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 ...

In Java, polymorphism results in code ... and non-abstract methods in addition to or instead of abstract methods. For example, an abstract Vehicle class might declare fields describing its make ...

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.