class ChatPacketListener {

    @PacketListener
    public void onChatOutgoing(PacketEvent<SPacketChat> event, Connection connection) {
        SPacketChat packet = event.getPacket(); // Immutable packet instance
        if (cancel) {
            event.setCancelled(true);
        } else if (transformPacket) {
            event.setPacket(new SPacketChat("lol I manipulated your packet"))
        } else if (replacePacket) {
            connection.sendPacket();
        }
    }

    @PacketListener
    public void onChatIncoming(PacketEvent<CPacketChatMessage> event, Connection connection) {
        CPacketChatMessage packet = event.getPacket(); // Immutable packet instance
        if (packet.getMessage().contains("shit")) {
            connection.sendPacket(new SPacketChat("Please watch your language"));
            event.setCancelled(true);
        } else if (packet.getMessage().equals("plis let me swear")) {
            connection.sendPacket(new SPacketChat("k lol"));
            PacketInterceptor.unregisterListener(this);
            event.setCancelled(true);
        }
    }

}

// usage example - register ChatPacketListener to one specific connection only.
// It's also possible to register global listeners by omitting the clientConnection argument.
@Listener
public void onPlayerJoin(ClientConnectionEvent.Join event) {
    Player player = event.getTargetEntity();
    ClientConnection clientConnection = PacketInterceptor.getClientConnection(player);
    PacketInterceptor.registerListener(new ChatPacketListener(), clientConnection);
}