Search button logic

I tried to figured out the logic when to enable and disable the search button. If we all the users to click the search button when one drop-down lists is not selected, the a null pointer exception error occur. So we have to make sure that all drop-down list have been clicked before the search button is enable. The logic for this is not so hard, but the problem is GWT widgets. Like the FlexTable problem yesterday I encountered. My logic for the FlexTable was right, but I the behavior of the FlexTable confused me (and lots of people). Hopefully, I can make it work tomorrow.