What is the difference between open class and abstract class?

Question

abstract class ServerMock(param: String) {
   protected var someVar = params + "123"

   fun justMyVar() = someVar
}

Usage example:

class BaseServer(param: String) : ServerMock(param) {
   val y = someVar
}

Can this class be marked as open and not abstract?

What is the difference between open and abstract class?

Answer 1

abstract class cannot be instantiated and must be inherited, abstract classes are open for extending by default. open modifier on the class allows inheriting it. If the class has not open modifier it is considered final and cannot be inherited.