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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

what is variations in raman spectra for nanomaterials
Jyoti Reply
I only see partial conversation and what's the question here!
Crow Reply
what about nanotechnology for water purification
RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
Damian
yes that's correct
Professor
I think
Professor
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
Rafiq
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
Damian
How we are making nano material?
LITNING Reply
what is a peer
LITNING Reply
What is meant by 'nano scale'?
LITNING Reply
What is STMs full form?
LITNING
scanning tunneling microscope
Sahil
how nano science is used for hydrophobicity
Santosh
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
Rafiq
what is differents between GO and RGO?
Mahi
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
Rafiq
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
Bob
The nanotechnology is as new science, to scale nanometric
brayan
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Damian
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 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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