Java 9 Interface vs Class

Private methods are not inherited by subclasses, so this feature doesn't affect implementation classes. I believe the private methods in interfaces allow us to share code between default methods.

Java interfaces still cannot have non-static members. That's a big difference and not multiple inheritance IMO.

Private interface methods in Java 9 behave exactly like other private methods: They must have a body (even in abstract classes) and can neither be called nor overridden by subclasses. As such they do not really interact with inheritance. Talking of which (and particularly multiple inheritance), there are (at least?) three kinds of it:

• Inheritance of types means that one type can be another type, e.g. String is an Object. Java allowed multiple inheritance of types from day one (via interfaces).
• Inheritance of behavior means that one type can inherit the behavior of another type. Before Java 8, only classes could implement methods, so there was only single inheritance of this kind. With Java 8 came default methods, which allowed interfaces to implement methods, thus giving Java multiple inheritance of behavior.
• Inheritance of state means that a type inherits another type's internal state (i.e. fields). As it stands (Java 9 and everything currently proposed for future Java versions), only classes can have state, so there is only single inheritance of this kind.

As you can see private interface methods do not add anything here.

Regarding your question of how interfaces and classes compare, there are two main differences: multiple inheritance and state. Interfaces support the former, classes can have the latter. Since state is kind-of important in typical OOP, classes will remain relevant. 😉

If there were a way for an interface to force an implementation to have a particular non-public field or straight-out define one itself, the game would change and interfaces could compete with classes.