�ݺ�ߣ

Abstraction, Inheritance, and
Polymorphism
in Java

“Object Orientation involving encapsulation,
inheritance, polymorphism, and abstraction, is an
important approach in programming and program
design. It is widely accepted and used in industry and
is growing in popularity in the first and second
college-level programming courses.”

Some other reasons to move on
to Java:
• Platform-independent software
• Relatively easy graphics and GUI
programming
• Lots of library packages
• Free compiler and IDEs

Some other reasons to move on
to Java:
• Colleges are teaching it
• Companies are using it
• Students want it
• (Teachers welcome it... ;)
• (Programmers drink it... :)

What are OOP’s claims to fame?
• Better suited for team development
• Facilitates utilizing and creating reusable
software components
• Easier GUI programming
• Easier program maintenance

OOP in a Nutshell:
• A program models a
world of interacting
objects
• Objects create other
objects and “send
messages” to each other
(in Java, call each
other’s methods)
• Each object belongs to a
class; a class defines
properties of its objects
• A class implements an
ADT; the data type of an
object is its class
• Programmers write classes
(and reuse existing classes)

Quiz:
How many classes we wrote
for this applet?
A. 1
B. 2
C. 5
D. 10
E. 17

DanceStudio
DanceModel
Music
DanceFloor
Controller
Model
View
MaleDancer
FemaleDancer
MaleFoot
FemaleFoot
Dancer
Foot

Good news:
The classes are fairly short
DanceStudio 92 lines MaleDancer 10 lines
DanceModel 50 lines FemaleDancer 10 lines
DanceFloor 30 lines Foot 100 lines
Music 52 lines MaleFoot 42 lines
Dancer 80 lines FemaleFoot 42 lines
• In OOP, the number of classes is not
considered a problem

In a project with 10 classes we need an IDE...

Abstraction
... relevant to the given project (with an
eye to future reuse in similar projects).
Abstraction means ignoring irrelevant
features, properties, or functions and
emphasizing the relevant ones...
“Relevant” to what?

Abstraction
MaleDancer FemaleDancer
Dancer

Encapsulation
Encapsulation means that all data members
(fields) of a class are declared private. Some
methods may be private, too.
The class interacts with other classes (called
the clients of this class) only through the
class’s constructors and public methods.
Constructors and public methods of a class
serve as the interface to class’s clients.

Encapsulation
MaleFoot FemaleFoot
Foot

public abstract class Foot
{
private static final int footWidth = 24;
private boolean amLeft;
private int myX, myY;
private int myDir;
private boolean myWeight;
// Constructor:
protected Foot(String side, int x, int y, int dir)
{
amLeft = side.equals("left");
myX = x;
myY = y;
myDir = dir;
myWeight = true;
}
Continued 
All fields are
private

Encapsulation ensures that
structural changes remain local
• Changes in the code create software
maintenance problems
• Usually, the structure of a class (as defined
by its fields) changes more often than the
class’s constructors and methods
• Encapsulation ensures that when fields
change, no changes are needed in other
classes (a principle known as “locality”)

True or False? Abstraction and
encapsulation are helpful for the
following:
 Team development ________
 Reusable software ________
 GUI programming ________
 Easier program maintenance ________

Answer:
 Team development ________
 Reusable software ________
 Easier program maintenance ________
T
T
T
(True if you are working on system
packages, such as Swing)
F

Inheritance
A class can extend another class,
inheriting all its data members and
methods while redefining some of them
and/or adding its own.
Inheritance represents the is a
relationship between data types. For
example: a FemaleDancer is a
Dancer.

Inheritance Terminology:
public class FemaleDancer extends Dancer
{
...
}
subclass
or
derived class
superclass
or
base class
extends

MaleDancer FemaleDancer
Dancer
Example:
Inheritance (cont’d)

public class FemaleDancer extends Dancer
{
public FemaleDancer(String steps[],
int x, int y, int dir)
{
leftFoot = new FemaleFoot("left", x, y, dir);
rightFoot = new FemaleFoot("right", x, y, dir);
leftFoot.move(-Foot.getWidth() / 2, 0);
rightFoot.move(Foot.getWidth() / 2, 0);
}
}
Constructors are not inherited. The
FemaleDancer class only adds a constructor:

MaleFoot FemaleFoot
Foot
Example:

public class FemaleFoot extends Foot
{
public FemaleFoot(String side, int x, int y, int dir)
{
super(side, x, y, dir); // calls Foot's constructor
}
//
public void drawLeft(Graphics g)
{
...
}
public void drawRight(Graphics g)
{
...
}
}
Supplies methods
that are abstract
in Foot:

Inheritance may be used to define a
hierarchy of classes in an application:
MaleFoot FemaleFoot
Foot
MaleLeftFoot MaleRightFoot FemaleLeftFoot FemaleRightFoot
Object

• You don’t need to have the source code of
a class to extend it
All methods of the base library
class are available in your
derived class

True or False? Inheritance is helpful for
the following:
 Reusable software ________
 Easier program maintenance ________

Answer:
 Reusable software ________
 GUI programming ________
 Easier program maintenance ________
F
T
???
T

Polymorphism
Polymorphism ensures that the
appropriate method is called for an
object of a specific type when the object
is disguised as a more general type.
Good news: polymorphism is already
supported in Java — all you have to do
is use it properly.

Polymorphism (cont’d)
Situation 1:
A collection (array, list, etc.) contains
objects of different but related types, all
derived from the same common base
class.

public abstract class Foot
{
...
public void draw(Graphics g)
{
...
if (isLeft())
drawLeft(g);
else
drawRight(g);
...
}
}
Polymorphism replaces old-fashioned use
of explicit object attributes and if-else
(or switch) statements, as in:

These slides and the Dance Studio code are
posted at:
http://www.skylit.com/oop/

�ݺ�ߣ

Abstract inheritpoly

More Related Content

Abstract inheritpoly