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Object-oriented systems use classes to express the above notions of abstract equivalence.

A class is an abstract description of a set of objects.
A class thus contain the descriptions of all the behaviors of the objects that it represents. In computer science parlance, we call the individual behaviors of a class its methods . In addition, a class may, but not always, contain descriptions of the internal data held by the objects, called its fields , as well as implementation details about its methods and fields.

Turning the description around, we can say that a class is a template or recipe for the creation of a particular type of object. That is, one can use a class to create (" instantiate ") objects of the type described by the class. Be careful not to make the very beginner's common mistake of equating classes and objects. A class is a specification of an set of objects, it is not the actual object.

In technical terms, a class defines a new type    in the system. Types are identifies used to differentiate different kinds of data. For instance, integers, characters, strings and arrays are all types of data.

Implementation in java

Classes are the fundamtental building blocks of Java programs. Defining a class is basically a matter of specifying a name for the class, its relationship to other classes and what sort of behaviors the class exhibits as well as what data it may contain.

SuppSuppose we wanted to define a class to describe a household pet. We could do it as such: class Pet { }

The word class above is called a keyword and can only be used to state that the following code is the definition of a class. The class keyword is immediately followed by the desired name of the class, Pet here. The curly braces, {...} , indicate the extent of the definition of the class. That is, any definitions of the class's behaviors and data will be found between the curly braces. Curly braces must therefore always appear as a matched set.

In general, in Java, curly braces mark the extent of a definition.

The accepted typographic standard in the Java community is that the opening curly brace is at the end of a line and the ending curly brace is at the beginning of its own line. Any code between the curly braces is indented.

Well, our class for pets is simple, but not very interesting because it doesn't do anything. We could say that our pet has a name, but personally, I have found that the behavior of most pets is not affected by the particular name we give them. That is, we can give our pet a name but that doesn't change the way they behave. Let's try something else.

Pets eat a certain amount of food per day. Pets have a certain weight. Let's create a model , that states that the number of hours that a pet sleeps per day is related to the amount of food they eat and their weight. Thus we could say that a pet has a behavior of sleeping a certain number of hours depending on its weight and the amount of food eaten.

/** * A class that models a household pet*/ class Pet{/*** The weight of the pet in lbs */double weight;/** * The number of hours the pet will sleep after eating* the given lbs of food. * @param lbsOfFood The number of lbs of food eaten.* @return The number of hours slept after eating the food. */double hoursSlept(double lbsOfFood) { return 24.0*lbsOfFood/weight;} }

But what about the pet's owner (a person, supposedly)? A person is not such a simple thing as a weight. Assigning a number to every possible person in the world is possible but not necessarily the most practical technique. There are many ways to represent people: the Registrar likes to think of you as a number with credit hours and unpaid bills, The CS department sees you as a 8-character login and unfinished graduation requirements, your doctor sees you as a collection of pulsating blood vessels, cells and bones, and your parents see you as this sweet little thing just out of diapers. A person is still a person, but the way that we choose to represent them may differ from situation to situation.

But here's the crux of the issue: does your pet care how you are internally represented? Or does your pet just want something that is a person to be their owner? /** * A class that models a household pet*/ class Pet{/*** The weight of the pet in lbs */double weight;/** * The pet's owner*/ Person owner;/*** The number of hours the pet will sleep after eating * the given lbs of food.* @param lbsOfFood The number of lbs of food eaten. * @return The number of hours slept after eating the food.*/ double hoursSlept(double lbsOfFood) {return 24.0*lbsOfFood/weight; }/*** Determines whether or not this pet is happy to see a * particular person.* @param p The person who the pet sees. * @return true if the Person is their owner, false otherwise.*/ boolean isHappyToSee(Person p) {return p == owner; }}

Here we've added a field of type Person , which is a class used to represent people. It doesn't matter how Person is implemented, just that it is a representation of a person. We could use the class definition of Person that was created in the module on Abstraction , or perhaps one that is fixed up a little bit, or perhaps a completely new one. The Pet doesn't care.

Thus we see that objects can contain objects. What does this say about the possible complexity of a system of objects?

Download code

To download a zipped file containing the code above, click one of the following links:

Both of the above codebases include a DrJava project file (.pjt) that can be opened from DrJava to easily manage all the code and test files.

Questions & Answers

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
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
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
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
I'm interested in nanotube
Uday
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
how to synthesize TiO2 nanoparticles by chemical methods
Zubear
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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