MIDP Programming - Java 2 Micro Edition Application Development - page 41

mainForm.append (amountWhole);
mainForm.append (amountFraction);
}
When the application is started, you request the display to show your money transfer form by calling
setCurrent() . As explained in the " MIDlets " section, the application manager notifies you about
the application start by calling the startApp() method. So you implement this method accordingly:
public void startApp() {
display.setCurrent (mainForm);
}
Please note that startApp() is called also when the MIDlet resumes from the paused state, so you
cannot move the initialization code from the constructor to this method.
Both pauseApp() and destroyApp() are declared as abstract in the MIDlet class, so you need
to implement these methods in your application, even if you do not have real content for them. You just
provide empty implementations, like in the HelloMidp example in the first section:
public void pauseApp() {
}
public void destroyApp (boolean unconditional) {
}
Selecting Elements Using ChoiceGroup s
In the previous section, you created a simple to enter information for transferring money between two
accounts. Now you will extend the application to allow the user to select different currencies. For this
purpose, you will now add a ChoiceGroup to your application.
The ChoiceGroup is an MIDP UI widget enabling the user to choose between different elements in a
Form . These elements consist of simple String s, but can display an optional image per element as
well. ChoiceGroup s can be of two different types. Corresponding type constants are defined in the
Choice interface. These constants are used in the List class as well; the List class allows an
additional third type. The three type constants are listed in Table 3.4 .
Table 3.4. Choice Type Constants
Constant Value
EXCLUSIVE Specifies a ChoiceGroup or List having only one element selected at the
s ame time.
IMPLICIT Valid for List s only. It lets the List send Command s to indicate state changes.
MULTIPLE In contrast to EXPLICIT , MULTIPLE allows the selection of multiple elements.
The ChoiceGroup constructor requires at least a label and a type value. Additionally, a String
array and an Image array containing the elements can be passed to the constructor. Elements can also
be added dynamically using the append() method. The append() method has two parameters, a
String for the label and an Image . In both cases, the image parameter may be null if no images
are desired.
In order to add a ChoiceGroup to the TeleTransfer application, you introduce a new variable
currency of type ChoiceGroup . By setting the type to EXCLUSIVE , you get a ChoiceGroup
where only one item can be selected at a time. You directly add elements for the United States (USD),
the European Union (EUR), and Japan (JPY) by passing a String array created inline. The
ChoiceGroup enables the user to choose between three currencies that are represented textually by