consider wider use of default methods

[jrtom]

Default methods in Java 8+ interfaces provide some additional support for users that need to implement our interfaces but don't want to (or can't) use our abstract classes.

In some cases, they may allow us to get rid of our abstract classes entirely.

[jbduncan]

Good idea! :+1:

A thing we may want to watch out for is if we have any abstract classes with final methods, we may not want to or be able to migrate those to default methods on interfaces, as a user could then override them.

I don't really have anything else to add. :)