Java expression parsers are lately showing a tendency to become non-free (at least in the paid sense) but help has arrived in Java 1.6 in the form of  support in the runtime for JavaScript. This makes evaluation of a mathematical expression very easy indeed, as JavaScript's eval can be called. The source code is HERE (make sure you single-quote the entire expression for command line execution)

import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;

public class Calc {
    public static void main(String[] args) throws Exception {
        if (args.length < 1) {
            System.err.println("Usage: java Calc <expression>");

        ScriptEngineManager mgr = new ScriptEngineManager();
        ScriptEngine engine = mgr.getEngineByName("JavaScript");