一、基本知识
1、Item类是Form类的派生类。
2、通过改变Item类的派生类的实例的状态，用户可以和应用程序进行交互。
3、itemStateChanged方法和普通触发器不同，在用户引起状态变化时自动调用的操作，程序本身引起的不会调用。
二、创建实践
1、以ChoiceGroup的应用为例，所有应用ITEM的MIDlet如果要处理ITEM的状态变化必须重写itemStateChanged方法
2、实际运行效果图
3、NETBEANS设计器的设计
4、代码（NETBEANS生成的大部分框架，笔者修改了其中几行，增加了itemStateChanged方法）
package hello;
import javax.microedition.midlet.*;
import javax.microedition.lcdui.*;
public class HelloMIDlet extends MIDlet implementsCommandListener,ItemStateListener{
privateboolean midletPaused = false;
//
privateCommand exitCommand;
private Formform;
privateChoiceGroup weather_CG;
//
publicHelloMIDlet() {
}
//
//
//
private voidinitialize() {
// write pre-initialize user code here
// write post-initialize user code here
}
//
//
public voidstartMIDlet() {
// write pre-action user code here
switchDisplayable(null, getForm());
// write post-action user code here
}
//
//
public voidresumeMIDlet() {
// write pre-action user code here
// write post-action user code here
}
//
//
public voidswitchDisplayable(Alert alert, Displayable nextDisplayable) {
// write pre-switch user code here
Display display = getDisplay();
if (alert == null) {
display.setCurrent(nextDisplayable);
} else {
display.setCurrent(alert, nextDisplayable);
}
// write post-switch user code here
}
//
//
public voidcommandAction(Command command, Displayable displayable) {
// write pre-action user code here
if (displayable == form) {
if (command == exitCommand) {
// write pre-action user code here
exitMIDlet();
// write post-action user code here
}
}
// write post-action user code here
}
//
//重写ItemStateChanged方法
public voiditemStateChanged(Item item) {
// write pre-action user code here
if (item == weather_CG) {
form.setTitle("你选择了"+weather_CG.getString(weather_CG.getSelectedIndex())+"天");
// write post-action user codehere
}
// write post-action user code here
}
//
//
publicCommand getExitCommand() {
if (exitCommand == null) {
// write pre-init user code here
exitCommand = new Command("/u9000/u51FA", Command.EXIT, 0);
// write post-init user code here
}
return exitCommand;
}
//
//
public FormgetForm() {
if (form == null) {
// write pre-init user code here
form = new Form("Welcome", new Item[] { getWeather_CG() });
form.addCommand(getExitCommand());
form.setCommandListener(this);
//增加初始天气选择情况显示
form.setTitle("你选择了晴天");
//增加ITEM的监听器
form.setItemStateListener(this); // write post-init user codehere
}
return form;
}
//
//
publicChoiceGroup getWeather_CG() {
if (weather_CG == null) {
// write pre-init user code here
weather_CG = new ChoiceGroup("/u5929/u6C14/u7C7B/u578B",Choice.EXCLUSIVE);
weather_CG.setLayout(ImageItem.LAYOUT_DEFAULT);
weather_CG.setFitPolicy(Choice.TEXT_WRAP_DEFAULT);
//选项框项的代码
weather_CG.append("晴",null);
weather_CG.append("阴",null);
weather_CG.append("雨",null);
weather_CG.append("雪",null);
weather_CG.setSelectedIndex(0,true);
// write post-init user code here
}
return weather_CG;
}
//
publicDisplay getDisplay () {
return Display.getDisplay(this);
}
public voidexitMIDlet() {
switchDisplayable (null, null);
destroyApp(true);
notifyDestroyed();
}
public voidstartApp() {
if (midletPaused) {
resumeMIDlet ();
} else {
initialize ();
startMIDlet ();
}
midletPaused = false;
}
public voidpauseApp() {
midletPaused = true;
}
public voiddestroyApp(boolean unconditional) {
}
}