[E-Lang] Re: inheritance, delegation, mental models for begin ners (was: down with `define' (was: newbie syntax: picayune points from a prejudiced programmer))

[Mark S. Miller]

At 05:20 PM Friday 3/2/01, Karp, Alan wrote:
>MarkM wrote:
>>
>> Btw, many of us, when doing class-based programming with inheritance,
>mostly 
>> stopped using concrete non-final classes since the Udanax Gold days.  When
>> we found we wanted to subclass a concrete class, we refactored so that we 
>> were subclassing from an abstract class.  [...]
>
>I must have been doing something wrong, because I tried to do this (at least
>I think I did), and I didn't like the result.  Say I have a class C with 2
>methods, M1 and M2.  One subclass C1 overrides M1, another C2 overrides M2.
>If C is a concrete class, then C1 gets C's M2, and C2 gets C's M1.  If C is
>abstract in that it has no implementation for M1 and M2, where do these
>implementations go?  I ended up with an extra abstract class between C and
>each of C1 and C2.  Of course, it's not terrible with only 2 methods; my
>case had 5.

We're having a terminology problem.  By "abstract class" I mean what Java 
means: A class that cannot be directly instantiated.  Only subclasses of it 
can be instantiated.  You seem to mean a class that has only abstract 
methods, or approximately what Java calls an interface.  Note that a Java 
abstract class need not have any abstract methods.  Merely by declaring the 
class abstract, you prevent it from being directly instantiated.

So regarding your example in Java, we refactor by leaving C alone but for 
declaring it abstract.  We might then supply yet another concrete final 
subclass C0 that only has a constructor and inherits all its instance 
behavior from C.  Why? So far we've only added noise and no signal.  Because 
we found the resulting code structure was more evolvable.  C is for 
subclassing.  Period.  C0, C1, and C2 are for instantiating.  Period.  No one 
class is maintained under the requirement to do both jobs.


        Cheers,
        --MarkM