// XMPPClient.java
    public XMPPClient(Context ctx, AbstractXMPPConnection conn, OnClientAPIResultListener listener) {
        super(false, "xmpp");
        this.conn = conn;
        this.listener = listener;
        this.ctx = ctx;
    }

    public AbstractXMPPConnection getConnection() {
        return conn;
    }

    public void start(String server, String jid, String password, ClientIdentityParams params) {
        this.server = server;
        this.jid = jid;
        SmackConfiguration.DEBUG = true;
        try {
            XMPPTCPConnectionConfiguration config =
                    Authenticator.buildAuthConfig(
                            server,
                            jid,
                            password,
                            true
                    );
            conn = new XMPPTCPConnection(config);
            conn.connect();
            conn.login(jid, password, Authenticator.generateXMPPResource());
            ...
        } catch (Exception e) {
            e.printStackTrace();
            listener.onFail(new HashMap<>(), e);
        }
    }

    // SuperChat.java
    @Override
    public void join(BaseClient client, String nickname) {
        if(client instanceof XMPPClient xmppClient) {
            try {
		// xmppClient.getConnection returns AbstractXMPPConnection
                mucm = MultiUserChatManager.getInstanceFor(xmppClient.getConnection());
                muc = mucm.getMultiUserChat(JidCreate.entityBareFrom((String) id));
                muc.join(Resourcepart.from(nickname));
                isJoined = true;
                requiredAuth = false;
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }