Small S-exp like arithmetic code interpreter in Java

ArithmeticCodeEvaluator.java

package org.komamitsu.test;

import java.util.StringTokenizer;

public class ArithmeticCodeEvaluator
{
    private static int eval(StringTokenizer tokenizer)
    {
        String first = tokenizer.nextToken();
        if (first.equals("+")) {
            return eval(tokenizer) + eval(tokenizer);
        }
        else if (first.equals("-")) {
            return eval(tokenizer) - eval(tokenizer);
        }
        else if (first.equals("*")) {
            return eval(tokenizer) * eval(tokenizer);
        }
        else if (first.equals("/")) {
            return eval(tokenizer) / eval(tokenizer);
        }
        else if (first.equals("(")) {
            int result = eval(tokenizer);
            String closeParen = tokenizer.nextToken();
            if (!closeParen.equals(")")) {
                throw new IllegalStateException("Expected ')', but got: " + closeParen);
            }
            return result;
        }

        return Integer.valueOf(first);
    }

    public static void main(String[] args)
    {
        String s = "(/ (* (+ 2 3) (- 10 4)) (+ 3 (/ 28 4)))";
        int result = eval(new StringTokenizer(s, "() "));

        System.out.println("Result: " + result);
    }
}