<< Chapter < Page Chapter >> Page >

3. the visitor pattern

The visitor pattern is a pattern for communication and collaboration between two union patterns: a "host" union and a "visitor" union.  An abstract visitor is usually defined as an interface in Java.  It has a separate method for each of the concrete variant of the host union.  The abstract host has a method (called the "hook") to "accept" a visitor and leaves it up to each of its concrete variants to call the appropriate visitor method.  This "decoupling" of the host's structural behaviors from the extrinsic algorithms on the host permits the addition of infinitely many external algorithms without changing any of the host union code.  This extensibility only works if the taxonomy of the host union is stable and does not change.  If we have to modify the host union, then we will have to modify ALL visitors as well!

 

NOTE: All the "state-less" visitors, that is visitors that contain no non-static fields should be singletons.  Visitors that contain non-static fields should not be singletons.

4.  fundamental object-oriented design methodology (foodm)

  1. Identify and separate the variant and the invariant behaviors.
  2. Encapsulate the invariant behaviors into a system of classes.
  3. Add "hooks" to this class to define communication protocols with other classes.
  4. Encapsulate the variant behaviors into a union of classes that comply with the above protocols.

The result is a flexible system of co-operating objects that is not only reusable and extensible, but also easy to understand and maintain.

Let us illustrate the above process by applying it to the design of the immutable list structure and its algorithms.

  1. Here, the invariant is the intrinsic and primitive behavior of the list structure, IList , and the variants are the multitude of extrinsic and non-primitive algorithms that manipulate it, IListAlgo .
  2. The recursive list structure is implemented using the composite design pattern and encapsulated with a minimal and complete set of primitive structural operations:  getFirst() and getRest()
  3. The hook method Object execute(IListAlgo ago,  Object inp) defines the protocols for operating on the list structure. The hook works as if a IList announces to the outside world the following protocol: If you want me to execute your algorithm, encapsulate it into an object of type IListAlgo, hand it to me together with its inp object as parameters for my execute(). I will send your algorithm object the appropriate message for it to perform its task, and return you the result.
    • emptyCase(...) should be the part of the algorithm that deals with the case where I am empty.
    • nonEmptyCase(...) should be the part of the algorithm that deals with the case where I am not empty.”
  4. IListAlgo and all of its concrete implementations forms a union of algorithms classes that can be sent to the list structure for execution.

Below is the UML class diagram of the resulting list design.  Click here to see the full documentation.   Click here to see the code .

The above design is nothing but a special case of the Visitor Pattern .  The interface IList is called the host and its method execute() is called a " hook " to the IListAlgo visitors .  Via polymorphism, IList knows exactly what method to call on the specific IListAlgo visitor.  This design turns the list structure into a (miniature) framework where control is inverted: one hands an algorithm to the structure to be executed instead of handing a structure to an algorithm to perform a computation. Since an IListAlgo only interacts with the list on which it operates via the list’s public interface, the list structure is capable of carrying out any conforming algorithm, past, present, or future. This is how reusability and extensibility is achieved.

Questions & Answers

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
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 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
How can I make nanorobot?
Lily
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
how can I make nanorobot?
Lily
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
Got questions? Join the online conversation and get instant answers!
Jobilize.com Reply

Get the best Algebra and trigonometry course in your pocket!





Source:  OpenStax, Principles of object-oriented programming. OpenStax CNX. May 10, 2013 Download for free at http://legacy.cnx.org/content/col10213/1.37
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Principles of object-oriented programming' conversation and receive update notifications?

Ask