<< Chapter < Page Chapter >> Page >

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

the diagram of the digestive system
Assiatu Reply
How does twins formed
William Reply
They formed in two ways first when one sperm and one egg are splited by mitosis or two sperm and two eggs join together
Oluwatobi
what is genetics
Josephine Reply
Genetics is the study of heredity
Misack
how does twins formed?
Misack
What is manual
Hassan Reply
discuss biological phenomenon and provide pieces of evidence to show that it was responsible for the formation of eukaryotic organelles
Joseph Reply
what is biology
Yousuf Reply
the study of living organisms and their interactions with one another and their environments
AI-Robot
the study of living organisms and their interactions with one another and their environment.
Wine
discuss the biological phenomenon and provide pieces of evidence to show that it was responsible for the formation of eukaryotic organelles in an essay form
Joseph Reply
what is the blood cells
Shaker Reply
list any five characteristics of the blood cells
Shaker
lack electricity and its more savely than electronic microscope because its naturally by using of light
Abdullahi Reply
advantage of electronic microscope is easily and clearly while disadvantage is dangerous because its electronic. advantage of light microscope is savely and naturally by sun while disadvantage is not easily,means its not sharp and not clear
Abdullahi
cell theory state that every organisms composed of one or more cell,cell is the basic unit of life
Abdullahi
is like gone fail us
DENG
cells is the basic structure and functions of all living things
Ramadan
What is classification
ISCONT Reply
is organisms that are similar into groups called tara
Yamosa
in what situation (s) would be the use of a scanning electron microscope be ideal and why?
Kenna Reply
A scanning electron microscope (SEM) is ideal for situations requiring high-resolution imaging of surfaces. It is commonly used in materials science, biology, and geology to examine the topography and composition of samples at a nanoscale level. SEM is particularly useful for studying fine details,
Hilary
cell is the building block of life.
Condoleezza Reply
what is cell divisoin?
Aron Reply
Diversity of living thing
ISCONT
what is cell division
Aron Reply
Cell division is the process by which a single cell divides into two or more daughter cells. It is a fundamental process in all living organisms and is essential for growth, development, and reproduction. Cell division can occur through either mitosis or meiosis.
AI-Robot
What is life?
Allison Reply
life is defined as any system capable of performing functions such as eating, metabolizing,excreting,breathing,moving,Growing,reproducing,and responding to external stimuli.
Mohamed
Got questions? Join the online conversation and get instant answers!
Jobilize.com Reply
Practice Key Terms 2

Get Jobilize Job Search Mobile App in your pocket Now!

Get it on Google Play Download on the App Store Now




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