If the methods of an abstract class shouldn't have a body, why does this exercise validate with the abstract class's methods having bodies?

Question about the task

Refactoring the Chain of Responsibility pattern

Resolved

I don't understand. Are we supposed to have bodies in the abstract class's methods or not? What is the recommended practice?

public abstract class AbstractLogger implements Logger{
    int level;
    Logger next;

    public AbstractLogger(int level) {
        this.level = level;
    }

    @Override
    public void inform(String message, int level) {
        if (this.level <= level) {
            info(message);
        }
        if (next != null) {
            next.inform(message, level);
        }
    }

    @Override
    public void setNext(Logger next) {
        this.next = next;
    }

    @Override
    public void info(String message) {

    }
}

Comments (4)

Popular
New
Old

You must be signed in to leave a comment

Thomas

Level 41 , Bayreuth, Germany

24 May 2021, 08:48

solution

You implemented the Logger interface, so all the methods the derived classes should have are already defined. Now you add in between an abstract class that implements some of the methods. The ones that the derived classes commonly use and that do not need to get modified anymore or just be one or two derived classes. For methods that need different implementations in all derived classes it makes no sense to code a default implementation in the abstract class. So you write default implementations for inform and setNext in the abstract class. The abstract class does not necessarily need to implement info as it is abstract, and, as already said, it makes no sense. The derived classes (all of them) now have to implement the info method. edit: btw. in newer Java versions (I think from SE 8 on) interfaces support default implementations. So you won't necessarily need the intermediate abstract class anymore.

Andrei

Level 41

24 May 2021, 09:05

Hi Thomas, thanks for the reply. Initially I was under the impression that abstract classes should only have methods with no bodies; I thought that, by default, they do not permit fully functional methods. I did some more research (Jenkov tutorials about abstract classes) and, as you mentioned, you can have fully functional methods in an abstract class, alongside empty bodied ones. Thanks for the tips! :)

Andrei

Level 41

24 May 2021, 09:05

Regarding the default implementations in interfaces, you mean that user would be able to create a body of the method inside the interface?

Thomas

Level 41 , Bayreuth, Germany

24 May 2021, 09:38

useful

yes, exactly. What you did with the interface and the default implementation in the abstract class you now can do solely within the interface. Yoiu just need to use the default keyword before the method. For me as learner however it seems to be more obvious (if you have heared about these principles) to separate interface and (default) implementations. But on the other hand it's more typing work and you need to look things up in even more files.