Building User Interfaces - Beginning Java ME Platform - page 102

mainForm = new Form("String Item Demo", new Item[] {
get_stringItem1(),
get_stringItem2(),
});
Passing a list of Item instances to the constructor inserts the instances in the same
order on the display, and is thus a common way to construct whole screens at a time.
The Form class really is a collection, though, and offers several methods to manipulate
the items it displays. I show you those in a later section in this chapter, “Collecting Visible
Items Using the Form Class.” For now, though, you should know that you can also add a
single item to the Form instance using its append and insert methods. The append method
adds the Item instance to the end of the list of items on the Form , while the insert method
inserts the item between two other items on the Form .
Introducing Items
For your user interface, the rubber hits the road with the Item class and its subclass.
Unless you're coding raw interface code using the Canvas or GameCanvas classes (which I
discuss later), each visible item in your user interface must implement the Item interface
or else be a wholly separate Displayable object such as an Alert , TextBox , or List . The Item
class encapsulates the following responsibilities:
• Command management : An Item can send commands to the Form 's command
listener in response to user keystrokes such as selections or other actions. In addi-
tion, an individual Item can have its own command listeners.
• Drawing : An Item knows how to draw itself.
• Event handling : An Item knows how to handle raw events such as keystrokes, so
your application doesn't have to.
• Layout preferences : An Item signals to its containing form its preferences about how
it should be laid out, and then the Form uses its layout policy to present all of the
items on the display in a cohesive way.
The Item class provides three methods for managing commands:
• addCommand : Lets you add a Command instance of type ITEM to the item. In turn,
the MIDP implementation presents this command when the item is active
(highlighted).
• removeCommand : Removes the indicated command from the item.
• setItemCommandListener : Sets a listener for ITEM commands on this item.