abstract class vs interface

1. https://www.infoworld.com/article/2928719/c-sharp/my-two-cents-on-abstract-class-vs-interface.html
2. http://www.geeksforgeeks.org/difference-between-abstract-class-and-interface-in-java/
3. https://beginnersbook.com/2013/05/abstract-class-vs-interface-in-java/
4. https://stackoverflow.com/a/18778220