Midlet Navigation Graphs in JML - Formal Methods: Foundations and Applications - page 19

Displayable

Canvas Screen

GameCanvas Alert Form List TextBox

Fig. 1. Class diagram: basic MIDP GUI elements

CommandListener

1 0..1

MIDlet Display Displayable

1 1 1 1

Command

1 0..*

Fig. 2. Class diagram: relation between midlets, displays, and screens

setCurrent(Alert alert, Displayable nextDisplayable) , which will dis-
play the alert , and then show the nextDisplayable .
In its turn, a Displayable object may have a CommandListener associ-
ated with it, which implements a method void commandAction(Command c,
Displayable d) to handle incoming command events occurring on some Disp-
layable d . Commands are user actions, such as buttons that the user can se-
lect on the screen. Fig. 2 shows the relevant class structure. Note that control
passes back and forth between the midlet and the platform. When a midlet
calls setCurrent(...) to change the display, it hands over control to the plat-
form; when after that a user action occurs, the platform hands back control to
the midlet by a call back to commandAction(...) . The behaviour of the mi-
dlet is determined by: (i) the current MIDlet and its Display , (ii) the current
Displayable shown on that Display , (iii) the Commands that the midlet pro-
vides (if any), (iv) the associated CommandListener (if any). The display and the
midlet should never change. The MIDP platform controls which Displayable is
shown, and offers a midlet API calls to change it; the midlet is in charge of the
Commands and CommandListeners and their associations to Displayables .

Sensitive Operations. The second relevant part of the MIDP infrastructure
are the APIs responsible for network communication and personal information
management. These operations are possibly security-sensitive. An unwanted or
uncontrolled network communication may result in (a) sensitive data being sent
out from the phone, or (b) unwanted network usage charges. Access to personal
information (e.g. the phone book) may result in unwanted information leakage.
The high level API structure for network communication is very simple. In
principle it only involves the Connector class that provides static methods for