一个代码上的小有关问题，请教下

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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);
            }
        }
    }
}