Programming Wireless Devices with the Java2 Platform

8.1 MIDP UI Compared to Desktop AWT

The differences between mobile devices and desktop computers are evident when comparing the designs of the MIDP lcdui with the Abstract Windowing Toolkit (AWT) of the Java^TM 2 Platform, Standard Edition.

AWT was designed and optimized for desktop computers. The assumptions that are appropriate for desktop computers are not appropriate for mobile devices. For example, when a user interacts with AWT, event objects are created dynamically. These objects are short-lived and exist only until each associated event is processed by the system. The event object then becomes garbage and must be reclaimed by the garbage collector of the Java virtual machine. The limited CPU and memory subsystems of a mobile information device (MID) typically cannot afford the overhead of unnecessary garbage objects. To limit the creation of such objects, lcdui events are simply method calls that pass the event parameters directly as the parameters of the method.

AWT also assumes certain desktop environment specific user interaction models. For instance, the component set of AWT was designed to work with a pointing device such as a mouse or pen. As mentioned earlier, this assumption is valid only for a small subset of MIDs, since most of the MIDP target devices have just a keypad for user input. The MIDP user interface API uses a layer of abstraction to separate applications from concrete input methods used in different devices. For example, a MIDP device with touch input allows the user to use this input method on high-level user interface components but it is not visible to applications. The different concrete input methods are transparent to the application code.

AWT has extensive support for window management, such as overlapping windows, window resizing, and so forth. In contrast, MIDP target devices have small displays that are not large enough for multiple overlapping windows. The lcdui API of MIDP has no window management capabilities. A lcdui user interface is constructed using a set of Displayables. This displayable model and the class structure of user interface are described in the next section.