code: public final void addInterface(final Class<?> intf) {
          if (intf == null) {
              throw new IllegalArgumentException("The argument 'intf' cannot be null!");
          }
          if (!intf.isInterface()) {
              throw new IllegalArgumentException("The argument 'intf' [" + intf.getName()
                      + "] is not an interface!");
          }
          interfaces.add(intf);
      }
docs: Adds a new interface that has this method.
  
  @param intf
  Interface to add - Cannot be <code>null</code> and must be an
  interface.