code: protected final void init(I inboundHandler, O outboundHandler) {
          validate(inboundHandler, outboundHandler);
          this.inboundHandler = inboundHandler;
          this.outboundHandler = outboundHandler;
      }
docs: Initialized this handler with the specified handlers.
  
  @throws IllegalStateException if this handler was not constructed via the default constructor or
  if this handler does not implement all required handler interfaces
  @throws IllegalArgumentException if the specified handlers cannot be combined into one due to a conflict
  in the type hierarchy