java - How can interfaces replace the need for multiple inheritance when have existing classes

Question

First of all... Sorry for this post. I know that there are many many posts on stackoverflow which are discussing multiple inheritance. But I already know that Java does not support multiple inheritance and I know that using interfaces should be an alternative. But I don't get it and see my dilemma:

I have to make changes on a very very large and complex tool written in Java. In this tool there is a data structure built with many different class objects with a linked member hierarchy. Anyway...

• I have one class Tagged which has multiple methods and returns an object tag depending on the object's class. It needs members and static variables.
• And a second class called XMLElement allows to link objects and in the end generate a XML file. I also need member and static variables here.
• Finally, I have these many many data classes which nearly all should extend XMLElement and some of them Tagged.

Ok ok, this won't work since it's only possible to extend just one class. I read very often that everything with Java is ok and there is no need for having multiple inheritance. I believe, but I don't see how an interface should replace inheritance.

1. It makes no sense to put the real implementation in all data classes since it is the same every time but this would be necessary with interfaces (I think).
2. I don't see how I could change one of my inheritance classes to an interface. I have variables in here and they have to be exactly there.

I really don't get it so please can somebody explain me how to handle this?

Answer 1

Actually, I have no good answer other than Java SHOULD have Multiple Inheritance. The whole point that interfaces should be able to replace the need for Multiple Inheritance is like the big lie that when repeated enough times becomes true.

The argument is that Multiple Inheritance causes all these problems (la-di-dah), yet I keep hearing those arguments from Java developers who have never used C++. I also don't EVER remember C++ programmers saying "Gee, I love C++, but if they would only get rid of Multiple Inheritance, it would become a great language". People used it when it was practical and didn't when it wasn't.

Your problem is a classic case of where Multiple Inheritance would be appropriate. Any suggestion to refactor the code is really telling you how to work around the PROBLEM that Java has no Multiple Inheritance.

Also all the discussion that "oh, delegation is better, la-di-dah" is confusing religion with design. There is no right way. Things are either more useful or less useful and that is all.

In your case Multiple Inheritance would be more useful and a more elegant solution.

As far as refactoring your code into a less useful form to satisfy all the religious people who have never used Multiple Inheritance and believe "Multiple Inheritance is bad", I guess you will have to downgrade your code because I don't see Java "improving" in that way any time soon. There are too many people repeating the religious mantra to the point of stupidity that I can't see it ever being added to the language.

Actually, my solution for you would be "x extends Tagged, XMLElement" and that would be all.

...but as you can see from the solutions provided above, most people think that such a solution would be WAY TOO COMPLEX AND CONFUSING!

I would prefer to venture into the "x extends a,b" territory myself, even if it is a very frightening solution that might overwhelm the abilities of most Java programmers.

What is even more amazing about the solutions suggested above is that everyone here who suggested that you refactor your code into "delegation" because Multiple Inheritance is bad, would, if they were confronted with the very same problem, would solve the problem by simply doing: "x extends a,b" and be done with it, and all their religious arguments about "delegation vs inheritance" would disappear. The whole debate is stupid, and it only being advanced by clueless programmers who only demonstrate how well they can recite out of a book and how little they can think for themselves.

You are 100% correct that Multiple Inheritance would help, and no, you are doing anything wrong in your code if you think Java should have it.