Building User Interfaces - Beginning Java ME Platform - page 111

The Form class's layout policy centers around rows, by default positioning each Item
next to the previous as long as multiple Item s fit on a single line. If there are more Item s
than fit on a single line, Item s are placed on rows one below the other. In the unlikely
event that there are more Item s than fit on the display, the MIDP implementation may
choose to provide a scrolling view of the Item s on the Form , or may paginate the Form ,
taking you to a new screen to view additional Item s on the Form .
The layout algorithm provides default layout constraints for each Item added to a
Form . For example, if the Form is laying out Item s left to right, an Item with an unspecified
layout policy will default to LAYOUT_LEFT , left-aligned on the Form . If the Item specifies
another layout policy, such as LAYOUT_CENTER , then the Form will attempt to accommodate
the desired layout for the Item on the Form . As the Item s are laid out on the Form , the layout
algorithm attempts to keep subsequent Item s next to each other, unless any of the follow-
ing occurs:
• The previous Item has a row break after it.
• The current Item has the LAYOUT_NEWLINE_BEFORE layout hint set.
• The current Item is a StringItem that begins with \n .
• The current Item is a ChoiceGroup , DateField , Gauge , or TextField , and the LAYOUT_2
hint is not set.
• The current Item has a layout flag that differs from the form's current alignment.
A row break occurs after an Item if any of the following occurs:
• The Item is a StringItem that ends with \n .
• The Item has the LAYOUT_NEWLINE_AFTER hint set.
• The Item is a ChoiceGroup , DateField , Gauge , or TextField , and the LAYOUT_2 hint is
not set.
If all of this seems confusing, don't panic: the behavior is actually fairly intuitive, and
you'll find in practice that a bit of experimentation quickly yields the layout you want.
The Form 's other responsibility is to keep a collection of the Item s it draws. While the
details of that collection are private to the implementation of the Form class, the Form class
provides the following methods:
• append : Appends an Item to the Form
• delete : Takes an index and deletes the Item at the specified index in its collection
from the Form