MIDP Programming - Java 2 Micro Edition Application Development - page 35

application running in the foreground and unnecessary battery consumption. The destroyApp()
method provides an unconditional parameter; if it is set to false, the MIDlet is allowed to refuse its
termination by throwing a MIDletStateChangeException . MIDlets can request to resume
activity by calling resumeRequest() . If a MIDlet decides to go to the paused state, it should
notify the application manager by calling notifyPaused() . In order to terminate, a MIDlet can
call notifyDestroyed() . Note that System.exit() is not supported in MIDP and will
throw an exception instead of terminating the application.
Note
Some devices might terminate a MIDlet under some circumstances without calling
destroyApp (), for example on incoming phone calls or when the batteries are exhausted. Thus,
it might be dangerous to rely on destroyApp() for saving data entered or modified by the user.
Display and Displayable
MIDlets can be pure background applications or applications interacting with the user. Interactive
applications can get access to the display by obtaining an instance of the Display class. A
MIDlet can get its Display instance by calling Display.getDisplay(MIDlet midlet) ,
where the MIDlet itself is given as parameter.
The Display class and all other user interface classes of MIDP are located in the package
javax.microedition.lcdui . The Display class provides a setCurrent() method that
sets the current display content of the MIDlet. The actual device screen is not required to reflect
the MIDlet display immediately—the setCurrent() method just influences the internal state of
the MIDlet display and notifies the application manager that the MIDlet would like to have the
given Displayable object displayed. The difference between Display and Displayable is
that the Display class represents the display hardware, whereas Displayable is something
that can be shown on the display. The MIDlet can call the isShown() method of Displayable
in order to determine whether the content is really shown on the screen.
HelloMidp Revisited
The HelloMidp example from Chapter 1 , "Java 2 Micro Edition Overview," is already a
complete MIDlet. Now that you have the necessary foundation, you can revisit HelloMidp from
an API point of view.
First, you import the necessary midlet and lcdui packages:
import javax.microedition.midlet.*;
import javax.microedition.lcdui.*;
Like all MIDP applications, the HelloMidp example is required to extend the MIDlet class:
public class HelloMidp extends MIDlet {
In the constructor, you obtain the Display and create a Form :
Display display;
Form mainForm;
public HelloMidp() {