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From: "Lew" <lew@1:261/38.remove-s5y-this>
Subject: Re: multiple inheritance
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Date: Thu, 02 Aug 2012 19:12:01 GMT
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  To: Joshua Cranmer
From: Lew <lewbloch@gmail.com>

Joshua Cranmer wrote:
> bob smith wrote:
>> Why doesn't Java support multiple inheritance?

Strictly speaking, Java does support multiple inheritance, just not from 
classes.

This is because multiple inheritance of implementation is silly.

> 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.

Quite so.

--
Lew

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