A simple point of sale system that I made for a restaurant owned by a friend of mine's family (Check out their awesome restaurant: Asli Dinning).
This app illustrates some Android development practices with Android Jetpack. Some of the libraries that I used worth to mentioning:
For charting parts, I used MPAndroidChart.
This app is almost completed, however a major feature is yet to be added. That is the connectivity to POS printers, which most likely will be located in the kitchen so that chefs can read printed orders. I think I will start off working with Epson POS printers and using their ePOS-Print API.