How to change the attributes of a field in a form based on a value of certain dropdown option in jsp?

i have scenerio where based on a padrticular value of some drop down , a specific set of attributes take action as in below example.i am using jsp and javascript

 <td width="20%"><select type="text" name="cars" style='WIDTH:428'            
       <option value="" selected>Select</option>
    <option value="volvo">Volvo</option>
        <option value="saab">Saab</option>
        <option value="mercedes">Mercedes</option>
        <option value="audi">Audi</option>
    </select></td></tr>

based on the above selected option i want to choose like this

 if (mercedes)
      <td ><input type="text" name="mercedes"  
         onchange="javascript:onefunction();"></td>
        else
       <td ><input type="text" name="othercars"    
       onchange="javascript: otherfunction();"></td>

please help?