public final class MyModelAdapter {
    private OnConnectionReady onConnectionReady;

    public MyModelAdapter setOnConnectionReady(OnConnectionReady connection) {
        this.onConnectionReady = connection;
        return this;
    }

    public interface OnConnectionReady {
        void onConnect(String message);
    }

    //more stuff..
}

public final class MyModel {
    private MyModelAdapter adapter;

    public void do() {
        adapter.setOnConnectionReady(this::whenAdapterIsConnected);
    }

    public void whenAdapterIsConnected(String message) {
        //make fun
    }
}