{Collected from oracle Website for my friends}
Lesson: Object-Oriented Programming Concepts
Lesson: Object-Oriented Programming Concepts
If you've never used an
object-oriented programming language before, you'll need to learn a few basic
concepts before you can begin writing any code. This lesson will introduce you
to objects, classes, inheritance, interfaces, and packages. Each discussion
focuses on how these concepts relate to the real world, while simultaneously
providing an introduction to the syntax of the Java programming language.
What Is an Object?
An 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.
What Is a Class?
A class is a blueprint
or prototype from which objects are created. This section defines a class that
models the state and behavior of a real-world object. It intentionally focuses
on the basics, showing how even a simple class can cleanly model state and
behavior.
What Is Inheritance?
Inheritance provides a
powerful and natural mechanism for organizing and structuring your software.
This section explains how classes inherit state and behavior from their
superclasses, and explains how to derive one class from another using the
simple syntax provided by the Java programming language.
What Is an Interface?
An interface is a
contract between a class and the outside world. When a class implements an
interface, it promises to provide the behavior published by that interface.
This section defines a simple interface and explains the necessary changes for
any class that implements it.
What Is a Package?
A package is a namespace
for organizing classes and interfaces in a logical manner. Placing your code
into packages makes large software projects easier to manage. This section
explains why this is useful, and introduces you to the Application Programming
Interface (API) provided by the Java platform.
Questions and Exercises: Object-Oriented
Programming Concepts
Use the questions and
exercises presented in this section to test your understanding of objects,
classes, inheritance, interfaces, and packages.
What Is an Object?
Objects are key to
understanding object-oriented technology. Look around right now and you'll
find many examples of real-world objects: your dog, your desk, your television
set, your bicycle.
Real-world objects share
two characteristics: They all have state and behavior. Dogs have state (name, color, breed, hungry) and behavior
(barking, fetching, wagging tail). Bicycles also have state (current gear,
current pedal cadence, current speed) and behavior (changing gear, changing
pedal cadence, applying brakes). Identifying the state and behavior for
real-world objects is a great way to begin thinking in terms of object-oriented
programming.
Take a minute right now
to observe the real-world objects that are in your immediate area. For each
object that you see, ask yourself two questions: "What possible states can
this object be in?" and "What possible behavior can this object
perform?". Make sure to write down your observations. As you do, you'll
notice that real-world objects vary in complexity; your desktop lamp may have
only two possible states (on and off) and two possible behaviors (turn on, turn
off), but your desktop radio might have additional states (on, off, current
volume, current station) and behavior (turn on, turn off, increase volume,
decrease volume, seek, scan, and tune). You may also notice that some objects,
in turn, will also contain other objects. These real-world observations all
translate into the world of object-oriented programming.
A software object.
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.
Consider a bicycle, for
example:
A bicycle modeled as a
software object.
By attributing state
(current speed, current pedal cadence, and current gear) and providing methods
for changing that state, the object remains in control of how the outside world
is allowed to use it. For example, if the bicycle only has 6 gears, a method to
change gears could reject any value that is less than 1 or greater than 6.
Bundling code into
individual software objects provides a number of benefits, including:
1.
Modularity: The source
code for an object can be written and maintained independently of the source
code for other objects. Once created, an object can be easily passed around
inside the system.
2.
Information-hiding: By
interacting only with an object's methods, the details of its internal
implementation remain hidden from the outside world.
3.
Code re-use: If an
object already exists (perhaps written by another software developer), you can
use that object in your program. This allows specialists to
implement/test/debug complex, task-specific objects, which you can then trust
to run in your own code.
4.
Pluggability and
debugging ease: If a particular object turns out to be problematic, you can
simply remove it from your application and plug in a different object as its
replacement. This is analogous to fixing mechanical problems in the real world.
If a bolt breaks, you replace it, not the entire machine.
5.
What Is a Class?
6. 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.
7. The following Bicycle class is one possible
implementation of a bicycle:
8.
9.
class Bicycle {
10.
11. int cadence = 0;
12. int speed = 0;
13. int gear = 1;
14.
15. void
changeCadence(int newValue) {
16. cadence =
newValue;
17. }
18.
19. void changeGear(int newValue)
{
20. gear = newValue;
21. }
22.
23. void speedUp(int
increment) {
24. speed = speed +
increment;
25. }
26.
27. void applyBrakes(int
decrement) {
28. speed = speed -
decrement;
29. }
30.
31. void printStates() {
32.
System.out.println("cadence:"+cadence+"
speed:"+speed+" gear:"+gear);
33. }
34.}
35. 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.
36. 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.
37. Here's a BicycleDemo class that creates two separate Bicycle objects and invokes their methods:
38.
39.class BicycleDemo {
40. public static void
main(String[] args) {
41.
42. // Create two different Bicycle objects
43. Bicycle bike1 =
new Bicycle();
44. Bicycle bike2 =
new Bicycle();
45.
46. // Invoke methods
on those objects
47.
bike1.changeCadence(50);
48. bike1.speedUp(10);
49. bike1.changeGear(2);
50.
bike1.printStates();
51.
52.
bike2.changeCadence(50);
53. bike2.speedUp(10);
54.
bike2.changeGear(2);
55.
bike2.changeCadence(40);
56. bike2.speedUp(10);
57.
bike2.changeGear(3);
58. bike2.printStates();
59. }
60.}
61.
62. The output of this test prints the ending pedal
cadence, speed, and gear for the two bicycles:
63.cadence:50 speed:10 gear:2
64.cadence:40 speed:20 gear:3
65. What Is Inheritance?
66. 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.
67. 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:
68. 
69.
A hierarchy of bicycle
classes.
70. 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:
71.class MountainBike extends Bicycle {
72.
73. // new fields and
methods defining a mountain bike would go here
74.
75.}
76. 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.
77. What Is an Interface?
78. As you've already learned, objects define their
interaction with the outside world through the methods that they expose.
Methods form the object's interface with the outside world; the buttons on the front
of your television set, for example, are the interface between you and the
electrical wiring on the other side of its plastic casing. You press the
"power" button to turn the television on and off.
79. 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:
80.interface Bicycle {
81.
82. void
changeCadence(int newValue); // wheel
revolutions per minute
83.
84. void changeGear(int
newValue);
85.
86. void speedUp(int
increment);
87.
88. void applyBrakes(int
decrement);
89.}
90. To implement this interface, the name of your
class would change (to a particular brand of bicycle, for example, such asACMEBicycle), and you'd use the implements keyword in the class declaration:
91.class ACMEBicycle implements Bicycle {
92.
93. // remainder of this
class implemented as before
94.
95.}
96. 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.
97.
98. Note:
99. To actually compile the ACMEBicycle class, you'll need to add the public keyword to the beginning of the implemented
interface methods. You'll learn the reasons for this later in the lessons on Classes and Objectsand Interfaces
and Inheritance.
What
Is a 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.The Java Platform API Specification contains the complete listing for all packages, interfaces, classes, fields, and methods supplied by the Java Platform 6, Standard Edition. Load the page in your browser and bookmark it. As a programmer, it will become your single most important piece of reference documentation.
Questions and Exercises:
Object-Oriented Programming Concepts
Questions
1.
Real-world objects
contain ___ and ___.
2.
A software object's
state is stored in ___.
3.
A software object's
behavior is exposed through ___.
4.
Hiding internal data
from the outside world, and accessing it only through publicly exposed methods
is known as data ___.
5.
A blueprint for a
software object is called a ___.
6.
Common behavior can be
defined in a ___ and inherited into a ___ using the ___ keyword.
7.
A collection of methods
with no implementation is called an ___.
8.
A namespace that
organizes classes and interfaces by functionality is called a ___.
9.
The term API stands for
___?
Exercises
1.
Create new classes for
each real-world object that you observed at the beginning of this trail. Refer
to the Bicycle class if you forget the required syntax.
2.
For each new class that
you've created above, create an interface that defines its behavior, then
require your class to implement it. Omit one or two methods and try compiling.
What does the error look like?
Answers
to Questions
1.
Real-world
objects contain state and behavior.
2.
A
software object's state is stored in fields.
3.
A
software object's behavior is exposed through methods.
4.
Hiding
internal data from the outside world, and accessing it only through publicly
exposed methods is known as data encapsulation.
5.
A
blueprint for a software object is called a class.
6.
Common
behavior can be defined in a superclass and inherited into a subclass using the extends keyword.
7.
A
collection of methods with no implementation is called an interface.
8.
A
namespace that organizes classes and interfaces by functionality is called a package.
9.
The
term API stands for Application Programming Interface.
Answers to Exercises
1.
Your
answers will vary depending on the real-world objects that you are modeling.
2.
Your
answers will vary here as well, but the error message will specifically list
the required methods that have not been implemented.
