Lazy loading images works only sometimes, sometimes it doesn't

I'm using lazy loading images based on this question. I'm implementing a search function that, when the user enters a keyword, the application will do a request and parse the result using SAX and then place the data into a ListView. Sometimes the images can be loaded into ImageViews of the ListView rows but sometimes it will load only one or two images and the application crashes.

Below is my LogCat output:

05-16 16:37:37.414: ERROR/AndroidRuntime(620): FATAL EXCEPTION: Thread-11
05-16 16:37:37.414: ERROR/AndroidRuntime(620): java.lang.NullPointerException
05-16 16:37:37.414: ERROR/AndroidRuntime(620):     at com.TPLibrary.Search.ImageLoader.getBitmap(ImageLoader.java:71)
05-16 16:37:37.414: ERROR/AndroidRuntime(620):     at com.TPLibrary.Search.ImageLoader.access$0(ImageLoader.java:68)
05-16 16:37:37.414: ERROR/AndroidRuntime(620):     at com.TPLibrary.Search.ImageLoader$PhotosLoader.run(ImageLoader.java:173)

Below is where the errors are occuring at:

private Bitmap getBitmap(String url) { // line 68
    //I identify images by hashcode. Not a perfect solution, but ok for the demo
    String filename = String.valueOf(url.hashCode()); // line 71
    File f = new File(cacheDir, filename);

    //from SD cache
    Bitmap b = decodeFile(f);
    if (b != null)
        return b;

    //from web
    try {
        Bitmap bitmap = null;
        InputStream is = new URL(url).openStream();
        OutputStream os = new FileOutputStream(f);
        Utils.CopyStream(is, os);
        os.close();
        bitmap = decodeFile(f);
        return bitmap;
    } catch (Exception ex){
        ex.printStackTrace();
        return null;
    }
}

class PhotosLoader extends Thread {
    public void run() {
        try {
            while(true)
            {
                //thread waits until there are any images to load in the queue
                if (photosQueue.photosToLoad.size() == 0)
                    synchronized(photosQueue.photosToLoad) {
                        photosQueue.photosToLoad.wait();
                    }
                if (photosQueue.photosToLoad.size() != 0) {
                    PhotoToLoad photoToLoad;
                    synchronized(photosQueue.photosToLoad) {
                        photoToLoad=photosQueue.photosToLoad.pop();
                    }
                    Bitmap bmp = getBitmap(photoToLoad.url); // line 173
                    cache.put(photoToLoad.url, bmp);
                    Object tag = photoToLoad.imageView.getTag();
                    if (tag != null && ((String)tag开发者_如何学编程).equals(photoToLoad.url)){
                        BitmapDisplayer bd =
                            new BitmapDisplayer(bmp, photoToLoad.imageView);
                        Activity a = (Activity)photoToLoad.imageView.getContext();
                        a.runOnUiThread(bd);
                    }
                }
                if (Thread.interrupted())
                    break;
            }
        } catch (InterruptedException e) {
            //allow thread to exit
        }
    }
}

EDIT

I have tracked down the NullPointerException that comes from the PhotoToLoad class below:

private class PhotoToLoad {
    public String url;
    public ImageView imageView;
    public PhotoToLoad(String u, ImageView i) {
        url = u; 
        imageView = i;
    }
}

How can I solve the problem?

It seems that you pass null to this method. Make sure that you handle null appropriately, or make sure you don't pass null there (first is better ;)).

String filename=String.valueOf(url.hashCode());

So, url is null at this point. So Either you integrate a check like

if (url == null) {
    Log.w("null has been passed to getBitmap()");
    return null;
}

or you have a look on every call where you pass null to this method making it crash.

Remove return null from your catch() block because this might be throwing an Exception and the Bitmap is getting NULL object.