MIDP2.0中Alert新特性有什么？
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; }