2个关于菜单的小疑点

package gesanri;import javax.microedition.lcdui.ChoiceGroup;import javax.microedition.lcdui.Command;import javax.microedition.lcdui.CommandListener;import javax.microedition.lcdui.Display;import javax.microedition.lcdui.Displayable;import javax.microedition.lcdui.Form;public class City implements CommandListener{    private Displayable displayable;    private Display display;    private Form form;    private Command back;    private ChoiceGroup province;    private ChoiceGroup city;    private String[][] provinceAndCity;        public City(Display display,Displayable displayable) {        this.display = display;        this.displayable = displayable;        display = Display.getDisplay(MainMidlet.instance);        provinceAndCity = new String[][]{{"直辖市","北京","上海","天津","重庆"},{"特别行政区","香港","澳门"},{"黑龙江","哈尔滨","齐齐哈尔","牡丹江","大庆","伊春","双鸭山","鹤岗","鸡西","佳木斯","七台河","黑河","绥化","大兴安岭"}};                form = new Form("新增城市");        back = new Command("返回",Command.BACK,1);        province = new ChoiceGroup("省份：",ChoiceGroup.POPUP);        city = new ChoiceGroup("城市：",ChoiceGroup.POPUP);        province.append("请选择", null);        for(int i = 0; i < provinceAndCity.length; i++)            province.append(provinceAndCity[i][0], null);        city.append("请选择", null);        form.append(province);        form.append(city);        form.addCommand(back);        form.setCommandListener(this);    }    public void show(){        display.setCurrent(form);    }        public void commandAction(Command c, Displayable dpa) {        if(c == back){            display.setCurrent(displayable);        }    }}