Display - Best practice

[Peter Miklosko]

What is best practice in handling movement from screen to screen in midlet?
My teacher at university wasn't able to explain this to me, could be due to that he chuck whole 300+ lines in one file.
Secondly none of the books or tutorials that I read so far went on discussing this.
So here is how I do it and I would like to know if my approach is correct and if not I would like to know how to handle this correctly.

ImageHttp.java

 package imageHttp;

import javax.microedition.lcdui.Display;
import javax.microedition.midlet.MIDlet;

import imageHttp.gui.MainMenu;

public class ImageHttp extends MIDlet{

    public static ImageHttp instance;
    private MainMenu mMenu;

    public ImageHttp(){
        instance = this;
    }

    public void startApp(){
        if(mMenu == null){
            mMenu = new MainMenu();
        }
        Display.getDisplay(this).setCurrent(mMenu);
    }

    public void pauseApp(){}

    public void destroyApp(boolean unconditional){}

    public static void quitApp(){
        instance.destroyApp(true);
        instance.notifyDestroyed();
        instance = null;
    }
}

MainMenu.java

package imageHttp.gui;

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;


public class MainMenu extends List implements CommandListener{
    private final Command exitCom;

    public MainMenu(){
        super("Image Up/Down-load", List.IMPLICIT);
        
        exitCom = new Command("Exit",Command.EXIT,0);
        addCommand(exitCom);
        setCommandListener(this);
    }

    public void commandAction(Command c, Displayable d){
        if(c == exitCom){
            imageHttp.ImageHttp.quitApp();
        }
        else{
            int index = getSelectedIndex();
            Displayable dis =  null;
            switch(index){
                case 0:
                    dis = new ImageUrlRequest();
                    break;
                case 1:
                    dis = new ImageServletRequest();
                    break;
                case 2:
                    dis = new ImageUpload();
                    break;
                default:
                    break;
            }
            Display.getDisplay(imageHttp.ImageHttp.instance).setCurrent(dis);
        }
    }
}

In my opinion this better solution then sending instance of Displayable argument to next class.

[Amila Jayatillaka]

Midlet Class

  
public Display getDisplay () {
        return Display.getDisplay(this);
}

In displayable classes, I'll get a instance of this midlet through constructor. And i'll call getDisplay() where ever i need...

[Peter Miklosko]

I was recommended following approach

package imageHttp;

import javax.microedition.lcdui.Display;
import javax.microedition.lcdui.Displayable;
import javax.microedition.midlet.MIDlet;

public class ImageHttp extends MIDlet{

    private Display disp = Display.getDisplay(this);
    public ImageHttp(){}

    public void startApp(){
        changeScreen(CoreUI.MAIN_MENU);
    }

    public void pauseApp(){}

    public void destroyApp(boolean unconditional){}

    public void quitApp(){
        destroyApp(true);
        notifyDestroyed();
    }

    public void changeScreen(String _uiClass){
        Displayable display;
        try{
            display = (Displayable)Class.forName(_uiClass).newInstance();
            disp.setCurrent(display);
            ((CoreUI)display).setUIManager(this);
        }
        catch(Exception e){
            e.printStackTrace();
        }
    }
}

package imageHttp;

public interface CoreUI {

    public final String MAIN_MENU = "imageHttp.gui.MainMenu";
    public final String URL_REQUEST = "imageHttp.gui.ImageUrlRequest";
    public final String SERVLET_REQUEST = "imageHttp.gui.ImageServletRequest";
    public final String UPLOAD = "imageHttp.gui.ImageUpload";

    void setUIManager(ImageHttp _mgr);
}

package imageHttp.gui;

import javax.microedition.lcdui.Command;
import javax.microedition.lcdui.CommandListener;
import javax.microedition.lcdui.Displayable;
import javax.microedition.lcdui.List;

import imageHttp.CoreUI;
import imageHttp.ImageHttp;

public class MainMenu extends List implements CommandListener, CoreUI{
    private final Command exitCom;
    private ImageHttp mgr;

    public MainMenu(){
        super("Image Up/Down-load", List.IMPLICIT);
        append("URL request down.", null);
        append("Servlet request down.", null);
        append("Http servlet up.", null);
        exitCom = new Command("Exit",Command.EXIT,0);
        addCommand(exitCom);
        setCommandListener(this);
    }

    public void setUIManager(ImageHttp _mgr){
        mgr = _mgr;
    }

    public void commandAction(Command c, Displayable d){
        if(c == exitCom){
            mgr.quitApp();
        }
        else{
            int index = getSelectedIndex();
            String dis = "";
            switch(index){
                case 0:
                    dis = CoreUI.URL_REQUEST;
                    break;
                case 1:
                    dis = CoreUI.SERVLET_REQUEST;
                    break;
                case 2:
                    dis = CoreUI.UPLOAD;
                    break;
                default:
                    break;
            }
            mgr.changeScreen(dis);
        }
    }
}