public static double evaluate(String expression)
    {
        Stack stack = new Stack();
        
        StringTokenizer tokenizer;
        tokenizer = new StringTokenizer (expression);
        while (tokenizer.hasMoreTokens()) {
            char word = tokenizer.nextToken().charAt(0);
                    double first = Double.parseDouble(stack.pop().toString());
                    double second = Double.parseDouble(stack.pop().toString());
                    
                switch (word) {
                    case '+': stack.push(first + second); break;

                    case '-': stack.push(first - second); break;

                    case '*': stack.push(first * second); break;

                    case '/': stack.push(first / second); break;

                    default: stack.push(word); break;
                }
 
        }
        
 
        // finish this method
        
        return stack.pop().toString();// change the 0.0 to the answer off of the stack
    }// end evaluate