From: Chauk-Mean P <chauk.mean@...>
Date: 2007-04-04T03:08:16+09:00
Subject: Re: New presentation on Ruby

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2007/4/3, Ilan Berci <coder68@yahoo.com>:

>>
>> As stated in slide 15 :
>> - I just said that Java untyped collections or generic collections for
>> type
>> Object DO support heterogeneous types.
>> But the drawback is that you NEED to downcast a retrieved element.
>> - Conversely, if you use a generic collection for a type X (e.g.
>> TalkingAnimal) different than Object, you can put only objects of type
>> X.
>> You cannot put an object that is not of type X (e.g Rabbit2)
>> You avoid most downcasts for retrieved elements. Thus you can retrieve a
>> TalkingAnimal and call talk.
>> But if you put an object of type Y that extends the capability of X
>> (e.gDuck2) and you want to call a method of Y (
>> e.g. walk), then you need to downcast to Y.
>
>Chuck, please update your java from 1.4 to 1.5JDK (5.0) and check out
>generics, there is no longer a NEED(why the upercase?)  to downcast from
>containers as it's done for you.

Ilan,

First, my first name is Chauk-Mean not Chuck :-)

Regarding Java 1.5, I think that I'm up to date.
Indeed, the second example from the slide 15 uses generics (please take a
second look at it) :

Map<String, TalkingAnimal> menagerie = new Map<String, TalkingAnimal>
...

Yes, a talking animal can talk directly and there is no need to downcast

TalkingAnimal duckObject = menagerie.get("my duck");
duckObject.talk

But if you want your object (which happens to be a duck ) to walk, you have
to downcast it to Duck2

Duck2 duck = (Duck2)duckObject;
duck.walk;

As I said, with generics you avoid most downcasts but there are cases like
this one which require downcasts.


> >I also fully realize that I didn't hear the talk that went with the
> >slides but you asked us for feedback on the slides knowing that we
> >didn't hear the presentation.  Be carefull what you wish for especially
> >on this list..


You're right. It seems that I have to make an annotated version with some
additional comments.

>As for your example on duck typing, I can illustrate the same thing in
>your slides by doing an exact port in Java which is known not to support
>duck typing (message passing)

>class Duck
> >{  public String quack() {return "quack";} }
>
> >class Bird
> >{  public String quack() {return "chirp";}}
>
> >d = new Duck(); d.quack();
> >b = new Bird(); b.quack();  //  does this mean java supports duck typing
> too?? It does if we go by your slide


Hey, you have to declare the type in Java.
And of course, it will work if you declare the type like :
Duck d = new Duck();
d.quack();

It's a detail that is important.

It's the same for C++.

Chauk-Mean.

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