Dijkstra的双栈算术表达式求值算法

public class Evaluate{
    public static void main(String[] args){
        Stack<String> ops = new Stack<String>();
        Stack<Double> vals = new Stack<Double>();
        Scanner in = new Scanner(System.in);
        while (in.hasnext()){
            String s = in.nextString();
            if    (s.equals("("))        ;
            else if    (s.equals("+"))    ops.push(s);
            else if (s.equals("-")) ops.push(s);
            else if (s.equals("*")) ops.push(s);
            else if (s.equals("/")) ops.push(s);
            else if (s.equals("sqart")) ops.push(s);
            else if (s.equals(")")){
                String op = ops.pop();
                double v = vals.pop();
                if    (op.equals("+")) v = vals.pop()+v;
                else if (op.equals("-")) v = vals.pop()-v;
                else if (op.equals("*")) v = vals.pop()*v;
                else if (op.equals("/")) v = vals.pop()/v;
                else if (op.equals("sqrt")) v = Math.sqrt(v);
                vals.push(v);
            }
            else vals.push(Double.parseDouble(s));
        }
        system.out.println(vals.pop());
    }
}