Re: Class ... implements ...

From:

Lew <noone@lewscanon.com>

Newsgroups:

comp.lang.java.help

Date:

Sat, 02 Apr 2011 17:30:17 -0400

Message-ID:

<in84h3$fa3$1@news.albasani.net>

Merciadri Luca wrote:

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):

Sure, it is an awful lot easier to do your homework once someone has given you
the answers. We know.

But it seems rude that you ask for answers to your homework and haven't even
tried to do the research yourself first. Maybe somewhat more actionable than
rude, even.

Perhaps if you tell us first what you are thinking or trying, it will help
counteract that unfortunate impression that you are trying to, ahem, cut corners.

1. C1 obj_1 = new C2();
2. C2 obj_2 = new C1();
3. I1 obj_3 = new C2();
4. I1 obj_4 = new I1();
5. C2 obj_5 = new I1();

I've found that 1. and 3. are valid, when the others (2., 4. and 5.)
are invalid. Am I right?

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.

--
Lew