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abstract with copy inheritance [message #574509]

Mon, 30 August 2010 10:08

Andreas Mertgen

Messages: 4
Registered: August 2010

Junior Member

Suppose we have a simple base class B with a method bar.

Let us look at the following team T0:

public abstract team class T0 {

protected abstract class R {
protected abstract void foo();
}

protected abstract class NR extends R playedBy B {
foo <- before bar;
}

}

Due to the abstract method foo, both roles and therefore the team have to be abstract.

Now let us have a look at a subclassing team T1:

public team class T1 extends T0 {

protected class R {
protected void foo(){...};
}

protected class NR {};

}

Here of course, we have to add the missing implementation of foo to make the role R non-abstract. However, role NR must be "overriden" as well to get rid of the abstract property.

I understand, that the mechanism has to handle the "abstract"-issue explicitly. But the resulting implementation with an empty role class seems to be a little odd. Is there a better way to deal with implicit inheritance like in this situation?

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Re: abstract with copy inheritance [message #984720 is a reply to message #574509]

Wed, 14 November 2012 23:05

Stephan Herrmann

Messages: 1853
Registered: July 2009

Senior Member

Ups, nobody ever answered this one?

I'm intrigued by the example, tempted to say we could simply fix the compiler.

But then I wonder: T1.NR has an implementation for foo, OK, but it could still be abstract even if it contains no abstract methods.
In Java, marking a class as abstract is more than acknowledging that it has one or more abstract methods, it could simply be a way to specify: "not intended for instantiation".

In this light the "empty" role is an relevant override (and should be marked with @Override) re-declaring the role from abstract to non-abstract.

Let's briefly try the other way around: assume that without overriding the implicit role T1.NR would be non-abstract: how would I achieve it to be abstract (to prohibit instantiation)? I would need to "override" the abstract role with an empty abstract role. This might be a contrived example but this solution certainly feels even more weird.

From this I conclude it is indeed better, to not implicitly "improve" the role from abstract to not-abstract. Even if this is likely what you want, if it ever happens that you don't want this "improvement", you'll be much bewildered by this unsolicited magic.

does this make sense?
Stephan

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