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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

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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