Displayable objects are the views in which the application interaction takes place. Application-defined operations in Displayable objects are implemented using Commands. The device implementation provides additional operations based on displayable type, as will be discussed in Section 8.6.3, “Device-Provided Operations.”
All Displayable objects have the following properties:
Zero or more Commands for application specific user operations on the Displayable
A CommandListener (set by the application with the method setCommandListener) that gets notified when the user selects any of the Commands
 Note that only a single CommandListener can be added because of the unicast listener model is used in the lcdui API.