Question

I want to understand the role interface plays in inheritance between two classes.

My understanding is that you can create superclass and subclass with the use of extends.

class Parent {
}
class Child extends Parent {
}

This is already sufficient in creating superclass and subclass.

When do we need interface? Do we need implements for Child or Parent?

class Parent {
}
class Child extends Parent implements MyInterface {
}
interface MyInterface {
}

Answer 1

Inheritance and interface implementation defines a is-a relationship. This means that is the subclass A extends a superclass B, A is-a B. If the superclass B implements the interface I, B is-a I. And since A is-a A, A is also a I.

For example, a Car (ssuperclass) is a Vehicle (interface). Since Audi extends Car, Audi is also a Vehicle.

A subclass can not be less than its parent, but it can be more. So a child may implement interfaces that its parent class doesn’t implement.

For example, all cars are not hybrid (interface). But this doesn’t prevent a Prius, which is a Car, to be a hybrid. A Prius is more than just a Car. It’s also a hybrid.