Downloaders without the base class having to implement UIL interface in the base class.

Downloader.java

abstract class Downloader {}

abstract class CacheableDownloader extends Downloader {
  abstract boolean isCached();
  abstract void setCacheIdentifier(String id)
}

class HTTPDownloader extends CacheableDownloader {
  void setCacheIdentifier(String id) {
    this.etag = id;
  }

  boolean isCached() {
    return serverEtag.equals(this.etag);
  }
}

class BluetoothDownloader extends Downloader {}


class IndexDownloader {
  
  private Downloader downloader;
  
}

class ApkDownloader {
  
  private Downloader downloader;
  
}

class UILDownloader implements uil.ImageDownloader {
  
  private Downloader downloader;
  
  public InputStream inputStream() {
    return downloader.getInputStream()
  }
  
}