RE: Java - Help Understanding Advice From Learning Java Third Edition
- From: james.homme@xxxxxxxxxxxx
- To: programmingblind@xxxxxxxxxxxxx
- Date: Sun, 27 Jul 2008 08:04:52 -0400
Hi Ken,
Let me see if I am understanding a little more. It sounds like what you are
saying is that I'm already using composition without knowing it. For
example, If I were to take the first ever program in the book and modify it
so that it used any class as a variable, then I would be using composition.
That's because every Java program has to be a class--even the simplest one.
In your example, if I had a little program with a variable that prints out
someone's name, then I would be using composition because composition is
defined as using a class to put a variable in another class.
Is that somewhere in the ballpark?
Thanks.
Jim
James D Homme, Usability Engineering, Highmark Inc.,
james.homme@xxxxxxxxxxxx, 412-544-1810
"it is only possible to live happily ever after on a day-to-day basis." --
Margaret Bonnano
Highmark internal only: For usability and accessibility:
http://highwire.highmark.com/sites/iwov/hwt093/
"Ken Perry"
<whistler@blinkso
ft.com> To
Sent by: programmingblind@xxxxxxxxxxxxx
programmingblind- cc
bounce@freelists.
org Subject
RE: Java - Help Understanding
Advice From Learning Java Third
07/27/2008 12:18 Edition
AM
Please respond to
programmingblind@
freelists.org
Ok what I call this is a book written by someone who knows what they are
talking about but not exactly how to get it across to people just getting
started. I am not even sure I can do it justice but I will give it a shot
by using examples. Using objects by composition means your going to
include the object in your class like you would any other variable. So for
example in a class you might have an int to store integers. In the same
way you might have a TreeView to store items that you want shown on the
screen. If you have a variable in your class like a TreeView in your class
you are using it as a composed object or a member variable.
Now if you have a class that you want to be a more advanced TreeView you
will inherrit a TreeView and add to it in your class. So the Treeview is
not a member variable instead it’s a parent of your AdvancedTreeView.
What this guy is trying to say is it is easier to use a persons class as a
composed object when you can because you don't have to add
functionality to the base class you just use what is there. Also if for
example someone upgrades the Treview class and you used it as a composed
object it should not break your code by much and should be easily fixable.
Now if they update a base class and you had a lot tied into into the parent
you might run into trouble for example
Lets say you used a TreeView class as a composed object. And did something
like
TreeView.AddItem("frog");
Most likely the creator of the treeview when changing the class will leave
the AddItem function the same but now he has decided to store all items in
his tree view in a hash instead of the list that he used first. I know he
would be stupid to do either just use it as an example. So he changes the
base member variable from List MyList. To Hash MyHash.
If you had inherited the TreeView to make a better TreeView and had used
the parents MyList variable in hundreds of things you were doing you would
have to go through and fix it. While if you composed it and only used the
methods like AddItem that the creator intended you to use you would not
even notice he or she had made a change.
Anyway I am sure I am rambling now so if I did no better then this guy I am
sorry and let me know I didn't and maybe I will come up with a better
example. If you think code does better then I could write you an example
of a class that uses composed objects all though I am betting you used
composed objects all the time for example in Java the String class is a
object that you use all the time as a composed object for example this is a
composed string
String myString="tada";
If on the other hand you needed to have a special function in the string
class you would have to inherit the string class and add a method to it to
lets see hmm a scramble all the letters. Then once you did that everyone
could use your class just like a String class but with the added
functionality of scramble.
Anyway I will stop now if you still don't have it let me know and I will
try again. I will say though this is where I banged my head back in 1992
when I decided to learn c++ over c. After you get over things like
Inheritance and virtual functions you will be in the clear.
Ken
-----Original Message-----
From: programmingblind-bounce@xxxxxxxxxxxxx
[mailto:programmingblind-bounce@xxxxxxxxxxxxx] On Behalf Of
james.homme@xxxxxxxxxxxx
Sent: Saturday, July 26, 2008 6:16 AM
To: programmingblind@xxxxxxxxxxxxx
Subject: Java - Help Understanding Advice From Learning Java Third Edition
Hi,
The following piece of text comes from the first chapter of this book,
which starts to talk about classes. I feel that it's important for me to
grasp what it's talking about, but I don't understand what it's trying to
tell me. Can anyone say it a different way and possibly help me understand
what it's driving at?
Specialize objects only when you have to—use composition instead of
inheritance . When you use an object in its existing form, as a piece of a
new object, you are composing objects. When you change or refine the
behavior of an object (by subclassing ), you are using inheritance . You
should try to reuse objects by composition rather than inheritance whenever
possible because when you compose objects, you are taking full advantage of
existing tools.
Inheritance
involves breaking down the barrier of an object and should be done only
when there's a real advantage. Ask yourself if you really need to inherit
the whole public interface of an object (do you want to be a "kind" of that
object) or whether you can just delegate certain jobs to the object and use
it by composition.
I feel almost like they are saying that you should import the code from the
objects you want to use into your program and make entirely new classes
that use the imported ones. Am I on the right track?
Thanks.
Jim
James D Homme, Usability Engineering, Highmark Inc.,
james.homme@xxxxxxxxxxxx, 412-544-1810
"it is only possible to live happily ever after on a day-to-day basis." --
Margaret Bonnano
Highmark internal only: For usability and accessibility:
http://highwire.highmark.com/sites/iwov/hwt093/Vjʱjín
__________
View the list's information and change your settings at
//www.freelists.org/list/programmingblind
Other related posts:
