Just an idea, I'm not sure if this works with servlets but this is how I've seen it in php ...

Code:
<select name='color[]' size=4 multiple>
<option value='blue'>Blue</option>
<option value='green'>Green</option>
<option value='red'>Red</option>
Notice the brackets in the name of the list: "color[]"
Perhaps the same trick needs to be applied in Java.

Never tried it before, but hey at least it's some inspiration for you, in case you're really stuck.