CIS3023: Programming Fundamentals for CIS Majors II
Summer 2010
Abstract Classes
“None of the abstract concepts comes closer to fulfilled utopia
than that of eternal peace.”
--Theodor W. Adorno
Course Lecture Slides
7 June 2010
Ganesh Viswanathan
Example
Drawing Application
Create different shapes: circle, rectangle, …
Circle
Rectangle
-color: String
-color: String
-filled: Boolean
-filled: Boolean
-dateCreated: java.util.Date
-dateCreated: java.util.Date
-radius: double
-width: double
+Circle()
-height: double
+Circle(radius: double)
+Rectangle()
+getRadius(): double
+Rectangle(width: double, height: double)
+setRadius(radius: double): void
+getWidth(): double
+getColor(): String
+setWidth(width: double): void
+setColor(color: String): void
+getHeight(): double
+isFilled(): boolean
+getColor(): String
+setFilled(filled: boolean): void
+setColor(color: String): void
+getDateCreated(): java.util.Date
+isFilled(): boolean
+setFilled(filled: boolean): void
+getArea(): double
+getDateCreated(): java.util.Date
+getPerimeter(): double
+getDiameter(): double
+setHeight(height: double): void
+getArea(): double
+getPerimeter(): double
G e o m e tr ic O b j e c t
- c o lo r : S t r i n g
T h e c o lo r o f t h e o b j e c t (d e f a u lt : w h i t e ) .
- f i ll e d : b o o le a n
In d i c a t e s w h e t h e r t h e o b j e c t i s fi lle d w i t h a c o lo r ( d e f a u lt : f a ls e ) .
- d a t e C r e a t e d : j a v a .u t i l.D a t e
T h e d a t e w h e n th e o b j e c t w a s c r e a t e d .
+ G e o m e tric O b je c t()
C re a te s a G e o m e tric O b je c t.
+ g e t C o lo r ( ) : S t r i n g
R e t u rn s th e c o lo r .
+ s e t C o lo r ( c o l o r : S t r i n g ) : v o i d
S e ts a n e w c o lo r.
+ i s F i lle d ( ) : b o o le a n
R e t u rn s th e f i lle d p r o p e r t y .
+ s e t F i ll e d ( f i ll e d : b o o le a n ) : v o i d
S e t s a n e w f i l le d p r o p e r t y .
+ g e t D a t e C r e a t e d ( ) : j a v a .u t i l.D a t e
R e t u rn s th e d a t e C r e a t e d .
+ t o S t r in g ( ) : S t r i n g
R e t u rn s a s t r in g r e p r e s e n t a t i o n o f th i s o b j e c t .
R e c ta n g le
C ir c le
- r a d iu s : d o u b le
+ C i rc le ( )
- w i d t h : d o u b le
- h e i g h t : d o u b le
+ C i rc le ( r a d i u s : d o u b le )
+ R e c ta n g le ( )
+ g e t R a d iu s ( ) : d o u b le
+ R e c ta n g le ( w i d th : d o u b le , h e i g h t : d o u b le )
+ s e t R a d iu s ( r a d iu s : d o u b le ) : v o i d
+ g e t W i d t h ( ) : d o u b le
+ g e t A r e a ( ) : d o u b le
+ s e t W i d th ( w i d th : d o u b le ) : v o i d
+ g e t P e r i m e t e r ( ) : d o u b le
+ g e t H e i g h t ( ) : d o u b le
+ g e t D i a m e t e r ( ) : d o u b le
+ s e t H e i g h t ( h e i g h t : d o u b le ) : v o i d
+ g e t A r e a ( ) : d o u b le
+ g e t P e r i m e t e r ( ) : d o u b le
So now we can instantiate a GeometricObject as
follows:
GeometricObject g = new GeometricObject();
But does it make sense ?
Solution – Abstract Classes!
Do not allow instantiation of GeometricObject
class
How?
Declare it as abstract as follows:
public abstract class GeometricObject {
…
}
6
Abstract Class
G e o m e tr ic O b j e c t
- c o lo r : S t r i n g
T h e c o lo r o f t h e o b j e c t (d e f a u lt : w h i t e ) .
- f i ll e d : b o o le a n
In d i c a t e s w h e t h e r t h e o b j e c t i s fi lle d w i t h a c o lo r ( d e f a u lt : f a ls e ) .
- d a t e C r e a t e d : j a v a .u t i l.D a t e
T h e d a t e w h e n th e o b j e c t w a s c r e a t e d .
+ G e o m e tric O b je c t()
C re a te s a G e o m e tric O b je c t.
+ g e t C o lo r ( ) : S t r i n g
R e t u rn s th e c o lo r .
+ s e t C o lo r ( c o l o r : S t r i n g ) : v o i d
S e ts a n e w c o lo r.
+ i s F i lle d ( ) : b o o le a n
R e t u rn s th e f i lle d p r o p e r t y .
+ s e t F i ll e d ( f i ll e d : b o o le a n ) : v o i d
S e t s a n e w f i l le d p r o p e r t y .
+ g e t D a t e C r e a t e d ( ) : j a v a .u t i l.D a t e
R e t u rn s th e d a t e C r e a t e d .
+ t o S t r in g ( ) : S t r i n g
R e t u rn s a s t r in g r e p r e s e n t a t i o n o f th i s o b j e c t .
R e c ta n g le
C ir c le
- r a d iu s : d o u b le
+ C i rc le ( )
- w i d t h : d o u b le
- h e i g h t : d o u b le
+ C i rc le ( r a d i u s : d o u b le )
+ R e c ta n g le ( )
+ g e t R a d iu s ( ) : d o u b le
+ R e c ta n g le ( w i d th : d o u b le , h e i g h t : d o u b le )
+ s e t R a d iu s ( r a d iu s : d o u b le ) : v o i d
+ g e t W i d t h ( ) : d o u b le
+ g e t A r e a ( ) : d o u b le
+ s e t W i d th ( w i d th : d o u b le ) : v o i d
+ g e t P e r i m e t e r ( ) : d o u b le
+ g e t H e i g h t ( ) : d o u b le
+ g e t D i a m e t e r ( ) : d o u b le
+ s e t H e i g h t ( h e i g h t : d o u b le ) : v o i d
+ g e t A r e a ( ) : d o u b le
+ g e t P e r i m e t e r ( ) : d o u b le
Want to add methods
to compute area and
perimeter of any
GeometricObject
Abstract Class
G e o m e tr ic O b j e c t
- c o lo r : S t r i n g
T h e c o lo r o f t h e o b j e c t (d e f a u lt : w h i t e ) .
- f i ll e d : b o o le a n
In d i c a t e s w h e t h e r t h e o b j e c t i s fi lle d w i t h a c o lo r ( d e f a u lt : f a ls e ) .
- d a t e C r e a t e d : j a v a .u t i l.D a t e
T h e d a t e w h e n th e o b j e c t w a s c r e a t e d .
+ G e o m e tric O b je c t()
C re a te s a G e o m e tric O b je c t.
+ g e t C o lo r ( ) : S t r i n g
R e t u rn s th e c o lo r .
+ s e t C o lo r ( c o l o r : S t r i n g ) : v o i d
S e ts a n e w c o lo r.
+ i s F i lle d ( ) : b o o le a n
R e t u rn s th e f i lle d p r o p e r t y .
+ s e t F i ll e d ( f i ll e d : b o o le a n ) : v o i d
S e t s a n e w f i l le d p r o p e r t y .
+ g e t D a t e C r e a t e d ( ) : j a v a .u t i l.D a t e
R e t u rn s th e d a t e C r e a t e d .
+ t o S t r in g ( ) : S t r i n g
R e t u rn s a s t r in g r e p r e s e n t a t i o n o f th i s o b j e c t .
R e c ta n g le
C ir c le
- r a d iu s : d o u b le
+ C i rc le ( )
Added methods to
compute area and
perimeter for any
GeometricObject!
- w i d t h : d o u b le
- h e i g h t : d o u b le
+ C i rc le ( r a d i u s : d o u b le )
+ R e c ta n g le ( )
+ g e t R a d iu s ( ) : d o u b le
+ R e c ta n g le ( w i d th : d o u b le , h e i g h t : d o u b le )
+ s e t R a d iu s ( r a d iu s : d o u b le ) : v o i d
+ g e t W i d t h ( ) : d o u b le
+ g e t A r e a ( ) : d o u b le
+ s e t W i d th ( w i d th : d o u b le ) : v o i d
+ g e t P e r i m e t e r ( ) : d o u b le
+ g e t H e i g h t ( ) : d o u b le
+ g e t D i a m e t e r ( ) : d o u b le
+ s e t H e i g h t ( h e i g h t : d o u b le ) : v o i d
+ g e t A r e a ( ) : d o u b le
+ g e t P e r i m e t e r ( ) : d o u b le
8
public abstract class GeometricObject {
private String color = "white";
private boolean filled;
protected GeometricObject() {
filled = false;
}
public String getColor() { return color; }
public void setColor(String color) {
this.color = color;
}
public boolean isFilled() { return filled; }
public void setFilled(boolean filled) {
this.filled = filled;
}
public String toString() {
return "color: " + color + " and filled: " + filled;
}
}
9
public class Circle extends GeometricObject {
private double radius;
public Circle() { radius = 1.0;
}
public Circle(double radius) { this.radius = radius;
public double getRadius() { return radius;
public void setRadius(double radius) {
this.radius = radius;
}
}
}
public double getArea() {
return radius * radius * Math.PI;
}
public double getPerimeter() {
return 2 * radius * Math.PI;
}
}
10
public class Rectangle extends GeometricObject {
private double width;
private double height;
public Rectangle() { width = 1.0; height = 1.0;
}
public Rectangle(double width, double height) {
this.width = width; this.height = height;
}
public double getWidth() {
return width;
}
public void setWidth(double w){ width = w; }
public double getHeight() { return height; }
public void setHeight(double h) { height = h;
}
public double getArea() {
return width * height;
}
public double getPerimeter() {
return 2 * (width + height);
}
}
11
public class Main {
public static void main(String[] args) {
GeometricObject[] objects = new GeometricObject[5];
objects[0] = new Circle(5);
objects[1] = new Rectangle(5, 3);
for(int i = 0; i < objects.length; i++){
if(objects[i] != null)
System.out.println("Area: " +
objects[i].getArea()
+ "Perimeter: " +
objects[i].getPerimeter());
}
}
}
Will give error!!
cannot find symbol: getArea( ) and getPerimeter( )
public abstract class GeometricObject {
private String color = "white";
private boolean filled;
Solution
protected GeometricObject() {
filled = false;
}
public String getColor() { return color; }
public void setColor(String color) {
this.color = color;
}
public boolean isFilled() { return filled; }
public void setFilled(boolean filled) {
this.filled = filled;
}
public String toString() {
return "color: " + color + " and filled: " + filled;
}
public abstract double getArea();
public abstract double getPerimeter();
}
13
public class Main {
public static void main(String[] args) {
GeometricObject[] objects = new GeometricObject[5];
objects[0] = new Circle(5);
objects[1] = new Rectangle(5, 3);
for(int i = 0; i < objects.length; i++){
if(objects[i] != null)
System.out.println("Area: " +
objects[i].getArea()
+ "Perimeter: " +
objects[i].getPerimeter());
}
}
}
Will compile successfully now!
public abstract class GeometricObject {
private String color = "white";
private boolean filled;
protected GeometricObject() {
filled = false;
}
public String getColor() { return color; }
public void setColor(String color) {
this.color = color;
}
Why not create
concrete
getArea( ) and
getPerimeter( )
methods with
no bodies??
public boolean isFilled() { return filled; }
public void setFilled(boolean filled) {
this.filled = filled;
}
public String toString() {
return "color: " + color + " and filled: " + filled;
}
public double getArea() { return 0; }
public double getPerimeter { return 0; }
}
15
Abstract Class Characteristics
• A class that contains abstract methods must be
abstract.
• However, it is possible to declare an abstract
class that contains no abstract methods.
• A subclass can be abstract even if its superclass
is concrete.
16
Abstract Class, contd.
• All derived classes of an abstract class will have
to implement its abstract method(s), or they too
will be abstract
• An abstract class cannot be instantiated
• An abstract class can be used as a data type
though.
GeometricObject
g;
GeometricObject[] geo = new GeometricObject[10];
17
Why is the following allowed?
GeometricObject[] geo = new GeometricObject[10];
Why allow abstract classes to be used as data
types if they can not be instantiated?
18
public class Main {
public static void main(String[] args) {
GeometricObject[] objects = new GeometricObject[5];
objects[0] = new Circle(5);
objects[1] = new Rectangle(5, 3);
for(int i = 0; i < objects.length; i++){
if(objects[i] != null)
System.out.println("Area: " +
objects[i].getArea()
+ "Perimeter: " +
objects[i].getPerimeter());
}
}
}
Allows you to take advantage of polymorphism!
19
public class TestGeometricObject {
public static void main(String[] args) {
Circle c = new Circle(5);
Rectangle r1 = new Rectangle(5, 3);
Rectangle r2 = new Rectangle(3, 3);
System.out.println(“c and r1 have equal area: "
+ equalArea(c, r1));
System.out.println(“r1 and r2 have equal area: "
+ equalArea(r1, r2));
}
// what should the definition of equalArea() look like?
public static boolean equalArea( ? obj1, ? obj2) {
return obj1.getArea() == obj2.getArea();
}
}
20
public class TestGeometricObject {
Option 1
public static void main(String[] args) {
Circle c = new Circle(5);
Rectangle r1 = new Rectangle(5, 3);
Rectangle r2 = new Rectangle(3, 3);
System.out.println(“c and r1 have equal area: "
+ equalArea(c, r1));
System.out.println(“r1 and r2 have equal area: "
+ equalArea(r1, r2));
}
// what should the definition of equalArea() look like?
public static boolean equalArea( Circle o1, Rectangle o2){
return o1.getArea() == o2.getArea();
}
public static boolean equalArea( Rectangle o1,
Rectangle o2){
return o1.getArea() == o2.getArea();
21
}
public class TestGeometricObject {
public static void main(String[] args) {
Circle c = new Circle(5);
Rectangle r1 = new Rectangle(5, 3);
Rectangle r2 = new Rectangle(3, 3);
System.out.println(“c and r1 have equal area: "
+ equalArea(c, r1));
System.out.println(“r1 and r2 have equal area: "
+ equalArea(r1, r2));
}
public static boolean equalArea(GeometricObject
obj1, GeometricObject obj2) {
return obj1.getArea() == obj2.getArea();
}
}
22
public class TestGeometricObject {
public static void main(String[] args) {
GeometricObject geoObject1 = new Circle(5);
GeometricObject geoObject2 = new Rectangle(5, 3);
System.out.println(“geoObject1 and geoObject2 have
area: " + equalArea(geoObject1, geoObject2));
System.out.println(“geoObject2 and geoObject3 have
area: " + equalArea(geoObject2, geoObject3));
}
public static boolean equalArea(Object obj1, Object obj2)
{ // why not declare the parameters as Object type?
return obj1.getArea() == obj2.getArea();
}
}
Will give an error as Object class has no getArea( ) method!
23
public class TestGeometricObject {
public static void main(String[] args) {
GeometricObject geoObject1 = new Circle(5);
GeometricObject geoObject2 = new Rectangle(5, 3);
System.out.println(“geoObject1 and geoObject2 have
area: " + equalArea(geoObject1, geoObject2));
System.out.println(“geoObject2 and geoObject3 have
area: " + equalArea(geoObject2, geoObject3));
}
public static boolean equalArea(Object obj1, Object obj2)
{ // why not declare the parameters as Object type?
GeometricObject g1 = (GeometricObject)obj1;
GeometricObject g2 = (GeometricObject)obj2;
return g1.getArea() == g2.getArea();
}
}
24
Get more info!
http://java.sun.com/docs/books/tutorial/java/IandI/abstract.html
http://mindprod.com/jgloss/interfacevsabstract.html