if (providers_.containsKey(class_name))
if (provider_classes_.containsKey(class_name))
  public void addProvider(String class_name)      throws IllegalArgumentException, ClassNotFoundException  {    if (class_name == null || class_name.length() == 0)    {      throw new IllegalArgumentException();    }    if (providers_.containsKey(class_name))    {      return;    }    Class provider_class = Class.forName(class_name);    DMDServiceProvider provider = null;    try    {      provider = (DMDServiceProvider) provider_class.newInstance();    }    catch (Exception exception)    {      exception.printStackTrace(System.err);      throw new ClassNotFoundException();    }    providers_.put(class_name, provider);  }