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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

can someone help me with some logarithmic and exponential equations.
Jeffrey Reply
sure. what is your question?
ninjadapaul
20/(×-6^2)
Salomon
okay, so you have 6 raised to the power of 2. what is that part of your answer
ninjadapaul
I don't understand what the A with approx sign and the boxed x mean
ninjadapaul
it think it's written 20/(X-6)^2 so it's 20 divided by X-6 squared
Salomon
I'm not sure why it wrote it the other way
Salomon
I got X =-6
Salomon
ok. so take the square root of both sides, now you have plus or minus the square root of 20= x-6
ninjadapaul
oops. ignore that.
ninjadapaul
so you not have an equal sign anywhere in the original equation?
ninjadapaul
Commplementary angles
Idrissa Reply
hello
Sherica
im all ears I need to learn
Sherica
right! what he said ⤴⤴⤴
Tamia
what is a good calculator for all algebra; would a Casio fx 260 work with all algebra equations? please name the cheapest, thanks.
Kevin Reply
a perfect square v²+2v+_
Dearan Reply
kkk nice
Abdirahman Reply
algebra 2 Inequalities:If equation 2 = 0 it is an open set?
Kim Reply
or infinite solutions?
Kim
The answer is neither. The function, 2 = 0 cannot exist. Hence, the function is undefined.
Al
y=10×
Embra Reply
if |A| not equal to 0 and order of A is n prove that adj (adj A = |A|
Nancy Reply
rolling four fair dice and getting an even number an all four dice
ramon Reply
Kristine 2*2*2=8
Bridget Reply
Differences Between Laspeyres and Paasche Indices
Emedobi Reply
No. 7x -4y is simplified from 4x + (3y + 3x) -7y
Mary Reply
is it 3×y ?
Joan Reply
J, combine like terms 7x-4y
Bridget Reply
im not good at math so would this help me
Rachael Reply
yes
Asali
I'm not good at math so would you help me
Samantha
what is the problem that i will help you to self with?
Asali
how do you translate this in Algebraic Expressions
linda Reply
Need to simplify the expresin. 3/7 (x+y)-1/7 (x-1)=
Crystal Reply
. After 3 months on a diet, Lisa had lost 12% of her original weight. She lost 21 pounds. What was Lisa's original weight?
Chris Reply
what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
China
Cied
types of nano material
abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
Porter
many many of nanotubes
Porter
what is the k.e before it land
Yasmin
what is the function of carbon nanotubes?
Cesar
what is nanomaterials​ and their applications of sensors.
Ramkumar Reply
what is nano technology
Sravani Reply
what is system testing?
AMJAD
preparation of nanomaterial
Victor Reply
Yes, Nanotechnology has a very fast field of applications and their is always something new to do with it...
Himanshu Reply
good afternoon madam
AMJAD
what is system testing
AMJAD
what is the application of nanotechnology?
Stotaw
In this morden time nanotechnology used in many field . 1-Electronics-manufacturad IC ,RAM,MRAM,solar panel etc 2-Helth and Medical-Nanomedicine,Drug Dilivery for cancer treatment etc 3- Atomobile -MEMS, Coating on car etc. and may other field for details you can check at Google
Azam
anybody can imagine what will be happen after 100 years from now in nano tech world
Prasenjit
after 100 year this will be not nanotechnology maybe this technology name will be change . maybe aftet 100 year . we work on electron lable practically about its properties and behaviour by the different instruments
Azam
name doesn't matter , whatever it will be change... I'm taking about effect on circumstances of the microscopic world
Prasenjit
how hard could it be to apply nanotechnology against viral infections such HIV or Ebola?
Damian
silver nanoparticles could handle the job?
Damian
not now but maybe in future only AgNP maybe any other nanomaterials
Azam
can nanotechnology change the direction of the face of the world
Prasenjit Reply
At high concentrations (>0.01 M), the relation between absorptivity coefficient and absorbance is no longer linear. This is due to the electrostatic interactions between the quantum dots in close proximity. If the concentration of the solution is high, another effect that is seen is the scattering of light from the large number of quantum dots. This assumption only works at low concentrations of the analyte. Presence of stray light.
Ali Reply
the Beer law works very well for dilute solutions but fails for very high concentrations. why?
bamidele 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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