Wireless Java

Chapter 6. MIDP Events

In AWT and Swing, events are generated when a user interacts with an application. For example, if the user selects Save from the File menu, the application is notified of this action and responds to the generated event. The same model holds true for the MIDP. However, as mentioned in the previous chapter, there are two MIDP user interface APIs: high-level and low-level. Therefore, there are two kinds of events: high-level (such as selecting an item from a list) and low-level (such as pressing a key on the device).

This chapter discusses event handling in the MIDP and shows, through examples, how to handle high-level and low-level MIDP events generated by the components of the previous chapter. We start with an explanation of a simple application of events: navigating between screens.

Screen Navigation

A MIDlet developer needs to provide ways for the user to navigate through the different screens that make up the MIDlet. Because we can only show one screen at a time, however, we need to tie a mechanism to each screen that indicates to the MIDlet that the user has completed working with the current Displayable screen. We can do this by using the Command class, which is part of the javax.microedition.lcdui package. Let’s take a closer look at the Command class now.

Commands

Just like a design pattern with the same name, the Command class encapsulates the semantic information of an action. Note that it only contains information about a command, not the actual ...