Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.java.programmer > #16952

Re: multiple inheritance

Show all headers | View raw

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

Back to comp.lang.java.programmer | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

multiple inheritance bob smith <bob@coolfone.comze.com> - 2012-08-01 19:28 -0700
  Re: multiple inheritance markspace <-@.> - 2012-08-01 20:07 -0700
    Re: multiple inheritance Gene Wirchenko <genew@ocis.net> - 2012-08-01 21:07 -0700
  Re: multiple inheritance Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-08-01 23:12 -0400
  Re: multiple inheritance Joshua Cranmer <Pidgeot18@verizon.invalid> - 2012-08-01 23:41 -0400
    Re: multiple inheritance Lew <lewbloch@gmail.com> - 2012-08-02 01:05 -0700
    Re: multiple inheritance Arne Vajhøj <arne@vajhoej.dk> - 2012-08-12 23:15 -0400
  Re: multiple inheritance Roedy Green <see_website@mindprod.com.invalid> - 2012-08-02 01:10 -0700
    Re: multiple inheritance Arne Vajhøj <arne@vajhoej.dk> - 2012-08-12 23:08 -0400
      Re: multiple inheritance Lew <noone@lewscanon.com> - 2012-08-12 20:19 -0700
        Re: multiple inheritance Arne Vajhøj <arne@vajhoej.dk> - 2012-08-17 22:33 -0400
      Re: multiple inheritance Leif Roar Moldskred <leifm@dimnakorr.com> - 2012-08-13 05:55 -0500
        Re: multiple inheritance markspace <-@.> - 2012-08-13 07:58 -0700
        Re: multiple inheritance Arne Vajhøj <arne@vajhoej.dk> - 2012-08-17 22:38 -0400
  Re: multiple inheritance Jan Burse <janburse@fastmail.fm> - 2012-08-02 10:12 +0200

csiph-web