Wireless Java

Creating Low-Level GUI Components

In the high-level API, you have no control of what is displayed on the screen and very little freedom to “play” with the components programmatically. The implementation is responsible for selecting the best approach for the device. Some applications, however, such as games, may need more control over what is drawn on the screen. The MIDP javax.microedition.lcdui package also provides a low-level API for handling such cases.

In order to directly draw lines, text, and shapes on the screen, you must use the Canvas class. The Canvas class provides a blank screen on which a MIDlet can draw. For example, let’s draw the string “HelloWorld” on the screen. There’s a simple way to do this: subclass the Canvas class, which is an abstract class that extends Displayable, and override the paint( ) method. The resulting class, MyCanvas, is shown in Example 5-1.

The implementation of the paint( ) method uses the drawing capabilities of the javax.microedition.lcdui.Graphics class. In the paint( ) method, the drawing color is set to red, then a rectangle is drawn in the current color. The methods getWidth( ) and getHeight( ) return the width and height of the Canvas, respectively. The next call to setColor( ) sets the drawing color to white; then the string “Hello World!” is drawn in the top left corner of the screen.

Example 5-1. Subclassing Canvas

import javax.microedition.lcdui.*; public class MyCanvas extends Canvas { public void paint(Graphics g) { g.setColor(255, ...