What Each of the Two Is For

Five things this topic established, and where each one was.

What the topic established, and which card it was on
The thing	Where
An interface names a capability and holds no per-object state	Naming What a Thing Can Do
An abstract class is a partly-built class, with fields and a constructor	What an Abstract Class Adds
The deciding question is whether there is shared state	Interface or Abstract Class
A `default` method lets an interface grow without breaking implementations	default Methods
One `extends`, any number of `implements`, because interfaces add no fields	Why You Can Implement Ten Interfaces

State is the whole distinction. Everything else — how many you may have, what a default method may touch, why constants in an interface are shared — follows from an interface having none.

Java
Object-Oriented Programming