to be clear from the beginning: I'm not loooking for saving a custom layer to a layer file (for that I found many examples), but I want to get my custom layers saved to the mxd-files so that this layers would be displayed if the user reopens the file.
I'm using as underlying library ArcBrutile and I'm displaying HERE-Maps. I found this (old) article about layers and nearly the same article for a current ArcGIS version for Java. The first one states that I need for saving to implement the "IPersistStream"-Interface, while the other one tells me that I need "IPersistVariant"-Interface. Originally the "IPersistVariant"-interface already was implemented for the layers. I tried to implement also the "IPersistStream"-Interface, but the layers still won't be saved to the mxd-file.
So how I'm able to save tiled custom layers to mxd?