Evaluate Reverse Polish Notation

public class Solution {
    public int evalRPN(String[] tokens) {
        if(tokens == null || tokens.length == 0)    return 0;
        Stack<Integer> stack = new Stack<Integer>();
        for(int i = 0; i < tokens.length; i++)
        {
            String str = tokens[i];
            if(isOperator(str))
            {
                int one = stack.pop();
                int two = stack.pop();
                stack.push(getResult(two, one, str));
            }
            else
            {
                stack.push(Integer.valueOf(str));
            }
        }
        return stack.isEmpty() ? 0 : stack.pop();
    }
    public int getResult(int one, int two, String str)
    {
        switch(str)
        {
            case "+": return one + two; 
            case "-": return one - two; 
            case "*": return one * two; 
            case "/": return one / two;
        }
        return 0;
    }
    public boolean isOperator(String str)
    {
        if(str.equals("+") || str.equals("-") || str.equals("*") || str.equals("/")) return true;
        return false;
    }
}