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A a = new B();

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  A a = new B(); Davej <galt_57@hotmail.com> - 2012-06-17 06:44 -0700
    Re: A a = new B(); markspace <-@.> - 2012-06-17 07:55 -0700
      Re: A a = new B(); Davej <galt_57@hotmail.com> - 2012-06-17 09:23 -0700
        Re: A a = new B(); David Lamb <dalamb@cs.queensu.ca> - 2012-06-17 12:56 -0400
        Re: A a = new B(); markspace <-@.> - 2012-06-17 10:02 -0700
    Re: A a = new B(); rossum <rossum48@coldmail.com> - 2012-06-17 18:00 +0100
      Re: A a = new B(); markspace <-@.> - 2012-06-17 11:03 -0700
    Re: A a = new B(); Roedy Green <see_website@mindprod.com.invalid> - 2012-06-17 14:02 -0700
      Re: A a = new B(); Joerg Meier <joergmmeier@arcor.de> - 2012-06-19 02:31 +0200
        Re: A a = new B(); Lew <noone@lewscanon.com> - 2012-06-18 22:12 -0700
        Re: A a = new B(); Roedy Green <see_website@mindprod.com.invalid> - 2012-06-19 04:57 -0700
          Re: A a = new B(); Joerg Meier <joergmmeier@arcor.de> - 2012-06-19 14:49 +0200
            Re: A a = new B(); Gene Wirchenko <genew@ocis.net> - 2012-06-19 09:18 -0700

#1876 — A a = new B();

Can this be valid? If so what is this called? Thanks.

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#1877

On 6/17/2012 6:44 AM, Davej wrote:
> Can this be valid? If so what is this called? Thanks.
>


I'm pretty sure this is a "do your own homework" sort of deal.

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#1878

On Jun 17, 9:55 am, markspace <-@.> wrote:
> On 6/17/2012 6:44 AM, Davej wrote:
>
> > Can this be valid? If so what is this called? Thanks.
>
> I'm pretty sure this is a "do your own homework" sort of deal.

It's not my homework, but I've never seen this before, so I don't even
know what to look up.

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#1879

On 17/06/2012 12:23 PM, Davej wrote:
> On Jun 17, 9:55 am, markspace<-@.>  wrote:
>> On 6/17/2012 6:44 AM, Davej wrote:
>>
>>> Can this be valid? If so what is this called? Thanks.
>>
>> I'm pretty sure this is a "do your own homework" sort of deal.
>
> It's not my homework, but I've never seen this before, so I don't even
> know what to look up.

Look up "inheritance". Sorry to be cryptic, but markspace is right that 
this does look like a lot of "help me cheat on my homework" questions 
we've seen over the years.

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#1881

On 6/17/2012 9:23 AM, Davej wrote:
> On Jun 17, 9:55 am, markspace <-@.> wrote:
>> On 6/17/2012 6:44 AM, Davej wrote:
>>
>>> Can this be valid? If so what is this called? Thanks.
>>
>> I'm pretty sure this is a "do your own homework" sort of deal.
>
> It's not my homework, but I've never seen this before, so I don't even
> know what to look up.
>


Yeah, I don't mean to be rude, but it really does look like a student 
question.  One you should be asking your instructor.  If you've never 
seen it before, where do you see it now?

I'd like some context before I give you any further pointers.  If 
there's a blog or some source code online, a link would be preferable. 
Also, this question is hugely remedial.  What is your experience with 
programming?

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#1880

On Sun, 17 Jun 2012 06:44:50 -0700 (PDT), Davej <galt_57@hotmail.com>
wrote:

>Can this be valid? If so what is this called? Thanks.
What is the relationship between class A and class B?

rossum

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#1882

On 6/17/2012 10:00 AM, rossum wrote:
> On Sun, 17 Jun 2012 06:44:50 -0700 (PDT), Davej <galt_57@hotmail.com>
> wrote:
>
>> Can this be valid? If so what is this called? Thanks.
> What is the relationship between class A and class B?


Don't forget that the first part of his question is "is this valid?" 
What if it isn't valid?  What if the code snipette doesn't compile?  Is 
there a case where it would not be correct?

The fact the the question and the example are *so* well chosen to sit on 
a knife edge of various presumptions makes the question look like 
homework.  It's too smart to be a dumb question.

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#1883

On Sun, 17 Jun 2012 06:44:50 -0700 (PDT), Davej <galt_57@hotmail.com>
wrote, quoted or indirectly quoted someone who said :

>Can this be valid? If so what is this called? Thanks.

what are A and B.  A might be an interface and B a class.  A might be
a superclass or subclass of B.

Newbies tend to write code of the form

A a = new A();

Old hands tend to write code of the form

SomeInterface a = new ConcreteClass();

-- 
Roedy Green Canadian Mind Products
http://mindprod.com
If you look in a computer programmer's freezer you will find all 
kinds of containers, but none of them labeled.  They do the same thing 
creating files without labeling the encoding.  You are just supposed to 
know.  Ditto with the MIME type, the separator and comment delimiters and 
column names in CSV files.  Ditto with the endian convention.  Imagine how 
much more civilised life would have been if Martha Stewart were the first 
programmer. 

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#1885

On Sun, 17 Jun 2012 14:02:03 -0700, Roedy Green wrote:

> On Sun, 17 Jun 2012 06:44:50 -0700 (PDT), Davej <galt_57@hotmail.com>
> wrote, quoted or indirectly quoted someone who said :
>>Can this be valid? If so what is this called? Thanks.
> what are A and B.  A might be an interface and B a class.  A might be
> a superclass or subclass of B.

I don't see how A could be a subclass of B. What would happen if you called
a.x(); where x() is a method only subclass A has ?

String a = new Object(); // <- This will not compile, even though String is
a subclass of Object

Liebe Gruesse,
		Joerg

-- 
Ich lese meine Emails nicht, replies to Email bleiben also leider
ungelesen.

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#1886

Joerg Meier wrote:
> Roedy Green wrote:
>> Davej wrote, quoted or indirectly quoted someone who said :
>>> Can this be valid? If so what is this called? Thanks.
>> what are A and B.  A might be an interface and B a class.  A might be
>> a superclass or subclass of B.
>
> I don't see how A could be a subclass of B. What would happen if you called
> a.x(); where x() is a method only subclass A has ?
>
> String a = new Object(); //<- This will not compile, even though String is
> a subclass of Object

That is an excellent question and observation.

As the OP originally posted it, switch for this example to:

  import java.util.ArrayList;
  import javax.management.relation.RoleList;

Downcasting from the general type (ArrayList) to the specific (RoleList):

  RoleList specifics = new ArrayList<Object>();

the code won't compile for the reasons you state - the compiler cannot know 
that the new instance will fit into the subtype reference. You overcome the 
compiler's objections with a cast:

  RoleList specifics = (RoleList) new ArrayList<Object>();

This will get you past the compiler but blow up at runtime.

Exception in thread "main" java.lang.ClassCastException: java.util.ArrayList 
cannot be cast to javax.management.relation.RoleList
	at eegee.Casterol.run(Casterol.java:25)
	at eegee.Casterol.main(Casterol.java:35)

Upcasting always works, of course, without special ops.

  ArrayList<Object> generals = new RoleList();
  List<Object> potus = new RoleList();

In fact, upcasting like that is rather the point.

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

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#1888

On Tue, 19 Jun 2012 02:31:09 +0200, Joerg Meier <joergmmeier@arcor.de>
wrote, quoted or indirectly quoted someone who said :

>>>Can this be valid? If so what is this called? Thanks.
>> what are A and B.  A might be an interface and B a class.  A might be
>> a superclass or subclass of B.
>
>I don't see how A could be a subclass of B. What would happen if you called
>a.x(); where x() is a method only subclass A has ?
>
>String a = new Object(); // <- This will not compile, even though String is
>a subclass of Object

I was enumerating the possibilities you might consider.  I was not
claiming they were all valid syntax.  I did not want to give the
answer away too easily.
-- 
Roedy Green Canadian Mind Products
http://mindprod.com
If you look in a computer programmer's freezer you will find all 
kinds of containers, but none of them labeled.  They do the same thing 
creating files without labeling the encoding.  You are just supposed to 
know.  Ditto with the MIME type, the separator and comment delimiters and 
column names in CSV files.  Ditto with the endian convention.  Imagine how 
much more civilised life would have been if Martha Stewart were the first 
programmer. 

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#1889

On Tue, 19 Jun 2012 04:57:50 -0700, Roedy Green wrote:

> On Tue, 19 Jun 2012 02:31:09 +0200, Joerg Meier <joergmmeier@arcor.de>
> wrote, quoted or indirectly quoted someone who said :
> 
>>>>Can this be valid? If so what is this called? Thanks.
>>> what are A and B.  A might be an interface and B a class.  A might be
>>> a superclass or subclass of B.
>>I don't see how A could be a subclass of B. What would happen if you called
>>a.x(); where x() is a method only subclass A has ?

>>String a = new Object(); // <- This will not compile, even though String is
>>a subclass of Object
> I was enumerating the possibilities you might consider.  I was not
> claiming they were all valid syntax.  I did not want to give the
> answer away too easily.

Why would you state something you know is impossible as a possibility ?
That would only serve to confused newbies. And if you were to enumerating
the possibilities whether legit or not, why not list A and B both being
interfaces ? Why not A being a class and B being and interface ?

Liebe Gruesse,
		Joerg

-- 
Ich lese meine Emails nicht, replies to Email bleiben also leider
ungelesen.

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#1890

On Tue, 19 Jun 2012 14:49:31 +0200, Joerg Meier <joergmmeier@arcor.de>
wrote:

[snip]

>Why would you state something you know is impossible as a possibility ?
>That would only serve to confused newbies. And if you were to enumerating

     The original question has the smell of being a homework problem.
Helping with homework: maybe; doing it for him: no.

[snip]

Sincerely,

Gene Wirchenko

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