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multiple inheritance

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  multiple inheritance "bob smith" <bob.smith@1:261/38.remove-s5y-this> - 2012-08-02 19:11 +0000
    Re: multiple inheritance "markspace" <markspace@1:261/38.remove-s5y-this> - 2012-08-02 19:11 +0000
      Re: multiple inheritance "Gene Wirchenko" <gene.wirchenko@1:261/38.remove-s5y-this> - 2012-08-02 19:12 +0000
    Re: multiple inheritance "Eric Sosman" <eric.sosman@1:261/38.remove-s5y-this> - 2012-08-02 19:11 +0000
    Re: multiple inheritance "Joshua Cranmer" <joshua.cranmer@1:261/38.remove-s5y-this> - 2012-08-02 19:12 +0000
      Re: multiple inheritance "Lew" <lew@1:261/38.remove-s5y-this> - 2012-08-02 19:12 +0000
      Re: multiple inheritance "Arne Vajhøj" <������
høj@1:261/38.remove-nlb-this> - 2012-08-13 18:36 +0000
    Re: multiple inheritance "Roedy Green" <roedy.green@1:261/38.remove-s5y-this> - 2012-08-02 19:12 +0000
      Re: multiple inheritance "Arne Vajhøj" <arne.vajhøj@1:261/38.remove-nlb-this> - 2012-08-13 18:36 +0000
        Re: multiple inheritance "Lew" <lew@1:261/38.remove-nlb-this> - 2012-08-13 18:36 +0000
          Re: multiple inheritance "Arne Vajhøj" <������
høj@1:261/38.remove-rj6-this> - 2012-08-18 18:36 +0000
        Re: multiple inheritance "Leif Roar Moldskred" <leif.roar.moldskred@1:261/38.remove-nlb-this> - 2012-08-13 18:36 +0000
          Re: multiple inheritance "markspace" <markspace@1:261/38.remove-nlb-this> - 2012-08-13 18:36 +0000
          Re: multiple inheritance "Arne Vajhøj" <������
høj@1:261/38.remove-rj6-this> - 2012-08-18 18:36 +0000
    Re: multiple inheritance "Jan Burse" <jan.burse@1:261/38.remove-s5y-this> - 2012-08-02 19:12 +0000

#16976 — multiple inheritance

From: bob smith <bob@coolfone.comze.com>

Why doesn't Java support multiple inheritance?

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#16977

  To: bob smith
From: markspace <-@.>

On 8/1/2012 7:28 PM, bob smith wrote:
> Why doesn't Java support multiple inheritance?
>

The diamond problem.  I'm not really up on the details however.

I can tell you from hanging out on the lambda-dev list (Java 8 features) that 
Brian Goetz has pushed back strongly on any sort of multiple inheritance.  
Apparently the diamond problem is a real bear and introduces real complexity 
into both the compiler and the user code that can cause big problems in the 
long run.

http://en.wikipedia.org/wiki/Diamond_problem

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#16981

  To: markspace
From: Gene Wirchenko <genew@ocis.net>

On Wed, 01 Aug 2012 20:07:58 -0700, markspace <-@.> wrote:

>On 8/1/2012 7:28 PM, bob smith wrote:
>> Why doesn't Java support multiple inheritance?

>The diamond problem.  I'm not really up on the details however.

     Such a lovely name.  The link does go into enough detail to
understand it.

>I can tell you from hanging out on the lambda-dev list (Java 8 features)
>that Brian Goetz has pushed back strongly on any sort of multiple
>inheritance.  Apparently the diamond problem is a real bear and
>introduces real complexity into both the compiler and the user code that
>can cause big problems in the long run.
>
>http://en.wikipedia.org/wiki/Diamond_problem

     It also has a number of different handlings in MI languages so
there is not an obvious solution.

Sincerely,

Gene Wirchenko

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#16978

  To: bob smith
From: Eric Sosman <esosman@ieee-dot-org.invalid>

On 8/1/2012 10:28 PM, bob smith wrote:
> Why doesn't Java support multiple inheritance?

     To discourage formation of a Kennefeller dynasty?

     Because diamonds are a girl's best friend but a programmer's
biggest headache?

     The web is full of pages discussing the pros and cons of
Java's choice.  Perhaps you should read a few of them and then (if so moved) 
post "Excuse E for omitting multiple inheritance seems unconvincing to me for 
reasons R1 and R2, despite supporting arguments S1 through S9. Here's a 
concrete example where I think R1 and R2 trump S* and overturn E; what do 
others think?"

--
Eric Sosman
esosman@ieee-dot-org.invalid

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#16980

  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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#16984

  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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#17822

  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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#16985

  To: bob smith
From: Roedy Green <see_website@mindprod.com.invalid>

On Wed, 1 Aug 2012 19:28:31 -0700 (PDT), bob smith
<bob@coolfone.comze.com> wrote, quoted or indirectly quoted someone
who said :

>Why doesn't Java support multiple inheritance?

1. It has interfaces which gives much of the ability at lighter weight..

2. Java originally was intended for set top boxes. That is too heavyweight a 
feature.

3. Study Eiffel. implementing it is quite tricky, especially when you get name  
clashes.

you might see it in Java 11 or so, or whatever language inherits the Java 
mantle.
.
--
Roedy Green Canadian Mind Products
http://mindprod.com
The greatest shortcoming of the human race is our inability to understand the 
exponential function.
 ~ Dr. Albert A. Bartlett (born: 1923-03-21 age: 89)
http://www.youtube.com/watch?v=F-QA2rkpBSY

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#17821

  To: Roedy Green
From: Arne Vajhoj <arne@vajhoej.dk>

On 8/2/2012 4:10 AM, Roedy Green wrote:
> On Wed, 1 Aug 2012 19:28:31 -0700 (PDT), bob smith
> <bob@coolfone.comze.com> wrote, quoted or indirectly quoted someone
> who said :
>
>> Why doesn't Java support multiple inheritance?
>
> 1. It has interfaces which gives much of the ability at lighter
> weight..

Only for a very limited type of types (those with no implementation at all).

> 2. Java originally was intended for set top boxes. That is too
> heavyweight a feature.

C++ is used a lot in embedded context, so that argument does not hold water.

> 3. Study Eiffel. implementing it is quite tricky, especially when you
> get name  clashes.
>
> you might see it in Java 11 or so, or whatever language inherits the
> Java mantle.

Not likely.

It is not desirable.

Arne

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#17823

  To: Arne Vajhøj
From: Lew <noone@lewscanon.com>

Arne Vajh-,j wrote:
> Roedy Green wrote:
>> 1. It has interfaces which gives much of the ability at lighter
>> weight..
>
> Only for a very limited type of types (those with no implementation
> at all).

That is true, but "very limited" might be misconstrued as "not very useful". 
That Java limits multiple inheritance to interfaces is a design decision of the 
language, and confers advantages. These advantages come to the fore when one 
follows various recommended practices such as those found in Joshua Bloch's 
/Effective Java/.

There are vanishingly few cases where one cannot mix in implementation through 
a combination of composition and single inheritance of implementation ('class' 
parent types) to accomplish with equal facility what multiple implementation 
inheritance would. Avoiding the sorts of downsides mentioned upthread is the 
motivation.

There are many times one wishes to guarantee the presence of a contractual 
method that is required by several interfaces. 'java.lang.Runnable' need not be 
the only interface to specify 'void run();'. Let's say you have a custom
'Raceable' interface that also specifies 'void run();'. There's every reason
to let an algorithm that expects a 'Raceable' to use some concrete type's
'run()' even if it also serves to keep 'Runnable''s promise. Multiple
inheritance of promises is easier to understand and keep bug free.

This ties into a programming approach I call "type-based programming". Given 
some concrete type

  public class FormulaOne implements Runnable, Raceable
  {
    @Override
    public void run() { ... }
  }

client code can freely say:

   FormulaOne fone = new FormulaOne();
   Raceable raceable = fone;
   Runnable runnable = fone;

and so forth. Only signatures are shared, so implementation won't be confused.

--
Lew
Honi soit qui mal y pense.
http://upload.wikimedia.org/wikipedia/commons/c/cf/Friz.jpg

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#18010

  To: Lew
From: =?UTF-8?B?QXJuZSBWYWpow7hq?= <arne@vajhoej.dk>

On 8/12/2012 11:19 PM, Lew wrote:
> Arne Vajh-,j wrote:
>> Roedy Green wrote:
>>> 1. It has interfaces which gives much of the ability at lighter
>>> weight..
>>
>> Only for a very limited type of types (those with no implementation
>> at all).
>
> That is true, but "very limited" might be misconstrued as "not very
> useful". That Java limits multiple inheritance to interfaces is a design
> decision of the language, and confers advantages.

Indeed.

Implementation inheritance is not in fashion in Java, so interfaces is the 
majority of cases.

A classic text:

http://www.artima.com/intv/gosling34.html


> There are vanishingly few cases where one cannot mix in implementation
> through a combination of composition and single inheritance of
> implementation ('class' parent types) to accomplish with equal facility
> what multiple implementation inheritance would. Avoiding the sorts of
> downsides mentioned upthread is the motivation.

There are some solutions to those problems. But Java was designed to be simple, 
so we got what we got.

Arne

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#17826

  To: Arne Vajhøj
From: Leif Roar Moldskred <leifm@dimnakorr.com>

Arne Vajh-,j <arne@vajhoej.dk> wrote:
> On 8/2/2012 4:10 AM, Roedy Green wrote:
>>
>> 1. It has interfaces which gives much of the ability at lighter
>> weight..
>
> Only for a very limited type of types (those with no implementation
> at all).

At times, I've wished that Java had automatic delegation (composition) in 
addition to inheritance and interface implementation, where the compiler 
automatically adds the public methods and fields from delegated objects, except 
when there's a conflict of names. Something like this, maybe:


public class ClassAlfa {
  public void alfaMethod( ) { ... }
  public void sharedMethod( ) { ... }
  public void anotherSharedMethod( ) { ... }
}

public class ClassBeta {
  public void betaMethod( ) { ... }
  public void sharedMethod( ) { ... }
  public void anotherSharedMethod( ) { ... }
}


public class Delegator {
  private delegate ClassAlfa ALFA;
  private delegate ClassBeta BETA;

  public void sharedMethod( ) {
    ALFA.sharedMethod( );
  }

  public void anotherSharedMethod( ) {
    ALFA.sharedMethod( );
    BETA.sharedMethod( );
  }
}

public class Example {
  public static void main( String[] args ) {
    private Delegator delegator;

    delegator.alfaMethod( );
    delegator.betaMethod( );
    delegator.sharedMethod( );
    delegator.anotherSharedMethod( );
  }
}

--
Leif Roar Moldskred

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#17827

  To: Leif Roar Moldskred
From: markspace <-@.>

On 8/13/2012 3:55 AM, Leif Roar Moldskred wrote:
> Arne Vajh-,j <arne@vajhoej.dk> wrote:
>> On 8/2/2012 4:10 AM, Roedy Green wrote:
>>>
>>> 1. It has interfaces which gives much of the ability at lighter
>>> weight..
>>
>> Only for a very limited type of types (those with no implementation
>> at all).
>
> At times, I've wished that Java had automatic delegation (composition)


"Automatic delegation" is what I want.  I've even invented my own little syntax 
for it:

public SomeClass extends Fubar implements List(myList), OtherThing {

   private AbstractList myList = new ArrayList();

...

}


where the parenthesis in the interface list is a forward declaration to a field 
that will be the delegate for that particular interface.  It really shouldn't 
be that hard to do, just a few synthetic methods.

I really rather upset that we aren't getting something like this with Java 8.  
I don't see any reason not to fix this right now, honestly.

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#18014

  To: Leif Roar Moldskred
From: =?UTF-8?B?QXJuZSBWYWpow7hq?= <arne@vajhoej.dk>

On 8/13/2012 6:55 AM, Leif Roar Moldskred wrote:
> Arne Vajh-,j <arne@vajhoej.dk> wrote:
>> On 8/2/2012 4:10 AM, Roedy Green wrote:
>>>
>>> 1. It has interfaces which gives much of the ability at lighter
>>> weight..
>>
>> Only for a very limited type of types (those with no implementation
>> at all).
>
> At times, I've wished that Java had automatic delegation (composition)
> in addition to inheritance and interface implementation, where the
> compiler automatically adds the public methods and fields from
> delegated objects, except when there's a conflict of names. Something
> like this, maybe:
>
>
> public class ClassAlfa {
>    public void alfaMethod( ) { ... }
>    public void sharedMethod( ) { ... }
>    public void anotherSharedMethod( ) { ... }
> }
>
> public class ClassBeta {
>    public void betaMethod( ) { ... }
>    public void sharedMethod( ) { ... }
>    public void anotherSharedMethod( ) { ... }
> }
>
>
> public class Delegator {
>    private delegate ClassAlfa ALFA;
>    private delegate ClassBeta BETA;
>
>    public void sharedMethod( ) {
>      ALFA.sharedMethod( );
>    }
>
>    public void anotherSharedMethod( ) {
>      ALFA.sharedMethod( );
>      BETA.sharedMethod( );
>    }
> }
>
> public class Example {
>    public static void main( String[] args ) {
>      private Delegator delegator;
>
>      delegator.alfaMethod( );
>      delegator.betaMethod( );
>      delegator.sharedMethod( );
>      delegator.anotherSharedMethod( );
>    }
> }

You IDE should be able to generate the code for you.

But it could save some code in some cases.

Arne

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#16986

  To: bob smith
From: Jan Burse <janburse@fastmail.fm>

bob smith schrieb:
> Why doesn't Java support multiple inheritance?
>

Java only doesn't allow multiple implementation inheritance. But it does allow 
multiple signature inheritance via interfaces.

So you can have where P and Q are interfaces:

     class A implements P;

     class B implements Q;

     class C implements P, Q;

If you want also implementation inheritance, you can use Scala, which does some 
rewriting to Java for you via delegates or other approaches.

Bye

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