Re: multiple inheritance in Java

From:

=?ISO-8859-1?Q?Arne_Vajh=F8j?= <arne@vajhoej.dk>

Newsgroups:

comp.lang.java.programmer

Date:

Thu, 04 Jul 2013 17:33:52 -0400

Message-ID:

<51d5ea42$0$302$14726298@news.sunsite.dk>

On 7/2/2013 8:13 AM, Eric Sosman wrote:

On 7/2/2013 6:02 AM, Stefan Ram wrote:

Eric Sosman <esosman@comcast-dot-net.invalid> writes:

Still, I don't see the relevance of multiple inheritance to
your question, which seems to boil down to: "What are the trade-offs
in exposing fields vs. using accessors?" I think we can debate (have
debated) that matter at length without involving M.I. -- so it seems
I'm still missing your intent.

   When an object O exposes two public fields of two classes P and Q,
   this has some of the benefits that mutiple inheritance from both
   P and Q would have, while one does not tediously has to code
   a lot of delegating methods in this object O.

   (When I really implement this, P and Q actually are interfaces,
   but the during the creation of O an implementation of those
   interfaces is assigned to their corresponding fields in O.)

I still don't see the relevance of M.I. to the question.
Thought experiment: How would anything change if you were to
remove Q from the scenario altogether?

This might also be called ?poor man's multiple inheritance?.

I'm not sure why anyone would use that term for "making a
field public."

I understand his ?poor man's multiple inheritance?.

Instead of:

public class A {
     public void ma() { ... }
}

public class B {
     public void mb() { ... }
}

public class C extends A, B {
}

C o = new C();
o.ma();
o.mb();

which is not valid in Java he wants:

public class A {
     public void a() { ... }
}

public class B {
     public void b() { ... }
}

public class C {
     public final A a;
     public final B b;
     public C() {
         a = new A();
         b = new B();
     }
}

C o = new C();
o.a.ma();
o.b.mb();

But it still has nothing to do with public field vs accessor
as the above also be done as (and IMHO should be done as):

public class A {
     public void a() { ... }
}

public class B {
     public void b() { ... }
}

public class C {
     private A a;
     private B b;
     public C() {
         a = new A();
         b = new B();
     }
     public A getA() {
         return a;
     }
     public B getB() {
         return b;
     }
}

C o = new C();
o.getA().ma();
o.getB().mb();

Arne