Talk:How can I find out when the value in the TextField has been changed?
This is a fairly basic article which demonstrates how to listen for changes to Item instances contained in a Form. As the article points out, this can be useful in order to implement functionality such as auto-complete, where the list of options needs to be updated in response to users entering text in a Form component such as a TextField.
A simple code example is provided which does a good job of demonstrating how to implement the ItemStateListener interface (and the itemStateChanged method). It also shows how to set the ItemStateListener for a form. Usefully, a link is also provided to download a midlet which demonstrates the use of an ItemStateListener implementation in an actual application.