class EchoServer {
    private final int port;

    public EchoServer(int port) {
        this.port = port;
    }

    public static void main(String[] args)
        throws Exception {
        if (args.length != 1) {
            System.err.println("Usage: " + EchoServer.class.getSimpleName() +
                " <port>"
            );
            return;
        }
        int port = Integer.parseInt(args[0]);
        new EchoServer(port).start();
    }

    public void start() throws Exception {<FILL_FUNCTION_BODY>}
}

        final EchoServerHandler serverHandler = new EchoServerHandler();
        EventLoopGroup group = new NioEventLoopGroup();
        try {
            ServerBootstrap b = new ServerBootstrap();
            b.group(group)
                .channel(NioServerSocketChannel.class)
                .localAddress(new InetSocketAddress(port))
                .childHandler(new ChannelInitializer<SocketChannel>() {
                    @Override
                    public void initChannel(SocketChannel ch) throws Exception {
                        ch.pipeline().addLast(serverHandler);
                    }
                });

            ChannelFuture f = b.bind().sync();
            System.out.println(EchoServer.class.getName() +
                " started and listening for connections on " + f.channel().localAddress());
            f.channel().closeFuture().sync();
        } finally {
            group.shutdownGracefully().sync();
        }
    
<no_super_class>