Question

I have a question like this :

Think a scenario like this

public class Animal {

    protected String Name;
    Boolean canWork;

}

public class Dog {

  Enum TailType

}

And I need to have both of this classes attributes in a class of the third level which extends the both classes .. but using interfaces I don’t think this can be achieved. Is it possible to do this using a design pattern or some else method ?

Summary : I want to have attributes from two classes to a concrete class

Answer 1

You can have Dog extend Animal, then extend Dog by the third class, but unless your 3rd class is Poodle then you may have a problem you don’t realize yet. That being inheritance is only appropriate when the relationship is a modeling criteria, and extending objects only to get their functionality is the wrong approach. Inheritance should follow the IS-A principle. That being your subclass IS-A base class in modeling terms. If it doesn’t pass that test you are using inheritance when you shouldn’t. After all you can use delegation to obtain their functionality. That meaning:

public class SomeClass {

    private Dog dog;

    public void bark() {
       dog.bark();  // this is reusing the functionality without extending
    }
}

Now SomeClass can call or invoke methods on Dog without extending it. Now the downside to this is a reference to Dog can’t point to SomeClass, but if SomeClass is-not-a Dog that’s probably good. However, if you have to allow Dog and SomeClass to share some typing so you can have a reference that points at either Dog or SomeClass then you can create an interface that both share:

public class SomeClass implements Barkable {
    private Dog dog;

    @Override
    public void bark() {
       dog.bark();
    }
}

public class Dog implements Barkable {

    @Override
    public void bark() {
       System.out.println( "Bark! Bark!" );
    }
}

With delegation/composition and interfaces you DON’T need multiple inheritance. It’s a really simple technique to apply and master and you’ll build systems that are much more flexible than relying on inheritance alone.