一个代码上的小问题，请教下
/*
* To change this template, choose Tools | Templates
* and open the template in the editor.
*/

import javax.microedition.midlet.*;
import javax.microedition.lcdui.*;

/**
* @author Administrator
*/
public class Tex extends MIDlet implements CommandListener{
A a;
private Display dis;
private TextBox tb;
private Command exit,input;
private List conL;
static final String[] conN = {
"any",
"Num",
"Email",
"shi zhi",
"phone",
"URL",
};
static final int[] conT = {
TextField.ANY,
TextField.NUMERIC,
TextField.EMAILADDR,
TextField.DECIMAL,
TextField.PHONENUMBER,
TextField.URL,
};

class A implements CommandListener{
public void commandAction(Command c, Displayable d) {
if(c == exit){
dis.setCurrent(conL);
}

throw new UnsupportedOperationException("Not supported yet.");
}}
public Tex(){
dis = Display.getDisplay(this);
exit = new Command("退出",Command.SCREEN,1);

tb = new TextBox("","",200,0);
conL = new List("选择类型",Choice.IMPLICIT,conN,null);
tb.addCommand(exit);
tb.setCommandListener(a);
conL.addCommand(exit);
conL.setCommandListener(this);


}

public void startApp() {
dis.setCurrent(conL);
}

public void pauseApp() {
}

public void destroyApp(boolean unconditional) {
}


public void commandAction(Command c, Displayable d) {
if(d.equals(conL)){

if(c == exit){
destroyApp(false);
notifyDestroyed();
}

int t = ((List)d).getSelectedIndex();
tb.setTitle(conN[t]);
tb.setConstraints(conT[t]);
dis.setCurrent(tb);

}
throw new UnsupportedOperationException("Not supported yet.");
}
}



以上程序运行后会出现一个List，选择后出现一个TexBox，可以根据选择的不同而使输入类型不同。
我在List界面上选择退出时会结束程序(这个倒是行的)
我想在TextBox界面上选择退出时，返回到List那个界面。
所以我尝试着定义了一个类A
用了上述的代码希望实现这个功能，但是没用啊
所以想要请教下应该怎么写才能实现我说的功能呢？

[解决办法]

Java code

/*  * To change this template, choose Tools | Templates  * and open the template in the editor.  */import javax.microedition.lcdui.Choice;import javax.microedition.lcdui.Command;import javax.microedition.lcdui.CommandListener;import javax.microedition.lcdui.Display;import javax.microedition.lcdui.Displayable;import javax.microedition.lcdui.List;import javax.microedition.lcdui.TextBox;import javax.microedition.lcdui.TextField;import javax.microedition.midlet.MIDlet;/** * @author Administrator */public class Tex extends MIDlet implements CommandListener {    private Display dis;    private TextBox tb;    private Command exit, input;    private List conL;    static final String[] conN = { "any", "Num", "Email", "shi zhi", "phone",            "URL", };    static final int[] conT = { TextField.ANY, TextField.NUMERIC,            TextField.EMAILADDR, TextField.DECIMAL, TextField.PHONENUMBER,            TextField.URL, };    public Tex() {        dis = Display.getDisplay(this);        exit = new Command("退出", Command.SCREEN, 1);        tb = new TextBox("", "", 200, 0);        conL = new List("选择类型", Choice.IMPLICIT, conN, null);        tb.addCommand(exit);        tb.setCommandListener(this);// 注册监听器        conL.addCommand(exit);        conL.setCommandListener(this);    }    public void startApp() {        dis.setCurrent(conL);    }    public void pauseApp() {    }    public void destroyApp(boolean unconditional) {    }    public void commandAction(Command c, Displayable d) {        if (d.equals(conL)) {            if (c == exit) {                destroyApp(false);                notifyDestroyed();            }            int t = ((List) d).getSelectedIndex();            tb.setTitle(conN[t]);            tb.setConstraints(conT[t]);            dis.setCurrent(tb);        } else if (d == tb) {// 当前是TextBox            if (c == exit) {// 按退出，返回到List                dis.setCurrent(conL);            }        }    }}