Adding the abstract keyword in front of the class definition will mark the class as abstract. An abstract class is a partially defined class; properties and methods that have no implementation must be implemented in a derived class, unless the derived class is meant to be an abstract class as well. Here is how you would define an abstract class in Kotlin:

    abstract class A { 
      abstract fun doSomething() 
    } 

Unlike interfaces, you have to mark the function abstract if you don't provide a body definition.

You cannot create an instance of an abstract class. The role of such a class is to provide a common set of methods that multiple derived classes share. The best example of such a case is the InputStream class. This will be very familiar to a developer who has already worked with Java. The JDK documentation says: "This abstract class is the superclass of all classes representing an input stream of bytes. Applications that need to define a subclass...