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From inheritance to composition

Coming back to Ballworld, we see that the updateState method in ABall is an abstract algorithm to update the state of the ball. So, just as in the pizza example, we can represent this algorithm, and just this algorithm , as an object. We can say that a ball has an algorithm to update its state. Another wa of saying this is to say that the ball has a strategy to update its state. We can represent this by using composition. Instead of having an abstract method to update the state, we model a ball as having a reference to an IUpdateStrategy object. The code for update thus becomes public void update(Observable o, Object g) {_strategy.updateState(this); // update this ball's state using the strategy location.translate (velocity.x, velocity.y); // move the ballbounce(); // bounce the ball off the wall if necessary paint((Graphics) g); // paint the ball onto the container} The ball hands a reference to itself, this , to the strategy so that the strategy knows which ball to update. The variant updating behaviors are now represented by concrete implementations of the IUpdateStrategy interface.

Composition-based ballworld

Ball is now a concrete class and its subclasses have been eliminated in favor of being composed with an abstract strategy.
(Note that the Randomizer class has been redesigned to eliminate its static methods. One new method has been added as well.)

There are a number of very important points to notice about this new formulation:

  • The modified ABall class now contains 100% concrete code and thus should not be abstract anymore.
    • ABall has been renamed to simply Ball .
    • Accessor methods for the strategy ( getStrategy and setStrategy ) have been added.
    • The Ball class is still 100% invariant code.
  • The CurveBall , StraightBall , etc. subclasses are no longer needed as their variant behaviors have been moved to the IUpdateStrategy subclasses.
    • Effectively what has happened is that the updateState method has been moved from the ABall subclasses and embodied into their own classes.
    • The IUpdateStrategy subclasses do not inherit anything from Ball , hence they do not contain any invariant code.
    • The strategies are thus 100% variant code.
  • The reference to the ball during the updating process has gone from a peristent communication link (implicitly, this ), to a transient communication link ( host ).
  • This composition-based model divides the code exactly between the variant and invariant behaviors--this is the key to the power and flexibility it generates.

This new composition-based model of Ballworld is an example of the Strategy Design Pattern . The strategy design pattern allows us to isolate variant behaviors on a much finer level than simple inheritance models.

Composing behaviors

So far, all of our redesigning has resulted in a system that behaves exactly as it did when we started. But what one finds very often in developing systems is that in order to make two steps forward, one must first make one step backwards in order to fundmentally change the direction in which they are going. So, even though it looks like our system has not progressed because it still does exactly the same thing, we are indeed in a very different position, architecturally. By freeing the variant behaviors from the invariant ones, we have generated a tremendous amount of flexibility.

Questions & Answers

Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
Renato
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
?
Kyle
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
Adin
why?
Adin
what school?
Kyle
biomolecules are e building blocks of every organics and inorganic materials.
Joe
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
research.net
kanaga
sciencedirect big data base
Ernesto
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
Bharti
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
Daniel
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
Maciej
characteristics of micro business
Abigail
for teaching engĺish at school how nano technology help us
Anassong
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
NANO
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
s.
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
Tarell
what is the actual application of fullerenes nowadays?
Damian
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
Tarell
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
Virgil
is Bucky paper clear?
CYNTHIA
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
NANO
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Harper
Do you know which machine is used to that process?
s.
how to fabricate graphene ink ?
SUYASH Reply
for screen printed electrodes ?
SUYASH
What is lattice structure?
s. Reply
of graphene you mean?
Ebrahim
or in general
Ebrahim
in general
s.
Graphene has a hexagonal structure
tahir
On having this app for quite a bit time, Haven't realised there's a chat room in it.
Cied
what is biological synthesis of nanoparticles
Sanket Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Source:  OpenStax, Principles of object-oriented programming. OpenStax CNX. May 10, 2013 Download for free at http://legacy.cnx.org/content/col10213/1.37
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