Re: Class ... implements ...

From	Lew <noone@lewscanon.com>
Newsgroups	comp.lang.java.help
Subject	Re: Class ... implements ...
Date	2011-04-06 12:08 -0400
Organization	albasani.net
Message-ID	<ini35r$30t$3@news.albasani.net> (permalink)
References	<87vcywpg2h.fsf@merciadriluca-station.MERCIADRILUCA> <in9q0h$12p$1@dont-email.me> <87zko7e4he.fsf@merciadriluca-station.MERCIADRILUCA>

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On 04/03/2011 11:31 AM, Merciadri Luca wrote:
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> Eric Sosman<esosman@ieee-dot-org.invalid>  writes:
>
>> On 4/2/2011 4:13 PM, Merciadri Luca wrote:
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>>> Hi,
>>>
>>> I'm given the following program snippet:
>>>
>>> ==
>>> class C1
>>> {
>>>    public int n;
>>> }
>>>
>>> interface I1
>>> {
>>>    int f();
>>> }
>>>
>>> class C2 extends C1 implements I1
>>> {
>>>    public int f()
>>>    {
>>>     n = 18;
>>>    }
>>> }
>>> ==
>>> I need to say if the following expressions are valid or not
>>> (justifications are trivial once I'm sure of the answer):
>>>
>>> 1. C1 obj_1 = new C2();
>>
>>      Invalid.
>>
>>> 2. C2 obj_2 = new C1();
>>
>>      Invalid.
>>
>>> 3. I1 obj_3 = new C2();
>>
>>      Invalid.
>>
>>> 4. I1 obj_4 = new I1();
>>
>>      Invalid.
>>
>>> 5. C2 obj_5 = new I1();
>>
>>      Invalid.
>> I predict you will get compile-time
>> errors for all five of them.
>
> Okay, I just tested and received the results I wrote before.
>
> ==
> $ javac Test.java
> Test.java:25: incompatible types
> found   : C1
> required: C2
> C2 obj_2 = new C1();
>             ^
> Test.java:27: I1 is abstract; cannot be instantiated
> I1 obj_4 = new I1();
>             ^
> Test.java:28: I1 is abstract; cannot be instantiated
> C2 obj_5 = new I1();
>             ^
> 3 errors
> ==
> so that the three operations I mentioned as invalid are effectively
> (at least to my little knowledge) invalid.
>
> Do you have an explanation about your way of reasoning? My compiler
> might be too old, or something like that.

This question was answered in your first answer:
> Which expressions put a subtype instance into a supertype variable?
>
> Which expressions put a supertype instance into a subtype variable?
>
> How can a subtype variable be sure that a supertype instance is of the correct type?
>
> Are you allowed in Java to instantiate an interface?
>
> Please answer as best you can.  You should use the Java tutorials to help you; don't just randomly guess.  We will be expecting your responses.

Did you even engage with those hints?  If not, why expect more?

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

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Thread

Class ... implements ... Merciadri Luca <Luca.Merciadri@student.ulg.ac.be> - 2011-04-02 22:13 +0200
  Re: Class ... implements ... Lew <noone@lewscanon.com> - 2011-04-02 17:30 -0400
    Re: Class ... implements ... Roedy Green <see_website@mindprod.com.invalid> - 2011-04-02 23:09 -0700
    Re: Class ... implements ... Merciadri Luca <Luca.Merciadri@student.ulg.ac.be> - 2011-04-03 12:00 +0200
  Re: Class ... implements ... Roedy Green <see_website@mindprod.com.invalid> - 2011-04-02 20:53 -0700
    Re: Class ... implements ... Stanimir Stamenkov <s7an10@netscape.net> - 2011-04-03 10:15 +0300
  Re: Class ... implements ... Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-04-03 08:42 -0400
    Re: Class ... implements ... Roedy Green <see_website@mindprod.com.invalid> - 2011-04-03 06:45 -0700
      Re: Class ... implements ... Merciadri Luca <Luca.Merciadri@student.ulg.ac.be> - 2011-04-03 17:29 +0200
        Re: Class ... implements ... Patricia Shanahan <pats@acm.org> - 2011-04-03 09:01 -0700
          Re: Class ... implements ... Merciadri Luca <Luca.Merciadri@student.ulg.ac.be> - 2011-04-03 18:10 +0200
            Re: Class ... implements ... Patricia Shanahan <pats@acm.org> - 2011-04-03 09:32 -0700
              Re: Class ... implements ... Merciadri Luca <Luca.Merciadri@student.ulg.ac.be> - 2011-04-03 18:48 +0200
        Re: Class ... implements ... Aéris <aeris@imirhil.fr> - 2011-04-03 18:07 +0200
          Re: Class ... implements ... Merciadri Luca <Luca.Merciadri@student.ulg.ac.be> - 2011-04-03 18:22 +0200
            Re: Class ... implements ... Aéris <aeris@imirhil.fr> - 2011-04-03 19:02 +0200
              Re: Class ... implements ... Merciadri Luca <Luca.Merciadri@student.ulg.ac.be> - 2011-04-03 20:15 +0200
      Re: Class ... implements ... Lew <noone@lewscanon.com> - 2011-04-06 12:01 -0400
    Re: Class ... implements ... Merciadri Luca <Luca.Merciadri@student.ulg.ac.be> - 2011-04-03 17:27 +0200
      Re: Class ... implements ... Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-04-03 14:26 -0400
      Re: Class ... implements ... Arved Sandstrom <asandstrom3minus1@eastlink.ca> - 2011-04-03 16:14 -0300
        Re: Class ... implements ... Merciadri Luca <Luca.Merciadri@student.ulg.ac.be> - 2011-04-03 21:33 +0200
          Re: Class ... implements ... Aéris <aeris@imirhil.fr> - 2011-04-03 22:13 +0200
            Re: Class ... implements ... Merciadri Luca <Luca.Merciadri@student.ulg.ac.be> - 2011-04-03 22:24 +0200
      Re: Class ... implements ... Lew <noone@lewscanon.com> - 2011-04-06 12:05 -0400
    Re: Class ... implements ... Merciadri Luca <Luca.Merciadri@student.ulg.ac.be> - 2011-04-03 17:31 +0200
      Re: Class ... implements ... Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-04-03 14:41 -0400
        Re: Class ... implements ... Merciadri Luca <Luca.Merciadri@student.ulg.ac.be> - 2011-04-03 21:45 +0200
          Re: Class ... implements ... Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-04-03 16:40 -0400
            Re: Class ... implements ... Merciadri Luca <Luca.Merciadri@student.ulg.ac.be> - 2011-04-03 22:58 +0200
              Re: Class ... implements ... Lew <noone@lewscanon.com> - 2011-04-06 12:13 -0400
          Re: Class ... implements ... Lew <noone@lewscanon.com> - 2011-04-06 12:10 -0400
        Re: Class ... implements ... Merciadri Luca <Luca.Merciadri@student.ulg.ac.be> - 2011-04-03 21:51 +0200
          Re: Class ... implements ... Patricia Shanahan <pats@acm.org> - 2011-04-03 13:02 -0700
            Re: Class ... implements ... Merciadri Luca <Luca.Merciadri@student.ulg.ac.be> - 2011-04-03 22:23 +0200
          Re: Class ... implements ... Lew <noone@lewscanon.com> - 2011-04-06 12:15 -0400
      Re: Class ... implements ... Lew <noone@lewscanon.com> - 2011-04-06 12:08 -0400

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