Abstract classes and interfaces in Java serve fundamentally different purposes. Learn the differences between these Java language elements and how to use them in your programs.

Classes divide into two types: concrete and abstract. I’ll briefly review three constructs: concrete classes, abstract classes, and interfaces. I start with the concrete class.

The release of Java SE 15 in Sept 2020 will introduce "sealed classes" as a preview feature. A sealed class is a class or interface which restricts which other classes or interfaces may extend it.