How do the new class modifiers work in Dart 3.0 (interface vs abstract)?

Question

They added a few new modifiers to classes. What is the difference between abstract class, interface, and abstract interface?

Answer 1

The official docs are pretty thorough, but here's super short summary. For a longer summary, check out the bottom of this post.

The first thing you need to get is the difference between implements and extends. These concepts existed in Dart 2.x, but get expanded on.

implements = Your sub-class has the exact same “shape” (fields/functions) as the parent class but must define all its own behavior. You do not inherit any functionality from the parent's functions or fields. It’s similar to other languages, but you can implement just about any class (regardless of modifier)

extends = Your sub-class can extend (override) behavior, inherit behavior, and implement behavior

Then this table summarizes the differences:

Here is the full table from the spec:

Declaration	Construct?	Extend?	Implement?	Mix in?	Exhaustive?
class	Yes	Yes	Yes	No	No
base class	Yes	Yes	No	No	No
interface class	Yes	No	Yes	No	No
final class	Yes	No	No	No	No
sealed class	No	No	No	No	Yes
abstract class	No	Yes	Yes	No	No
abstract base class	No	Yes	No	No	No
abstract interface class	No	No	Yes	No	No
abstract final class	No	No	No	No	No
mixin class	Yes	Yes	Yes	Yes	No
base mixin class	Yes	Yes	No	Yes	No
abstract mixin class	No	Yes	Yes	Yes	No
abstract base mixin class	No	Yes	No	Yes	No
mixin	No	No	Yes	Yes	No
base mixin	No	No	No	Yes	No