Write Java Class for evaluating arithmetic expressions
<p>I have to write a class in Java that supports representing arithmetic expressions with 2 methods- eval and toString with the following black-box use:</p>

<pre><code>Expression e = 
new Multiplication( 
    new Addition( 
        new Double(2.5), 
        new Double(3.5)), 
    new UnaryMinus( 
        new Integer(5))); 
System.out.println(e.eval());   // should print out -30.0 
System.out.println(e.toString());   // should print out ((2.5 + 3.5) * (-(5))) 
</code></pre>

<p>How can I design such class? Which tools? Which Design pattern?</p>
