Martin Fowler: UML Distilled, 3rd Edition

I have been reading Martin Fowler's book, "UML Distilled, 3rd Edition" since last few months (my book reading has been very slow, keeping in mind the time I spend on web these days, to do most of my learnings).

This is a great UML book (and has only, 175 pages but very good), and I recommend it to anybody wanting to know about UML (Unified Modeling Language).

OO multiple inheritance, and Java

I have been thinking again, about multiple inheritance and why Java doesn't support it. I wrote a bit about this topic, some time ago.

There are so, so many resources on web about this, and it's actually very easy to find the answer to this, via a simple web search. Here is an article from where I started to know an answer to this, http://www.javaworld.com/javaqa/2002-07/02-qa-0719-multinheritance.html, which pointed me to this white paper by James Gosling and Henry McGilton. Really, I did not read this white paper by Java creators earlier (it never came across my eyes :)), in spite being familiar and working with Java since long time. Sometimes, we find gems on web in an unexpected ways (I mean, this paper is a gem for me :)). I'll try to read this paper (hopefully fully, and being able to understand it) over the next few days.

And here is a link in this white paper, which explains why Java doesn't support multiple inheritance. The following white paper link is also interesting, which gives a complete overview of C and C++ features, that were omitted in Java language (Java has been influenced from C and C++).

My personal opinion is, that if we must need to use multiple inheritance, we should just try to write programs in C++. On the contrary, my experience in using Java for about a decade, convinces me, that Java is suitable to solve almost any business application problem, and absence of multiple inheritance in Java, is not an hindrance to design good programming abstractions for problem domain. The advantages like Java's byte code portability and web friendliness far outweigh, any disadvantages caused by absence of multiple inheritance. On numerous occasions, I have created Java byte code on Windows, and used it without modification on Unix based systems (and vice versa). This is something which is built into the Java language, and it is cool!

Multiple inheritance in Java

I have always missed true multiple inheritance in Java (like, in C++). For e.g., we are not able to define a class as follows in Java:

class X extends A, B, C {

}

Though, I do not see any inheritance use case which cannot be solved by the current Java facilities, but I would love to have this facility in Java. I think, the most latest Java version (1.7) doesn't have this feature.

One workaround I can see, for multiple inheritance, is to define a class like following:

class X {
A a;
B b;
C c;
}

i.e., we could create private class members inside X (whose functionality we want to use in class X).

Though this might serve purpose for some of the cases, but it's not true multiple inheritance! This I think, is actually aggregation pattern.

Of course, Java has multiple inheritance of interfaces. But that is inheritance of method signatures, and not of implementation.

I guess, keeping the number of base classes to one, Java is much simpler syntactically, and has a simpler compiler implementation. Though I agree, that having a simple syntax (as the current Java inheritance facilities) which is powerful enough, and can solve many use cases is better, than having a complex syntactical facility, which might serve even more uses cases, but could also lead to semantically difficult programs, which may be difficult to maintain and debug, as complexity of the problem domain increases.