AddNumber.java

public double evaluate(String s) {
        String[] elements = getStringArray(s);
        double result = 0.0;
        for (int i = 0; i < elements.length; i++) {
            String c = elements[i];
            if (c.equals("+") && i >= 2) {
                result = parseDouble(elements[i - 1]) + parseDouble(elements[i - 2]);
            }
        }
        return result;
    }