code: public void addInterface(Class<?> newInterface) {
          if (!newInterface.isInterface()) {
              throw new IllegalArgumentException(newInterface + " is not an interface");
          }
          additionalInterfaces.add(newInterface);
      }
docs: Adds an additional interface that the proxy should implement. The default
  implementation will be to forward invocations to the bean instance.
  
  @param newInterface an interface