making English Calculator in java

Question

I am making an English Calculator and that is, for example, when the String is "1 plus 2", I split the words into 3 parts using split. The split elements are 1 & plus & 2, and the last job is just to add the numbers. It worked well when I used Switch, but I tried to use For method for practice and it prints out 0.0 for "1 plus 2" sentence... I think there is a problem in arraylist but have no one to ask.. Can I please get an idea of how to fix it?

Also, I`d like to know, for example, saying that the String is, "4 divided by 2". The elements are four, but the question is, how can I split the two words, "divided by" at same time? Thanks!

class Calculator {

    public static double calculate(String s) {

        List<String> list = new ArrayList<>(Arrays.asList(s.split(" ")));
        //System.out.println(list.size());
        double value1, value2;
        double result = 0.0;
        String operator;
        int i = 0;
        for(i = 0; i < list.size(); i++)
        {
            if(list.size() == 3)
            {
                operator = list.get(1);
                value1 = Double.parseDouble(list.get(0));
                value2 = Double.parseDouble(list.get(2));

                if(operator == "plus")
                {
                    result = value1 + value2;
                } else if(operator == "minus")
                {
                    result = value1 - value2;
                } else if(operator == "times")
                {
                    result = value1 * value2;
                } else if(operator == "mod")
                {
                    result = value1 % value2;
                }               
            }   
        }
        return result;
    }
}

class MyPractice
{
    public static void main(String[] args)
    {
        Calculator cal = new Calculator();
        System.out.println(cal.calculate("1 plus 2"));
    }
}