J2ME in a Nutshell

Chapter 5. The Low-Level MIDlet User Interface API

The high-level API provides enough functionality for you to create, with relatively little effort, MIDlets with user interfaces that work unchanged across a wide range of devices. The price to be paid for this, however, is that you are restricted to using the components provided in the javax.microedition.lcdui package, and you have very little control over the appearance of your MIDlet.

The low-level API gives you almost exactly the opposite situation. To use it, you need to put much more effort into creating the user interface, but in return you get pixel-level access to the screen, you have control over colors (or shades of gray) and fonts; and you can respond directly to the user’s key presses or pointer actions. This section takes a detailed look at the low-level API, which is useful for writing simple games or drawing charts. It may be used on its own or mixed with screens built using Form and the other classes covered in the previous chapter.

The Canvas Class

Canvas is the basic building block of the low-level API. Because it is derived directly from Displayable, it inherits the ability to have associated Commands, but it does not provide a title or the ability to contain other components. Canvas gives you direct access to the screen of a MIDP device, apart from the area used to draw Command buttons or labels, as shown in Figure 5-1. In the figure, the black area is the part of the screen occupied by the Canvas itself. ...