I know that multiple inheritance is not allowed in Java and C#. Many books just say, multiple inheritance is not allowed. But it can be implemented by using interfaces. Nothing is discussed about why it is not allowed. Can anybody tell me precisely why it is not allowed?
Source: Tips4all
The short answer is: because the language designers decided not to.
ReplyDeleteBasically, it seemed that both the .NET and Java designers did not allow multiple inheritance because they reasoned that adding MI added too much complexity to the languages while providing too little benefit.
For a more fun and in-depth read, there are some articles available on the web with interviews of some of the language designers. For example, for .NET, Chris Brumme (who worked at MS on the CLR) has explained the reasons why they decided not to:
Different languages actually have different expectations for how MI
works. For example, how conflicts are
resolved and whether duplicate bases
are merged or redundant. Before we can
implement MI in the CLR, we have to do
a survey of all the languages, figure
out the common concepts, and decide
how to express them in a
language-neutral manner. We would also
have to decide whether MI belongs in
the CLS and what this would mean for
languages that don't want this concept
(presumably VB.NET, for example). Of
course, that's the business we are in
as a common language runtime, but we
haven't got around to doing it for MI
yet.
The number of places where MI is truly appropriate is actually quite
small. In many cases, multiple
interface inheritance can get the job
done instead. In other cases, you may
be able to use encapsulation and
delegation. If we were to add a
slightly different construct, like
mixins, would that actually be more
powerful?
Multiple implementation inheritance injects a lot of complexity into the
implementation. This complexity
impacts casting, layout, dispatch,
field access, serialization, identity
comparisons, verifiability,
reflection, generics, and probably
lots of other places.
You can read the full article here.
For Java, you can read this article:
The reasons for omitting multiple
inheritance from the Java language
mostly stem from the "simple, object
oriented, and familiar" goal. As a
simple language, Java's creators
wanted a language that most developers
could grasp without extensive
training. To that end, they worked to
make the language as similar to C++ as
possible (familiar) without carrying
over C++'s unnecessary complexity
(simple).
In the designers' opinion, multiple
inheritance causes more problems and
confusion than it solves. So they cut
multiple inheritance from the language
(just as they cut operator
overloading). The designers' extensive
C++ experience taught them that
multiple inheritance just wasn't worth
the headache.
Multiple inheritance of implementation is what is not allowed.
ReplyDeleteThe problem is that the compiler/runtime cannot figure out what to do if you have a Cowboy and an Artist class, both with implementations for the draw() method, and then you try to create a new CowboyArtist type. What happens when you call the draw() method? Is someone lying dead in the street, or do you have a lovely watercolor?
I believe it's called the double diamond inheritance problem.
The main (although by no means the only) reason people steer away from MI is the so called "diamond problem" leading to ambiguity in your imlpementation. This wikipedia article discusses it and explains better than I could. MI can also lead to more complex code, and a lot of OO designers claim that you do't need MI, and if you do use it your model is probably wrong. I'm not sure I agree with this last point, but keeping things simple is always a good plan.
ReplyDeleteIn C++ multiple inheritance was a major headache when used improperly. To avoid those popular design issues multiple interfaces "inheritance" was forced instead in modern languages (java, C#).
ReplyDeleteAnother reason is that single-inheritance makes casting trivial, emitting no assembler instructions (other than checking for the compatibility of the types where required). If you had multiple-inheritance, you'd need to figure out where in the child class a certain parent starts. So performance is certainly a perk (although not the only one).
ReplyDeleteBack in the old days ('70s) when Computer Science was more Science and less mass production the programmers had time to think about good design and good implementation and as a result the products (programms) had high quality ( eg. TCP/IP design and implementation ).
ReplyDeleteNowadays, when everybody is programming, and the managers are changing the specs before deadlines, subtle issues like the one descriped in the wikipedia link from Steve Haigh post are difficult to track; therefore, the "multiple inheritance" is limited by compiler design. If you like it, you can still use C++ .... and have all the freedom you want :)
Because Java has a greatly different design philosophy from C++. (I'm not going to discuss C# here.)
ReplyDeleteIn designing C++, Stroustrup wanted to include useful features, regardless of how they could be misused. It's possible to screw up big-time with multiple inheritance, operator overloading, templates, and various other features, but it's also possible to do some very good things with them.
The Java design philosophy is to emphasize safety in language constructs. The result is that there are things that are a lot more awkward to do, but you can be a lot more confident that the code you're looking at means what you think it does.
Further, Java was to a large extent a reaction from C++ and Smalltalk, the best known OO languages. There are plenty of other OO languages (Common Lisp was actually the first one to be standardized), with different OO systems that handle MI better.
Not to mention that it's entirely possible to do MI in Java, using interfaces, composition, and delegation. It's more explicit than in C++, and therefore is clumsier to use but will get you something you're more likely to understand at first glance.
There is no right answer here. There are different answers, and which one is better for a given situation depends on applications and individual preference.
Multiple Inheritance is
ReplyDeletehard to understand
hard to debug (for example, if you mix classes from multiple frameworks that have identically-named methods deep down, quite unexpected synergies can occur)
easy to mis-use
not really that useful
hard to implement, especially if you want it done correctly and efficiently
Therefore, it can be considered a wise choice to not include Multiple Inheritance into the Java language.
I take the statement that "Multiple inheritance is not allowed in Java" with a pinch of salt.
ReplyDeleteMultiple Inheritance is defined when a "Type" inherits from more than one "Types". And interfaces are also classified as types as they have behavior. So Java does have multiple inheritance. Just that it is safer.