Alert类继承自Screen，它的作用是通知用户发生异常信息。在高级用户界面API中有三个类是不能有父容器的，他们必须要占据屏幕，其中一个就是Alert,另外两个是List和TextBox。在MIDP1.0中,Alert的灵活性比较差，表示的信息比较死板。在MIDP2.0中添加了一个重要的特性就是能够在Alert上添加指示器，因此可以和Guage结合起来使用。
和Alert结合使用的Guage有以下要求1.必须是非交互性的，2.不能同时在其他的容器内，3.Guage的label必须为null，4.Guage不能和Command和CommandListener关联。比如按照如下方式创建一个Guage实例
int max = ... // maximum value
int initial = ... // initial value
Gauge gauge = new Gauge( null, false, max, initial );
通过调用方法setIndicator()可以把Alert和Guage结合起来
Alert a = new Alert( "My alert" );
a.setIndicator( gauge );
由于MIDP的用户界面类都是线程安全的，因此你可以在其他的线程内改变guage的值，Alert会自动的重新绘制屏幕来更新guage的当前值。
在MIDP2.0中，我们可以显式的对Alert添加Command，当然如果你还要实现CommandListener的commandAction()方法来告诉系统当Command 被按下的时候该做什么。在MIDP1.0中，有个默认的Command和Alert关联在一起的，如果我们在Alert中显式的添加了Command的话，那么这个默认的Command就被取代，如果添加的Command被删除，默认的Command会自动恢复和Alert的关联。如果我们在Alert 上添加了两个以上的Command，那么它的Timeout会自动设置为FOREVER。
下面的应用程序很好的说明了如何使用不同的Alert表示不同的信息
import javax.microedition.lcdui.*;
import javax.microedition.midlet.*;
// A simple MIDlet for testing various alerts.
public class AlertTest extends MIDlet implements CommandListener
{
// An abstract class for our alert tests.
public abstract class AlertRunner
{
protected static final int ONE_SECOND = 1000;
protected static final int FIVE_SECONDS = 5000;
public AlertRunner(String title)
{
_title = title;
}
public abstract Alert doAlert();
public String getTitle()
{
return _title;
}
public String toString()
{
return getTitle();
}
private String _title;
}
// An alert test for a simple timed alert.
public class TimedAlert extends AlertRunner
{
public TimedAlert(String title)
{
super(title);
}
public Alert doAlert()
{
Alert a = new Alert(getTitle());
a.setString("Times out after 5 seconds...");
a.setTimeout(FIVE_SECONDS);
showAlert(a);
return a;
}
}
// An alert test for a simple modal alert.
public class ModalAlert extends AlertRunner
{
public ModalAlert(String title)
{
super(title);
}
public Alert doAlert()
{
Alert a = new Alert(getTitle());
a.setString("Waits to be dismissed");
a.setTimeout(Alert.FOREVER);
showAlert(a);
return a;
}
}
// An alert test that displays an alert with
// a gauge whose value is increased every second.
// The alert can be dismissed only after the gauge
// reaches its maximum value.
public class ProgressAlert extends AlertRunner implements CommandListener,
Runnable
{
private static final int MAX = 10;
public ProgressAlert(String title)
{
super(title);
}
public Alert doAlert()
{
_gauge = new Gauge(null, false, MAX, 0);
_gauge.setValue(0);
_done = false;
_alert = new Alert(getTitle());
_alert.setString("Counts to " + MAX
+ " and then lets you dismiss it");
_alert.setCommandListener(this);
_alert.setIndicator(_gauge);
// Set a _very_ long timeout
_alert.setTimeout(ONE_SECOND * 3600);
showAlert(_alert);
new Thread(this).start();
return _alert;
}
public void commandAction(Command c, Displayable d)
{
if (_done || _gauge.getValue() >= MAX)
{
showList();
}
}
private void done()
{
_alert.addCommand(new Command("Done", Command.OK, 1));
_done = true;
}
// A thread that bumps the value of the counter
// every second.
public void run()
{
int val = _gauge.getValue();
try
{
while (val < MAX)
{
Thread.sleep(ONE_SECOND);
_gauge.setValue(++val);
}
} catch (InterruptedException e)
{
}
done();
}
private Alert _alert;
private int _counter;
private boolean _done;
private Gauge _gauge;
}
// An alert test that displays a continuously
// running gauge before automatically timing out.
public class BusyAlert extends AlertRunner
{
public BusyAlert(String title)
{
super(title);
}
public Alert doAlert()
{
_gauge = new Gauge(null, false, Gauge.INDEFINITE,
Gauge.CONTINUOUS_RUNNING);
_alert = new Alert(getTitle());
_alert.setString("Runs for 5 seconds and "
+ "times out automatically");
_alert.setIndicator(_gauge);
_alert.setTimeout(FIVE_SECONDS);
showAlert(_alert);
return _alert;
}
private Alert _alert;
private Gauge _gauge;
}
// Standard MIDlet code. Displays a list of
// available alert tests and runs the test once
// it's been chosen.
private Display display;
public static final Command exitCommand = new Command("Exit", Command.EXIT,
1);
public AlertTest()
{
}
public void commandAction(Command c, Displayable d)
{
if (c == exitCommand)
{
exitMIDlet();
} else if (c == List.SELECT_COMMAND)
{
int index = _alertList.getSelectedIndex();
_alertRunners[index].doAlert();
}
}
protected void destroyApp(boolean unconditional)
throws MIDletStateChangeException
{
exitMIDlet();
}
public void exitMIDlet()
{
notifyDestroyed();
}
public Display getDisplay()
{
return display;
}
protected void initMIDlet()
{
// The list of alert tests....
_alertRunners = new AlertRunner[] { new TimedAlert("Timed alert"),
new ModalAlert("Modal alert"),
new ProgressAlert("Progress alert"),
new BusyAlert("Busy alert") };
_alertList = new List("Alert Testing", List.IMPLICIT);
_alertList.setCommandListener(this);
for (int i = 0; i < _alertRunners.length; ++i)
{
_alertList.append(_alertRunners[i].toString(), null);
}
showList();
}
protected void pauseApp()
{
}
private void showAlert(Alert a)
{
getDisplay().setCurrent(a, _alertList);
}
private void showList()
{
getDisplay().setCurrent(_alertList);
}
protected void startApp() throws MIDletStateChangeException
{
if (display == null)
{
display = Display.getDisplay(this);
initMIDlet();
}
}
private List _alertList;
private AlertRunner[] _alertRunners;
}