When default methods were introduced in Java 8, some developers thought they would be the same as abstract classes. That’s not true, however, because interfaces can’t have state. A default ...

public abstract class Ellipse2D extends RectangularShape ... Declaring a local class within an enclosing class instance method (EnclosingClass.java, version 3) class EnclosingClass { void ...

protected and private concrete methods, while interfaces have all fields as automatically public, static and final. Interfaces, however, support multiple inheritances where abstract classes don't.

You cannot prevent other developers from extending certain classes. A method might accept a specific type and its subclasses, but you want to restrict usage to a select few implementations of your own ...