深入分析J2ME平台MIDP低级事件处理机制

　　这些都是按键基本值，当然有些手机是触摸屏的。事件的传递是通过如下方法来实现的。

　　开发者应该继承Canvas类的同时覆盖相应的方法来实现低级事件的处理，低级事件的处理是连续的、串行的，在前面的事件返回之前后面的事件不会执行，当然也有例外，你可以参考API doc得到答案。下面是一个例子，它的功能就是在屏幕上显示出你按下键的键值，界面非常简单因此没有抓图。代码如下所示：

//LowLevelMIDlet.java

import javax.microedition.lcdui.Display;
import javax.microedition.midlet.MIDlet;
import javax.microedition.midlet.MIDletStateChangeException;

/*
 * Created on 2004-6-24
 *
 * TODO To change the template for this generated file go to
 * Window - Preferences - Java - Code Style - Code Templates
 */

/**
 * @author P2800
 *
 * TODO To change the template for this generated type comment go to Window -
 * Preferences - Java - Code Style - Code Templates
 */
public class LowLevelMIDlet extends MIDlet
{

    private Display display;
    private KeyCanvas keyCanvas;

    /*
     * (non-Javadoc)
     *
     * @see javax.microedition.midlet.MIDlet#startApp()
     */
    protected void startApp() throws MIDletStateChangeException
    {
        // TODO Auto-generated method stub
        display = Display.getDisplay(this);
        keyCanvas = new KeyCanvas();
        display.setCurrent(keyCanvas);

    }

    /*
     * (non-Javadoc)
     *
     * @see javax.microedition.midlet.MIDlet#pauseApp()
     */
    protected void pauseApp()
    {
        // TODO Auto-generated method stub

    }

    /*
     * (non-Javadoc)
     *
     * @see javax.microedition.midlet.MIDlet#destroyApp(boolean)
     */
    protected void destroyApp(boolean arg0) throws MIDletStateChangeException
    {
        // TODO Auto-generated method stub

    }

}

//KeyCanvas.java

import javax.microedition.lcdui.Canvas;
import javax.microedition.lcdui.Command;
import javax.microedition.lcdui.CommandListener;
import javax.microedition.lcdui.Displayable;
import javax.microedition.lcdui.Graphics;

/*
 * Created on 2004-6-25
 *
 * TODO To change the template for this generated file go to
 * Window - Preferences - Java - Code Style - Code Templates
 */

/**
 * @author P2800
 *
 * TODO To change the template for this generated type comment go to Window -
 * Preferences - Java - Code Style - Code Templates
 */
public class KeyCanvas extends Canvas implements CommandListener
{
    private String key = "Press any key";

    /*
     * (non-Javadoc)
     *
     * @see javax.microedition.lcdui.Canvas#paint(javax.microedition.lcdui.Graphics)
     */
    protected void paint(Graphics g)
    {
        // TODO Auto-generated method stub
        g.setColor(255, 255, 255);
        g.fillRect(0, 0, getWidth(), getHeight());
        g.setColor(0, 0, 0);
        g.drawString(key, getWidth() / 2, getHeight() / 2, Graphics.TOP
                | Graphics.HCENTER);

    }

    public void keyPressed(int keyCode)
    {
        key = keyCode + " is pressed";
        repaint();
    }

    public void keyReleased(int keyCode)
    {
        key = keyCode + " is released";
        repaint();
    }

    /*
     * (non-Javadoc)
     *
     * @see javax.microedition.lcdui.CommandListener#commandAction(javax.microedition.lcdui.Command,
     *      javax.microedition.lcdui.Displayable)
     */
    public void commandAction(Command arg0, Displayable arg1)
    {
        // TODO Auto-generated method stub

    }

}