此程序可计算两位数以上的表达式

import java.util.Stack;

public class ExpressionEvaluator {

    public int evaluate(String s) {
        Stack<Integer> numbers = new Stack<>();
        Stack<Character> operators = new Stack<>();
        int i = 0;
        char c = s.charAt(i);

        while (c==' '){
            i++;
            c = s.charAt(i);
        }
        if (!In(c)) {
            int d = c -'0';
            i++;
            while (i<s.length()&&!In(s.charAt(i))&&s.charAt(i)!=' '){
                d=d*10+(s.charAt(i)-'0');
                i++;
            }
            numbers.push(d);
        } else {
            operators.push(c);
            i++;
        }
        while ( i != s.length()) {

            c = s.charAt(i);
            if(c == ' '){
                i++;
                continue;
            }
            if (!In(c)) {
                int d = c -'0';
                i++;
                while (i<s.length()&&!In(s.charAt(i))&&s.charAt(i)!=' '){
                    d=d*10+(s.charAt(i)-'0');
                    i++;
                }
                numbers.push(d);

            } else if (operators.isEmpty()) {
                operators.push(c);
                i++;
            } else {
                switch (Precede(operators.peek(), c)) {
                    case '<':
                        operators.push(c);
                        i++;
                        break;
                    case '>':
                        char theta = operators.pop();
                        int a = numbers.pop();
                        int b = numbers.pop();
                        numbers.push(evalSingleOp(b, a, theta));
                        break;
                    case '=':
                        operators.pop();
                        i++;
                        break;
                }
            }
        }

        while (!operators.isEmpty()){
            char theta = operators.pop();
            int a = numbers.pop();
            int b = numbers.pop();
            numbers.push(evalSingleOp(b, a, theta));
        }

        return numbers.pop();
    }

    public char Precede(char a, char b) {//判断操作符的优先级
        switch (a) {//先算最厉害的
            case '+':
            case '-':
                if ((b == '*') || (b == '/') || (b == '('))
                    return '<';
                else
                    return '>';
            case '/':
            case '*':
                if (b == '(')
                    return '<';
                else
                    return '>';
            case '(':
                if (b == ')')
                    return '=';
                else
                    return '<';
            case ')':
                return '>';
            default:
                throw new RuntimeException("运算符优先级错误");
        }
    }

    public boolean In(char ch) {//判断是操作数还是操作符
        int i;
        char[] p = new char[]{'+', '-', '*', '/', '(', ')'};
        for (i = 0; i < 6; i++)
            if (ch == p[i]) return true;
        return false;
    }

    private int evalSingleOp(int a, int b, char op) {//计算表达式
        switch (op) {
            case '+':
                return a + b;
            case '-':
                return a - b;
            case '*':
                return a * b;
            case '/':
                return a / b;
            default:
                throw new IllegalStateException("Unexpected operator: " + op);
        }
    }

    public static void main(String[] args) {
        ExpressionEvaluator evaluator = new ExpressionEvaluator();
        System.out.println("(1+3)*((2+5)-4)*2="+evaluator.evaluate("(1+3)*((2+5)-4)*2"));
        System.out.println("1+3*2+5-4*2="+evaluator.evaluate("1+3*2+5-4*2"));
        System.out.println("100*2*3="+evaluator.evaluate("100*2*3"));
    }
}

结果如图