RE: Java - Help Understanding Advice From Learning Java Third Edition
- From: "Ken Perry" <whistler@xxxxxxxxxxxxx>
- To: <programmingblind@xxxxxxxxxxxxx>
- Date: Sat, 26 Jul 2008 21:18:55 -0700
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
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