Question

According to the Java docs for Socket the socket has an instance of a class called SocketImpl.

However if you look at that class it clearly states that SocketImpl is an abstract class.

As far i know you cannot have an instance of an abstract class! could anyone tell me what is going on here?

Link: Socket Java Docs

Qoute from Docs:

The actual work of the socket is performed by an instance of the SocketImpl class.

Link to SocketImpl class: SocketImpl

Answer 1

The actual concrete object is an instance of a class which extends SocketImpl:

public class ConcreteSocketImpl extends SocketImpl {
    ... (implement all the abstract methods from SocketImpl)
}

And since inheritance defines an is a relationship, any instance of ConcreteSocketImpl is a SocketImpl.

That’s a fundamental principle of object orientation. If you couldn’t have any instance of an abstract class, abstract classes and interfaces would make no sense at all, and would be completely useless.