I think you're spot on with the lack of a default method in olden days Java. It was certainly the driver behind having an AbstractBaseWhatever class back when.

I think you're spot on with the lack of a default method in olden days Java. It was certainly the driver behind having an AbstractBaseWhatever class back when. Though there could still be utility in having a base class that had some functionality acting on internal state that was shared by lots of implementations of that. Another use like Priceable and CartItem in modern Java is Connection and AutoCloseable (database connection objects). Anything AutoCloseble can be automatically closed after a try/catch block is done (with some specifics I glossed over on how). And a Connection is that, some specific form of DB connection. You also have a DB ResultSet that is AutoCloseable. So by having another interface extending another, or a class implementing the interfaces you can drive very related functionality on classes that may do very different things. CartItem could extend/implement the one or two method Priceable, along with all its other methods.