basic calculator (stack problem)

Implement a basic calculator to evaluate a simple expression string.

The expression string may contain open ( and closing parentheses ), the plus + or minus sign -, non-negative integers and empty spaces .

You may assume that the given expression is always valid.

Some examples:

"1 + 1" = 2
" 2-1 + 2 " = 3
"(1+(4+5+2)-3)+(6+8)" = 23

public class Solution {
    public int calculate(String s) {
        
        int len = s.length();
        Stack<String> stack = new Stack<String>();
        int i = 0;
        while(i < len)
        {
            if(s.charAt(i) == ' ') 
                i++;
            else if (s.charAt(i) == '(' || s.charAt(i) == '+' || s.charAt(i) == '-')
            {
                stack.push(s.substring(i,i+1));
                i++;
            }
            else if(s.charAt(i) >= '0' && s.charAt(i) <= '9')
            {
                int start = i;
                while(i < len && s.charAt(i) >= '0' && s.charAt(i) <= '9') i++;
                stack.push(s.substring(start, i));
            }
            else
            {
                ArrayList<String> tmp = new ArrayList<String>();
                while(!stack.empty())
                {
                    String top = stack.pop();
                    if(top.equals("(")) break;
                    tmp.add(0, top);
                }
                int sumtmp = 0;
                sumtmp += Integer.valueOf(tmp.get(0));
                for(int j = 1; j < tmp.size(); j=j+2)
                {
                    if(tmp.get(j)=="-")
                        sumtmp -= Integer.valueOf(tmp.get(j+1));
                    else
                        sumtmp += Integer.valueOf(tmp.get(j+1));
                }
                stack.push(Integer.toString(sumtmp));
                i++;
            }
        }
        ArrayList<String> tmp = new ArrayList<String>();
        while(!stack.empty())
        {
            String top = stack.pop();
            tmp.add(0, top);
        }
        int result = 0;
        result += Integer.valueOf(tmp.get(0));
        for(int j = 1; j < tmp.size(); j=j+2)
        {
            if(tmp.get(j)=="-")
                result -= Integer.valueOf(tmp.get(j+1));
            else
                result += Integer.valueOf(tmp.get(j+1));
        }
        return result;
    }
}

从25 到 30这段代码，i.e.,

while(!stack.empty())
 {
        String top = stack.pop();
        if(top.equals("(")) break;
        tmp.add(0, top);
 }

为什么不能写成这样

 while(stack.peek()!="(")
 {
     String top = stack.pop();
     tmp.add(0, top);
 }

写成这样就会报错

Exception in thread "main" java.util.EmptyStackException
at java.util.Stack.peek(Stack.java:102)
at helloworld.calculate(helloworld.java:43)
at helloworld.main(helloworld.java:83)

可是我能确定执行这段代码时stack是不为空的啊~