A recent question on Kotlin’s Reddit came to my attention lately: 'Why is it bad to write extension functions for classes that you own?' One of the answer was that it was the opposite: it’s a good practice, and for several reasons. Among them were two very important ones that improve the design of one’s code: Able to call the function on a nullable. For a class with generic types, add a function that is only available when a specific type is used.
Java 8 introduced default methods in interfaces. This post describes what they are, and how they can change the design of APIs. A nominal design Earlier, in Java, interfaces could only have contracts - method signatures with no implementation. In order to add some implementation, a class was required, whether abstract or not. Hence, traditional API design then followed this hierarchy: The root interface defines the contractAn intermediate class implements common behavior i.e. BarIf