(Java) Object Oriented Programming Concepts
Lesson Object-Oriented Programming (OOP)
- Object
An the object is a software bundle of related state and behavior. Software objects are
often used to model the real-world objects that you find in everyday life. This
lesson explains how state and behavior are represented within an object,
introduces the concept of data encapsulation, and explains the benefits of
designing your software in this manner.
Software objects are conceptually similar to
real-world objects: they too consist of state and related behavior. An object
stores its state in fields (variables in some programming languages) and
exposes its behavior through methods (functions in some programming
languages). Methods operate on an object's internal state and serve as the primary
mechanism for object-to-object communication. Hiding internal state and
requiring all interaction to be performed through an object's methods is known
as data encapsulation — a fundamental principle of object-oriented
programming.
2.
Class
In the real
world, you'll often find many individual objects all of the same kind. There
may be thousands of other bicycles in existence, all of the same make and
model. Each bicycle was built from the same set of blueprints and therefore
contains the same components. In object-oriented terms, we say that your
bicycle is an instance of the class of objects known as bicycles.
A class is the blueprint from which individual objects are created.
class Bicycle {
int cadence
= 0;
int speed =
0;
int gear =
1;
void
changeCadence(int newValue) {
cadence
= newValue;
}
void
changeGear(int newValue) {
gear =
newValue;
}
void speedUp(int increment) {
speed =
speed + increment;
}
void
applyBrakes(int decrement) {
speed =
speed - decrement;
}
void
printStates() {
System.out.println("cadence:" +
cadence + " speed:" +
speed + " gear:" + gear);
}
}
The syntax of the Java programming language will look new to you, but the
design of this class is based on the previous discussion of bicycle objects.
The fields cadence, speed, and gear represent the object's state, and the methods (changeCadence, changeGear, speedUp etc.) define
its interaction with the outside world.
You may have noticed that the Bicycle class does
not contain a main method. That's because it's not a complete
application; it's just the blueprint for bicycles that might be used in
an application. The responsibility of creating and using new Bicycle objects
belongs to some other class in your application.
class BicycleDemo {
public
static void main(String[] args) {
//
Create two different
//
Bicycle objects
Bicycle
bike1 = new Bicycle();
Bicycle
bike2 = new Bicycle();
//
Invoke methods on
// those
objects
bike1.changeCadence(50);
bike1.speedUp(10);
bike1.changeGear(2);
bike1.printStates();
bike2.changeCadence(50);
bike2.speedUp(10);
bike2.changeGear(2);
bike2.changeCadence(40);
bike2.speedUp(10);
bike2.changeGear(3);
bike2.printStates();
}
}
The output of this test prints the ending pedal cadence, speed, and gear
for the two bicycles:
cadence:50 speed:10 gear:2
cadence:40 speed:20 gear:3
3.
Inheritance
Different kinds of objects often have a certain amount in common
with each other. Mountain bikes, road bikes, and tandem bikes, for example, all
share the characteristics of bicycles (current speed, current pedal cadence,
current gear). Yet each also defines additional features that make them
different: tandem bicycles have two seats and two sets of handlebars; road
bikes have drop handlebars; some mountain bikes have an additional chain ring,
giving them a lower gear ratio.
Object-oriented
programming allows classes to inherit commonly used state and behavior
from other classes. In this example, Bicycle now becomes
the superclass of MountainBike, RoadBike, and TandemBike. In the Java
programming language, each class is allowed to have one direct superclass, and
each superclass has the potential for an unlimited number of subclasses:
The syntax
for creating a subclass is simple. At the beginning of your class declaration,
use the
extends keyword, followed by the name of the class to inherit from:
class MountainBike extends Bicycle {
// new fields and methods defining
// a mountain bike would go here
}This gives
MountainBike all the same
fields and methods as Bicycle, yet allows
its code to focus exclusively on the features that make it unique. This makes
code for your subclasses easy to read. However, you must take care to properly
document the state and behavior that each superclass defines, since that code
will not appear in the source file of each subclass.
4.
Interface
In its most common form, an interface is a group of related methods with empty bodies. A bicycle's behavior, if specified as an interface, might appear as follows:
interface Bicycle {
// wheel revolutions per minute
void changeCadence(int newValue);
void changeGear(int newValue);
void speedUp(int increment);
void applyBrakes(int decrement);
}To implement this interface, the name of your class would change (to a particular brand of bicycle, for example, such as
ACMEBicycle), and you'd
use the implements keyword in the class declaration:class ACMEBicycle implements Bicycle {
int cadence = 0;
int speed = 0;
int gear = 1;
// The compiler will now require that methods
// changeCadence, changeGear, speedUp, and applyBrakes
// all be implemented. Compilation will fail if those
// methods are missing from this class.
void changeCadence(int newValue) {
cadence = newValue;
}
void changeGear(int newValue) {
gear = newValue;
}
void speedUp(int increment) {
speed = speed + increment;
}
void applyBrakes(int decrement) {
speed = speed - decrement;
}
void printStates() {
System.out.println("cadence:" +
cadence + " speed:" +
speed + " gear:" + gear);
}
}Implementing an interface allows a class to become more formal about the behavior it promises to provide. Interfaces form a contract between the class and the outside world, and this contract is enforced at build time by the compiler. If your class claims to implement an interface, all methods defined by that interface must appear in its source code before the class will successfully compile.
5. Package
A package is a namespace that organizes a set of related classes
and interfaces. Conceptually you can think of packages as being similar to
different folders on your computer. You might keep HTML pages in one folder,
images in another, and scripts or applications in yet another. Because software
written in the Java programming language can be composed of hundreds or thousands
of individual classes, it makes sense to keep things organized by placing
related classes and interfaces into packages.
The Java platform provides an enormous class library (a set of
packages) suitable for use in your own applications. This library is known as
the "Application Programming Interface", or "API" for
short. Its packages represent the tasks most commonly associated with
general-purpose programming. For example, a
String object
contains state and behavior for character strings; a File object
allows a programmer to easily create, delete, inspect, compare, or modify a
file on the filesystem; a Socket object
allows for the creation and use of network sockets; various GUI objects control
buttons and checkboxes and anything else related to graphical user interfaces.
There are literally thousands of classes to choose from. This allows you, the
programmer, to focus on the design of your particular application, rather than
the infrastructure required to make it work.
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