Event in servlets

I am using a drop box constructed in a servlet page... i want that on selecting an entry in the dropbox an event be generated and handled by the same servlet to show related entries from the database... on the same page without pressing any submit button....
please help....
o.println("<form action=\" orderentry\">");
o.println("Customer Code:<select name=\"custcode\" > <option>A-1</option> <option>A-9</option> <option>A-10</option> <option>A-11</option> </select>");
o.println("</form>");