Groups > comp.lang.java.programmer > #17056

Re: multiple inheritance

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From	"Lew" <lew@1:261/38.remove-yy0-this>
Subject	Re: multiple inheritance
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  To: Joshua Cranmer
From: "Lew" <lew@1:261/38.remove-s5y-this>

  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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Thread

multiple inheritance "bob smith" <bob.smith@1:261/38.remove-yy0-this> - 2012-08-03 18:54 +0000
  Re: multiple inheritance "markspace" <markspace@1:261/38.remove-yy0-this> - 2012-08-03 18:54 +0000
    Re: multiple inheritance "Gene Wirchenko" <gene.wirchenko@1:261/38.remove-yy0-this> - 2012-08-03 18:54 +0000
  Re: multiple inheritance "Eric Sosman" <eric.sosman@1:261/38.remove-yy0-this> - 2012-08-03 18:54 +0000
  Re: multiple inheritance "Joshua Cranmer" <joshua.cranmer@1:261/38.remove-yy0-this> - 2012-08-03 18:54 +0000
    Re: multiple inheritance "Lew" <lew@1:261/38.remove-yy0-this> - 2012-08-03 18:54 +0000
    Re: multiple inheritance "Arne Vajhøj" <������
høj@1:261/38.remove-odu-this> - 2012-08-13 19:38 +0000

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