<>();

    static
    {
        operators.add('+');
        operators.add('-');
        operators.add('*');
        operators.add('/');
        operators.add('(');
        operators.add(')');
    }

    public static String infix2postfix(String exp) throws Exception {
        Stack<Character> ops = new Stack<Character>();
        StringBuilder sb = new StringBuilder();
        List<String> result = new ArrayList<>();

        int i = 0;
        String digit = new String();
        while (i < exp.length()) {
            char c = exp.charAt(i);
            if (!operators.contains(c)) {
                digit += c;
            } else {
                if (!digit.isEmpty())
                    result.add(digit);
                digit = "";

                if (ops.empty()) {
                    ops.push(c);
                } else if (c == ')') {
                    while (!ops.empty() &&  ops.peek() != '(') {
                        result.add(ops.pop().toString());
                    }
                    if (ops.empty()) {
                        throw new Exception("no (");
                    } else {
                        ops.pop();  // pop (
                    }

                } else if (op_priority(ops.peek()) < op_priority(c)) {
                    ops.push(c);
                } else if (ops.peek() == '(') {
                    ops.push(c);
                } else {
                    result.add(ops.pop().toString());
                    ops.push(c);
                    digit += exp.charAt(i + 1);
                    ++i;

                }
            }
            ++i;
        }

        if (digit != "") {
            result.add(digit);
        }

        while (ops.size() > 0) {
            result.add(ops.pop().toString());
        }
        for (String s : result) {
            sb.append(s);
            sb.append(" ");
        }
        return sb.toString();
    }

    public static double calc_temp(Character op, double v1, double v2) {
        switch (op) {
            case '+':
                return v1 + v2;
            case '-':
                return v1 - v2;
            case '*':
                return v1 * v2;
            case '/':
                return v1 / v2;
        }
        return 0;
    }

    public static double calc(String postfix) throws Exception{
        Stack<Double> vstack = new Stack<>();
        for (String v : postfix.split(" ")) {
            if (v.length() == 1 && !Character.isDigit(v.charAt(0))) {
                if (vstack.size() >= 2) {
                    double temp = calc_temp(v.charAt(0), vstack.pop(), vstack.pop());
                    vstack.push(temp);
                }
                else {
                    throw new Exception("error");
                }
            } else {
                System.out.println(v);
                vstack.push(Double.parseDouble(v));
            }
        }
        return vstack.pop();
    }

    public static void main(String[] args) {

        try {
            String exp = infix2postfix("(2+3)*4+1.3*(2+1)");
            System.out.println(exp);
            System.out.println(calc(exp.trim()));
        } catch (Exception ex) {
            System.out.println(ex.getMessage());
            System.out.println(ex.getCause());
            ex.printStackTrace();
        }
        System.out.println();
        System.out.println("Hello World!");
    }
}