Just a nice listenerjava.java

    private List<WeakReference<IFileDownloader.IFileDownloadCallBack> > callbackList
            = Collections.synchronizedList(new ArrayList<WeakReference<IFileDownloader.IFileDownloadCallBack>>());

    public void addCallback(IFileDownloader.IFileDownloadCallBack cb) {
        if (cb != null) {
            for (int i=0; i < callbackList.size(); ++i) {
                IFileDownloader.IFileDownloadCallBack callback = callbackList.get(i).get();
                if (callback == cb) {
                    return;
                }
            }
            callbackList.add(new WeakReference<IFileDownloader.IFileDownloadCallBack>(cb));
        }
    }

    public void removeCallback(IFileDownloader.IFileDownloadCallBack cb) {
        if (cb != null) {
            for (int i=0; i < callbackList.size(); ++i) {
                IFileDownloader.IFileDownloadCallBack callback = callbackList.get(i).get();
                if (callback == null) {
                    callbackList.remove(i--);
                } else if (callback == cb) {
                    callbackList.remove(i);
                }
            }
        }
    }