问J2ME按键事件处理

EN

package hello;

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

public class HelloMIDlet extends MIDlet {

    public Display display;

    public HelloMIDlet() {
        display = Display.getDisplay(this);
    }

    public void startApp() {
        Form f = new F(display);
        display.setCurrent(f);
    }

    public void pauseApp() {
    }

    public void destroyApp(boolean unconditional) {
    }

}

package hello;

import javax.microedition.lcdui.Alert;
import javax.microedition.lcdui.AlertType;
import javax.microedition.lcdui.Display;
import javax.microedition.lcdui.Form;
import javax.microedition.lcdui.Item;
import javax.microedition.lcdui.ItemStateListener;
import javax.microedition.lcdui.TextField;

public class F extends Form implements ItemStateListener {
    private TextField pin = new TextField("PIN :","",4,TextField.PASSWORD);
    private Alert alert;
    private Display disp;
    public F(Display d)
    {
        super("");
        disp = d;
        this.setItemStateListener(this);
        this.append(pin);
    }
    public void itemStateChanged(Item item) {
        if (item == pin)
        {
            for (int i=0; i<pin.getString().length(); i++)
            {
                if (String.valueOf(pin.getString().charAt(i)).equals(new String(" ")))
                    displayAlert();
            }
        }
    }
    private void displayAlert()
    {
        alert = new Alert("Info","No space please !",null, AlertType.ERROR);
        disp.setCurrent(alert, this);
    }
}