Abstraction, Inheritance, and
Polymorphism
in the Dance Studio in Java
Maria Litvin
Phillips Academy, Andover, MA
mlitvin@andover.edu
Why is The College Board using
Java?
Java:
Three reasons:
• OOP
• OOP
• OOP
The Recommendations of
the AP Computer Science
Ad Hoc Committee:
“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:
•
•
•
•
•
•
•
•
•
•
Internet applets
Platform-independent software
Relatively easy graphics and GUI programming
Lots of library packages
Free compiler and IDEs
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
• Each object belongs to a
world of interacting
class; a class defines
objects
properties of its objects
• Objects create other
• A class implements an
objects and “send
ADT; the data type of an
messages” to each other
object is its class
(in Java, call each
• Programmers write classes
other’s methods)
(and reuse existing classes)
Java != C++
• It may be tempting to
teach only conventional
(procedural)
programming in Java
class MyMath
{
public static long
factorial(int n)
{
long f = 1;
int i;
for (i = 2; i <= n; i++)
f *= i;
return f;
}
public static void
main(String[] args)
{
System.out.println("10! = "
+ factorial(10));
}
• It would mean to miss the
point
}
OOP
Case Study: Dance Studio
Exercise 1:
Run the Dance Studio applet.
Exercise 2:
Set up a project in an IDE (e.g. BlueJ) for
the Dance Studio applet. Compile and run.
Quiz:
How many classes were written
for this applet?
A. 1
B. 2
C. 5
D. 10
E. 17
Controller
DanceStudio
Music
DanceModel
View
Model
Dancer
DanceFloor
MaleDancer
FemaleDancer
Foot
MaleFoot
FemaleFoot
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
Quiz:
How many Java/Swing library classes are
used, explicitly or implicitly, in this applet?
A. 4
B. 8
C. 12
D. 20-50
E. 100-500
Answer: E
(hundreds)
/*
* @(#)JButton.java 1.83 00/03/14
* Copyright 1997-2000 Sun Microsystems, Inc.
* All Rights Reserved.
* ...
*/
package javax.swing;
import java.util.EventListener;
import java.awt.*;
import java.awt.event.*;
import java.awt.image.*;
import javax.swing.plaf.*;
import javax.swing.event.*;
import javax.accessibility.*;
import java.io.ObjectOutputStream;
import java.io.ObjectInputStream;
import java.io.IOException;
...
Abstraction
Abstraction means ignoring irrelevant
features, properties, or functions and
emphasizing the relevant ones...
“Relevant” to what?
... relevant to the given project (with an
eye to future reuse in similar projects).
Abstraction
Dancer
MaleDancer
FemaleDancer
public abstract class Dancer
{
// Fields (data members):
protected Foot leftFoot;
protected Foot rightFoot;
private String mySteps[];
private int stepIndex = -1;
public void setSteps(String steps[])
{
mySteps = steps;
stepIndex= -1;
}
Continued

Dancer (cont’d...)
// Methods (member functions):
public void nextStep()
{
...
}
public void makeStep(String sideStr,
String weightStr, String dirStr)
// e.g. makeStep("left", "weightboth", "F");
{
...
}
public void draw(Graphics g)
{
leftFoot.draw(g);
rightFoot.draw(g);
}
Continued

Dancer (cont’d...)
//
//
//
//
Private Methods
(callable only from constructors and methods
of this class, i.e. "private" to the programmer
who wrote this class)
private String[] parseNextStep()
{
...
}
}
How is Dancer used in
Dance Studio?
public class DanceModel
{
...
private Dancer dancers[];
public DanceModel(DanceFloor floor, int floorDir,
String steps1[], String[] steps2)
{
...
dancers = new Dancer[2];
dancers[0] = new MaleDancer(x1, y1, dir1);
dancers[0].setSteps(steps1);
dancers[1] = new FemaleDancer(x2, y2, dir2);
dancers[1].setSteps(steps2);
...
}
Continued

DanceModel (cont’d...)
public void nextBeat()
{
for (int k = 0; k < dancers.length; k++)
dancers[k].nextStep();
...
}
DanceStudio
Music
DanceModel
Dancer
DanceFloor
MaleDancer
FemaleDancer
Foot
MaleFoot
FemaleFoot
Exercise 3:
In the Dance Studio applet, change the
DanceModel class to place four dancers
(two couples) on the dance floor, as shown:
Hint:
• In DanceModel, add a MaleDancer and a
FemaleDancer as follows:
Dancer’s
position
(in pixels)
x
y
Dancer’s
orientation
0
6
2
4
dancer[...] = new MaleDancer(x, y, dir);
Exercise 4:
(a)
(b)
Compile and run the Banner applet
Modify Walker.java so that instead of a
flying banner, a “man” or a “woman”
walks across.
Hint:
• Use MaleDancer or FemaleDancer— they can
“walk” too...
MaleDancer walker = new MaleDancer(x, y, dir);
• Make him/her take one step with one foot, then
two steps with the other foot, etc. To make a
step:
"left" or "right"
walker.makeStep(side, "weightboth", "F");
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
Foot
MaleFoot
FemaleFoot
public abstract class Foot
{
private static final int footWidth = 24;
private
private
private
private
boolean amLeft;
int myX, myY;
int myDir;
boolean myWeight;
All fields are
private
// Constructor:
protected Foot(String side, int x, int y, int dir)
{
amLeft = side.equals("left");
myX = x;
myY = y;
myDir = dir;
myWeight = true;
}
Continued

Foot (cont’d...)
// Public methods -- accessors:
public static int getWidth()
{
return footWidth;
}
public boolean isLeft()
{
return amLeft;
}
Continued

Foot (cont’d...)
public int getX()
{
return myX;
}
public int getY()
{
return myY;
}
public int getDir()
{
return myDir;
}
public boolean hasWeight()
{
return myWeight;
}
Continued

Foot (cont’d...)
// Public methods -- modifiers:
public void setWeight(boolean weight)
{
myWeight = weight;
}
public void move(int dx, int dy)
{
...
}
public void turn(int dir)
{
// dir > 0 -- clockwise; dir < 0 -- counterclockwise
myDir = (8 + myDir + dir) % 8;
}
Continued

Foot (cont’d...)
// Public methods -- drawing:
public void draw(Graphics g)
{
...
if (isLeft())
drawLeft(g);
else
drawRight(g);
...
}
// Public methods -- abstract:
public abstract void drawLeft(Graphics g);
public abstract void drawRight(Graphics g);
}
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”)
Encapsulation (cont’d)
Example:
Suppose in Foot we decided to use
private String mySide;
instead of
private boolean amLeft;
Before:
After:
public abstract class Foot
{
...
private boolean amleft;
...
// Constructor:
protected Foot(String side,
int x, int y, int dir)
{
amLeft =
side.equals("left");
...
}
public boolean isLeft()
{
return amLeft;
}
...
public abstract class Foot
{
...
private String mySide;
...
// Constructor:
protected Foot(String side,
int x, int y, int dir)
{
mySide = side;
...
}
public boolean isLeft()
{
return
side.equals("left");
}
...
True or False? Abstraction and
encapsulation are helpful for the
following:
 Team development ________
 Reusable software ________
 GUI programming ________
 Easier program maintenance ________
Answer:
T
 Team development ________
T
 Reusable software ________
F
 GUI programming ________
 Easier
(True if you are working on system
packages, such as Swing)
T
program maintenance ________
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:
subclass
or
derived class
superclass
extends
or
base class
public class FemaleDancer extends Dancer
{
...
}
Inheritance (cont’d)
Example:
Dancer
MaleDancer
FemaleDancer
Inheritance (cont’d)
Constructors are not inherited. The
FemaleDancer class only adds a constructor:
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);
}
}
Inheritance (cont’d)
Example:
Foot
MaleFoot
FemaleFoot
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:
Object
Foot
MaleFoot
MaleLeftFoot
MaleRightFoot
FemaleFoot
FemaleLeftFoot
FemaleRightFoot
Inheritance is also used to extend library
classes (or someone else’s classes)
public class DanceFloor extends JPanel
{
private DanceModel currentDance;
public void updateDance(DanceModel dance)
{
...
}
...
public void paintComponent(Graphics g)
{
...
}
}
Added
field
Added
method
Redefined
method
All methods of the base library
class are available in your
derived class
• You don’t need to have the source code of
a class to extend it (e.g., we don’t need to
have JPanel.java to write your
DanceFloor class that extends JPanel)
Example:
... DanceFloor extends JPanel
public class DanceStudio extends JApplet
implements ActionListener
{
public void init()
{
...
floorPanel = new DanceFloor();
floorPanel.setBackground(Color.white);
...
}
...
The setBackground method is inherited from the
Swing class JPanel (along with its 287 other
methods)
True or False? Inheritance is helpful for
the following:
 Team development ________
 Reusable software ________
 GUI programming ________
 Easier program maintenance ________
Answer:
F
 Team development ________
T
 Reusable software ________
T
 GUI programming ________
???
 Easier program maintenance ________
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.
Example:
public class DanceModel
{
...
public DanceModel(...)
{
dancers = new Dancer[2];
dancers[0] = new MaleDancer(...);
dancers[1] = new FemaleDancer(...);
...
}
...
}
 
Example (cont’d):
public class DanceFloor extends JPanel
{
...
public void paintComponent(Graphics g)
{
...
Dancer dancer;
for (int k = 0;
(dancer = currentDance.getDancer(k)) != null; k++)
dancer.draw(g);
}
}
Polymorphism (cont’d)
Situation 2:
Abstract class’s fields are instantiated in
concrete subclasses.
Example:
public abstract class Dancer
{
protected Foot leftFoot;
protected Foot rightFoot;
...
public void draw(Graphics g)
{
leftFoot.draw(g);
rightFoot.draw(g);
}
}
What kind of feet, male
or female? This is
unknown at compile
time
The correct “draw” will
be called for the male or
female feet, depending
on the particular dancer
Example (cont’d):
public class FemaleDancer extends Dancer
{
public FemaleDancer(int x, int y, int dir)
{
leftFoot = new FemaleFoot("left", x, y, dir);
rightFoot = new FemaleFoot("right", x, y, dir);
...
}
}
Polymorphism replaces old-fashioned use
of explicit object attributes and if-else
(or switch) statements, as in:
public abstract class Foot
{
...
public void draw(Graphics g)
{
...
if (isLeft())
drawLeft(g);
else
drawRight(g);
...
}
}
Java: