怎样确定choiceGroup中的一个选项被选中解决方法

/* * * Copyright (c) 2007, Sun Microsystems, Inc. * * All rights reserved. * * Redistribution and use in source and binary forms, with or without * modification, are permitted provided that the following conditions * are met: * *  * Redistributions of source code must retain the above copyright *    notice, this list of conditions and the following disclaimer. *  * Redistributions in binary form must reproduce the above copyright *    notice, this list of conditions and the following disclaimer in the *    documentation and/or other materials provided with the distribution. *  * Neither the name of Sun Microsystems nor the names of its contributors *    may be used to endorse or promote products derived from this software *    without specific prior written permission. * * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. */import javax.microedition.lcdui.*;import javax.microedition.midlet.MIDlet;/** * The alert demo displays a list of alerts that will be displayed once the user * clicks a list item. These alerts try to present the full range of alert types * supported in MIDP. *  * @version 2.0 */public class ChoiceGroupDemo extends MIDlet implements CommandListener,        ItemStateListener {    private static final Command CMD_EXIT = new Command("Exit", Command.EXIT, 1);    private Display display;    private boolean firsttime;    private Form mainForm;    public ChoiceGroupDemo() {        firsttime = true;    }    protected void startApp() {        if (firsttime) {            display = Display.getDisplay(this);            mainForm = new Form("Choice Group");            mainForm.setItemStateListener(this);            mainForm.append("These are the available choice group types");            // these are the images and strings for the choices.            Image[] imageArray = null;            try {                // load the duke image to place in the image array                Image duke = Image.createImage("/midp/uidemo/Icon.png");                // these are the images and strings for the choices.                imageArray = new Image[] { duke, duke, duke, duke };            } catch (java.io.IOException err) {                // ignore the image loading failure the application can recover.            }            String[] stringArray = { "Option A", "Option B", "Option C",                    "Option D" };            // create the list of choice groups.            ChoiceGroup[] groups = { new ChoiceGroup("Exclusive",                    ChoiceGroup.EXCLUSIVE, stringArray, imageArray),            // new ChoiceGroup("Exclusive", ChoiceGroup.EXCLUSIVE, stringArray,            // imageArray),            // new ChoiceGroup("Multiple", ChoiceGroup.MULTIPLE, stringArray,            // imageArray),            // new ChoiceGroup("Pop-Up", ChoiceGroup.POPUP, stringArray,            // imageArray)            };            for (int iter = 0; iter < groups.length; iter++) {                mainForm.append(groups[iter]);            }            mainForm.addCommand(CMD_EXIT);            mainForm.setCommandListener(this);            firsttime = false;        }        display.setCurrent(mainForm);    }    public void commandAction(Command c, Displayable d) {        if (c == CMD_EXIT) {            destroyApp(false);            notifyDestroyed();        }    }    protected void destroyApp(boolean unconditional) {    }    protected void pauseApp() {    }    public void itemStateChanged(Item item) {        ChoiceGroup cg = (ChoiceGroup) item;        System.out.println("selectedIndex is " + cg.getSelectedIndex());    }}