Interfaces

We’ve seen in earlier sections how classes can form a hierarchy, in which subclasses are specialized versions of more general superclasses. In languages like C#, Java and Kotlin, a particular restriction applies to this class hierarchy: each class cannot have more than one superclass.

We’ve also seen that these superclasses can be abstract, with ‘missing’ method implementations that have to be provided in subclasses.

Interfaces are similar to abstract classes in this respect, but are not subject to the same limitations. A class can have at most one abstract class as its superclass, but it can implement multiple interfaces.

After completing this section, you will understand why interfaces are useful in their own right, and how they complement abstract classes. You will also know how to use them in Kotlin code.