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From: "Joshua Cranmer" <joshua.cranmer@1:261/38.remove-yy0-this>
Subject: Re: multiple inheritance
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Date: Fri, 03 Aug 2012 18:54:12 GMT
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  To: bob smith
From: "Joshua Cranmer" <joshua.cranmer@1:261/38.remove-s5y-this>

  To: bob smith
From: Joshua Cranmer <Pidgeot18@verizon.invalid>

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.

--
Beware of bugs in the above code; I have only proved it correct, not tried it.
-- Donald E. Knuth

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