Form field validation can access values of the nearby input controls, which is useful for performing complex context-sensitive form validation. The same technique can be also used for auto-completion, populating combobox/listbox, and so on. The example below is a bit contrived, but all the input elements are named "name" (for city name and state name), and we use @RelativePath so that the validation of the state name refers to the capital name, and the validation of the city name refers to the state name.

To implement this you need to provide a doCheckXXX method, where XXX is the name of your field. This should return a FormValidation object.