One of the earliest and most fundamental principle one learns while coding is programming by interface. Definition Interface-based programming defines the application as a collection of components, in which Application Programming Interface (API) calls between components may only be made through abstract interfaces, not concrete classes. Instances of classes will generally be obtained through other interfaces using techniques such as the Factory pattern. — Wikipedia https://en.

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.

If you listen to other language communities - such as Python or Ruby, it seems Java developers have a strong tendency of over-engineering. Perhaps they’re just jealous of our superior platform (wink), perhaps there is some very slight reason that they believe so. I do believe so. And it’s quite interesting that I realized it by doing code review - while I may be guilty of over-engineering myself when writing code. But I’m working on it.