multiple inheritance

[bob smith]

From: "bob smith" <bob.smith@1:261/38.remove-yy0-this>

From: "bob smith" <bob.smith@1:261/38.remove-s5y-this>

From: bob smith <[email protected]>

Why doesn't Java support multiple inheritance?

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[markspace]

To: bob smith
From: "markspace" <markspace@1:261/38.remove-yy0-this>

To: bob smith
From: "markspace" <markspace@1:261/38.remove-s5y-this>

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

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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[Eric Sosman]

To: bob smith
From: "Eric Sosman" <eric.sosman@1:261/38.remove-yy0-this>

To: bob smith
From: "Eric Sosman" <eric.sosman@1:261/38.remove-s5y-this>

To: bob smith
From: Eric Sosman <[email protected]>

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
(e-mail address removed)

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[Joshua Cranmer]

To: bob smith
From: "Joshua Cranmer" <joshua.cranmer@1:261/38.remove-yy0-this>

To: bob smith
From: "Joshua Cranmer" <joshua.cranmer@1:261/38.remove-s5y-this>

To: bob smith
From: Joshua Cranmer <[email protected]>

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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[Gene Wirchenko]

To: markspace
From: "Gene Wirchenko" <gene.wirchenko@1:261/38.remove-yy0-this>

To: markspace
From: "Gene Wirchenko" <gene.wirchenko@1:261/38.remove-s5y-this>

To: markspace
From: Gene Wirchenko <[email protected]>

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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[Lew]

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

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

To: Joshua Cranmer
From: Lew <[email protected]>


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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