Is there any LazyLoader for images to load image in ListField in BlackBerry?

Question 1

You can try using this link : http://www.coderholic.com/blackberry-webbitmapfield/

You have to create a separate class named as WebBitmapField as suggested in above link.

How to use that class in your list field image objects:

For every image url create WebBitmapField object

photoList_vector is the vector through which populate elements in list field

WebBitmapField web = new WebBitmapField("http://www.image1.png"); 

photoList_vector.addElement(web);

web = new WebBitmapField("http://www.image2.png"); 

photoList_vector.addElement(web);

Now use this vector to work on your list field......

In the above lines we try to ensure that when we simultaneously send multiple requests to get the images then each image corresponds to a particular WebBitmapField Object.

Each object is then added to vector so that it can be added to the list field.

Each url send is tied to an object of WebBitmapField.

So though request is send in a separate thread it gets tied to its associated object only

Hope it helps :)

Question 2

Several problems with your code:

The BitmapLazyLoader class looks like a consumer. It holds a Thread reference. This alone is very confusing, since Runnables are intended to be passed to a Thread constructor, but Runnables should not know about the thread for the sake of encapsulation. Letting this apart, this class attempts to spawn a thread only once, but as you are creating an instance of Runnable each time a row is drawn, you'll end up spawning a considerable number of threads. This will probably end in a TooManyThreadsException being thrown as in BlackBerry the max number of threads is limited to 16 per app. Even if you don reach the limit, performance will degrade as BlackBerries, which sport a single core CPU, you shouldn't have more than 2-3 threads running at the same time. EVEN if you could spawn infinite threads, in BlackBerry you can only have X connections opened at the same time (I think X is 5 for the whole OS, not sure about this). So first of all modify the code to ensure only a single worker thread is downloading images. (and if possible, extract the thread instantiation and launch out of the Runnable class).
When the bitmap is downloaded, you are not doing anything with it. Look at the ImageDownloadCompleted method, it is empty. (BTW, the convention for methods is to start with lowercase) So you should store the bitmap somewhere and call invalidate on your list, which in turn will paint the stored bitmaps.

Hope it helps.

Question 3

I have worked on this problem, earlier, and I am posting my technique here, though its not ideal solution, as it was coupled very much with Screen class, but still might be helpful.

First in your screen class have one array for bitmaps having size equal to list field items.

public class TempScreen extends MainScreen{
    Bitmap[] images=null;
    String[] urls={"image1_url", "image2_url".....};
    public TempScreen()
    {

        images=new Bitmap[urls.length];

    }

now in drawListRow method of ListFieldCallBack, check for the following:

 public void drawListRow(ListField list, Graphics g, int index, int y, int width){
     if(bitmap[index]==null)
     {
         //Load placeholder image
     }
     else
         //Load Bitmap    
 }

Now create a thread class to download the images:

public class ImageDownloader implements Runnable
{
    public void run()
    { 
        for(int i=0; i<size;i++)
        { 
             if(images[i]==null) 
             { 
                  images[i]=downloadImage(url[i]);//replace downloadImage method to whatever method       you have to download the bitmap from url 
                  UiApplication.getUiApplication().invokeLater(new Runnable(){
                      public void run()
                      {
                          list.invalidate()
                      }
                  });
              }
         }
    }
}

Now in constructor of the screen class, after setting callback to listfield, start thread:

Thread downloader=new Thread(new ImageDownloader());
downloader.start();

Edit: Change TempScreen constructor to following:

public TempScreen()
    {

        images=new Bitmap[urls.length];
        size = urls.length;
        mylist = new ListField();
        mylist.setCallback(this);
        mylist.setSize(4);
        mylist.setRowHeight(getFont().getHeight() * 3);
        add(mylist);
        Thread downloader=new Thread(new ImageDownloader());
        downloader.start();
    }