code: @SuppressWarnings("unchecked")
      public static <T> T getSingletonService(Class<T> intf) {
          T ret = null;
  
          synchronized (singletons) {
              if (singletons.containsKey(intf)) {
                  ret = (T) singletons.get(intf);
              } else {
                  List<T> services = getServices(intf);
  
                  if (!services.isEmpty()) {
                      ret = services.get(0);
                  }
  
                  singletons.put(intf, ret);
              }
          }
  
          return ret;
      }
docs: This method identifies a service implementation that implements
  the supplied interface, and returns it as a singleton, so that subsequent
  calls for the same service will get the same instance.
  
  @param intf The interface
  @return The service, or null if not found
  
  @param <T> The service interface