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The module uses the Ballworld program to demonstrate key ideas in abstract classes, polymorphism, inheritance and other coding techniques. In particular, this module will contrast a composition-based architecture with an inheritance-based architecture.

In this module we will explore what is gained by modifying the inheritance-based Ballworld system into a composition-based system.

In the inheritance-based Ballworld system, we were able to generate quite a bit of flexibility and extensibility. For instance, we could develop new kinds of balls and add them into the system without recompiling the rest of the code. This was accomplished by having the invariant ball creation and management code deal only with the abstract ball while the variant behaviors were encapsulated into the concrete subclasses.

The problem with inheritance

Inheritance seems to work quite well, but suppose we want to do more than just put different kinds of balls on the screen? What if we wanted to be able to change how a ball behaves, after it has been created? What if we want create balls that do a multiple of different behaviors, such as change color and radius? While working solutions using an inheritance-based system do exist, they are cumbersome, inefficient and most importantly, inconsistent with any sort of clear abstract model of what balls should be like.

The problem lies in the very nature of inheritance. When we attempted to separate the variant from the invariant behaviors, we overlooked a crucial aspect of inheritance. In our model, the superclass represented the invariant behaviors of a ball while the subclasses represented the variant behaviors. The separation seemed clear enough in the UML diagram, except that when one has an actual object instance , both the superclass and subclass behaviors are bound into a single entity. A ball object cannot change its variant updateState behavior because it is inextricably bound with to the invariant behaviors. A ball object cannot be composed of multiple updateState behaviors because that code cannot be isolated from the rest of the ball's code. If you want a curving behavior, you have to get it packaged in a whole ball object--you can't get just the behavior.

A clear sympton of this problem is the common code to call the superclass constructor found in all the subclasses' constructors. This tells us that the superclass is really right there in the subclass with everything else. The fact that the code is repeated from class to class says that it is invariant code in the middle of what we want to be variant code.

The inheritance-based model of Ballworld does not separate the variant and the invariant at the proper place. There is invariant code mixed together with the variant code.
That's why they can't be separated and the invariant behaviors are dragged along with the variant behaviors. This is what makes dynamically changing behaviors and multiply composed behaviors so difficult in this system.

Pizzas and shapes

To understand what we can do to remedy the problems with our inheritance-based model, let's digress for a bit and consider a simple model of pizzas. Here, we have a pizza which has a price and has a shape. A shape, be it a circle, square, rectangle of triangle, is capable of determining its own area. A pizza, when requested to calculate its price per square inch, simply takes its price and divides it by the area of its shape. To obtain that area, the Pizza delegates to the IShape , since it is the shape that knows how to calculate its area, not the pizza.

Pizzas and shapes

A pizza has-a shape, which is able to calculate its area.
Delegation is the handing of a calculation off to another object for it process. Here, the pizza is only interested in the result of the area calculation, not how it is performed.
To the pizza, the shape represents an abstract algorithm to calculate the area.
The Pizza and the IShape classes represent the invariant processes involved with calculating the price per square inch ration, while the concrete Circle , Square , Triangle and Rectangle classes represent the variant area calculations for different shapes. What wee see from this example is that
objects can be used to represent pure behavior, not just tangible entities.
Interfaces are particularly useful here as they are expressly designed to represent pure, abstract behavior.

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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