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Subject: Re: multiple inheritance
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Date: Mon, 13 Aug 2012 18:36:20 GMT
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  To: Joshua Cranmer
From: =?UTF-8?B?QXJuZSBWYWpow7hq?= <arne@vajhoej.dk>

On 8/1/2012 11:41 PM, Joshua Cranmer wrote:
> On 8/1/2012 10:28 PM, bob smith wrote:
>> Why doesn't Java support multiple inheritance?
>
> Because multiple inheritance is really, really, really complicated and
> confusing for most users.
>
> The short answer is the diamond problem:
>
> class A { int varA; };
> class B : A { int varB; };
> class C : A { int varC; };
> class D : B, C { int varD; };
>
> There are two main points of contention in this kind of hierarchy:
> 1. How many copies of varA should D have? Intuitively, one is probably
> what most people would expect, but the implementations of B and C would
> have to cooperate in realizing that their superclass may be shared with
> D. It also incurs a penalty in runtime costs
> 2. How does initialization/override order get resolved? Is it "BFS"-y
> (like D, B, C, A) or "DFS"-y (D, B, A, C)? There are even more
> convoluted orders in practice (C3 appears to be the most common
> nowadays), but this is the sort of stuff that tends to cause nasty sorts
> of little edge cases in practice.
>
> It is rare in practice that you need true multiple inheritance, in the
> sense of inheritance of implementation; multiple inheritance of
> interface is common, and this is as far as Java goes.

It should be noted that Scala with its trait has come up with a solution that 
allows pulling in multiple traits with implementation code without the diamond 
problem.

Arne

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