MIDP Programming - Java 2 Micro Edition Application Development - page 42

the abbreviations specified in the String array. The last parameter of the constructor is set to null
because you do not want Image s to be displayed at this time:
ChoiceGroup currency = new ChoiceGroup
("Currency", Choice.EXCLUSIVE,
new String[] {"USD", "EUR", "JPY"} , null);
You still need to add the currency ChoiceGroup to your main Form . As for the text fields, this is
done via the append() method of Form :
mainForm.append (currency);
Figure 3.4 shows the TeleTransfer application extended to choose a currency using a
ChoiceGroup .
Figure 3.4. The TeleTransfer MIDlet extended to enable the user to choose a
currency.
Receiving Changes from Interactive UI Items
If you run the new version of the TeleTransfer MIDlet, you can change the currency using the
ChoiceGroup , but the TextField labels for Dollar and Cent are not changed accordingly. You
need a way to notify the application if a selection is made in the currency ChoiceGroup .
Receiving changes of interactive high-level UI items in MIDP is based on a listener model similar to
AWT. Classes implementing the ItemStateListener interface are able to receive notifications for
the following events:
• State changes of a ChoiceGroup
• Value adjustments of an interactive Gauge
• TextField value changes
• DateField changes
The events are sent to the method itemStateChanged() of the ItemStateListener , where
the item that has changed is given as a parameter. In order to actually receive these events, the
ItemStateChangeListener must be registered using the setItemStateListener()
method of the corresponding Form .
Now that you know about item state change events, you can add the desired functionality to your
TeleTransfer MIDlet. First, you need to add the ItemStateListener interface to the class
declaration:
public class TeleTransfer extends MIDlet implements ItemStateListener
{