Download Chapter 9: Objects and Classes

COS240 O-O Languages
AUBG, COS dept
Lecture 15
Title:
Abstract Classes and
Interfaces in Java
Reference: COS240 Syllabus
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
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Lecture Contents:
 Review
on Inheritance and Polymorphism
 Abstract Classes
 Why Abstract Methods?
 Interfaces
 Example: the Comparable Interface
 Example: the Comparable Interface
 Interfaces vs. Abstract Classes
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Inheritance
 Review
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3
Polymorphism
 Review
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4
Interfaces & Abstract Classes
Instead
of Introduction
Look at next slide
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Interfaces & Abstract Classes
A
Super class defines common behavior for
related subclasses.
 An interface can be used to define common
behavior for classes, incl. unrelated classes
 An abstract class cannot be used to create
objects. An abstract class can contain
abstract methods, which are implemented
in concrete subclasses.
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Interfaces & Abstract Classes
 Brief
presentation
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Interfaces & Abstract Classes
The Interface concept
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Interface
No method bodies – just method header.
Must provide implementation when used.
public interface Driver {
void turnWheel(double angle);
void pressAccelerator(double amount);
void pressBrake(double amount);
}
public class BusDriver implements Driver {
// must include implementation for each of the
three methods from Driver
}
9
May also have
public class BusDriver extends Person implements Driver {
// must include implementation for each of the
three methods from Driver
}
This is a “back door” approach to multiple inheritance
for Java via extends and implements.
Single inheritance via extends
and extra inheritance via implements.
10
Interfaces & Abstract Classes
The Abstract Class concept
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Abstract Classes
Similar for C#
• A class where some methods are unspecified (like in
an interface).
– The unspecified methods are declared abstract.
– The class also is declared abstract.
• An abstract class must be sub-classed (i.e.
extended) - you cannot instantiate objects of an
abstract type.
• Useful if you want to specify an “interface” along with
some default behaviors.
12
Abstract Class Example
May have defined
methods and abstract
methods.
abstract class CacheFile {
String filename;
byte[] contents;
void flush() {
// Write file to disk
}
void refresh() {
// Load file from disk
}
}
These methods are defined
These methods are abstract
abstract String deserialize();
because how you want
abstract byte[] serialize(String s); to store data into the file is
application-dependent
13
Abstract Classes & Interfaces
 Comprehensive
presentation
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Inheritance hierarchy
If you move from super class down to sub class,
classes become more specific and more concrete.
 If you move from sub class up to super class, classes
become more general and less specific
 Sometimes super class is so abstract that it cannot
have any specific instances.
 Such a class is referred to as an ABSTRACT CLASS

Cn
Cn-1
.....
C2
C1
Since o is an instance of C1, o is also an
Object
instance of C C
…, C , and Cn
2, 3,Education,n-1
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GeometricObject – Circle, Rectangle
Object
↑
GeometricObject
↑
↑
Circle
Rectangle
 Common
properties for both Circle and Rect:
– Color, filled/nofilled, dateCreated
– specific property: radius
 Rectangle – specific properties: width, height
 Circle
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GeometricObject – Circle, Rectangle
GeometricObject
↑
↑
Circle
Rectangle
 Common
properties for both Circle and Rect:
– Color, filled/nofilled, dateCreated
 Circle
– specific behavior:
– getArea(), getPerimeter()
 Rectangle
– specific behavior:
– getArea(), getPerimeter()
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GeometricObject – Circle, Rectangle
GeometricObject
↑
↑
Circle
Rectangle
 Common
properties for both Circle and Rect:
– Color, filled/nofilled, dateCreated
 Circle
– specific property and behavior:
– Radius, getArea(), getPerimeter()
 Rectangle
– specific properties and behavior:
– width, height, getArea(), getPerimeter()
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Inheritance hierarchy
Is it reasonable to generalize getArea(), getPerimeter()
methods?
 From one side: Since we can compute area and perimeter
for all geometric objects, it is better to define getArea(),
getPerimeter() as methods in super class GeometricObject.
 From other side: Both methods cannot be implemented in
the base class because their implementation depends on
specific properties (radius or height/width) of the geometric
object defined in the sub classes.
 Such methods are referred to as abstract methods and are
denoted in the super class using modifier abstract.
 Class with abstract methods becomes an abstract class and
is must to be also denoted abstract. See next slide.

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Open file ProgGeometricObjectAbstractClass.java
abstract class GeometricObject {
…
public abstract double getArea();
public abstract double getPerimeter();
}
You cannot create instances of abstract classes using new
operator.
 Constructors in abstract classes are protected because they
are used only by subclasses.
 Super class defines common features (incl. methods
getArea() and getPerimeter()). Because you don’t know
how to compute area and perimeter of geometric objects,
both methods are defined as abstract methods. These
methods are implemented/overridden in the subclasses.

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Open file ProgGeometricObjectAbstractClass.java
class Circle extends GeometricObject {
…
public double getArea()
{ return Math.PI * radius * radius; }
public double getPerimeter()
{ return 2 * Math.PI * radius; }
public String toString()//overridden m.
{return "Circle:\n"+super.toString();}
} // end of class
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Open file ProgGeometricObjectAbstractClass.java
class Rectangle extends GeometricObject {
…
public double getArea() {
return width * height;
}
public double getPerimeter() {
return 2 * (width + height);
}
public String toString(){//overridden m.
{return "RecRec:\n"+super.toString();}
} // end of class
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Open file ProgGeometricObjectAbstractClass.java
 Why
do we need abstract methods? What benefits if
any?
 Browse the main() method:
– It creates two geometric objects – circle, rectangle
– Invokes equalArea() method – have a careful look
at the formal parameters type
– Invokes displayGeometricObject() method –
have a careful look at the formal parameter type
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Open file ProgGeometricObjectAbstractClass.java
public class ProgGeometricObjectAbstractClass {
public static void main(String args[]) {
GeometricObject geo1 = new Circle(5.);
GeometricObject geo2 = new Rectangle(5., 3.);
System.out.println("The two object have same area? "+ equalArea(geo1, geo2) );
displayGeometricObject(geo1);
displayGeometricObject(geo2);
} // end of main
public static boolean equalArea( GeometricObject o1, GeometricObject o2) {
return o1.getArea() == o2.getArea();
}
public static void displayGeometricObject(GeometricObject o) {
System.out.println("The area is " + o.getArea() );
System.out.println("The perimeter is " + o.getPerimeter() );
}
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
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Open file ProgGeometricObjectAbstractClass.java
 Thanks
to declaring abstract methods in super class,
it is valid to create a connection btw super class and
sub classes through
getArea()/getPerimeter() methods and
apply the polymorphic approach or in other words:
 Reference variable of a super class type (formal
parameter) can point to an object of its sub class
type (actual argument – look at its actual type).
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
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Open file ProgGeometricObjectAbstractClass.java
 Equalarea()
method to have two parameters
GeometricObject and to compare to equality
the area of a circle and the area of a rectangle.
public static boolean equalArea(GeometricObject o1,
GeometricObject o2) {
return o1.getArea() == o2.getArea();
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
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Open file ProgGeometricObjectAbstractClass.java

displayGeometricObject() method to have
a parameter GeometricObject and to display:
– the area and perimeter of a circle or
– the area and perimeter of a rectangle

(depends on actual argument) when method called.
public static void
displayGeometricObject(GeometricObject o) {
System.out.println("The area is " + o.getArea() );
System.out.println(“Perimeter is "+o.getPerimeter());
} // end of method
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Inheritance, polymorphism and abstract classes

Given the source:
GeometricObject geo1 = new Circle(5.);
GeometricObject geo2 = new Rectangle(5., 3.);
GeometricObject[] o = { new Circle(2.), new Circle(3.),
new Rectangle(5., 3.),
new Rectangle(4.,6.) };
for (int i=0; i< o.length; i++)
System.out.println("The two objects have same area?"+
equalArea(geo1,o[i]));

Q.: Can you guess/predict the expected output?
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Inheritance, polymorphism and abstract classes

Given the source:
GeometricObject geo1 = new Circle(5.);
GeometricObject geo2 = new Rectangle(5., 3.);
GeometricObject[] o = { new Circle(2.), new Circle(3.),
new Rectangle(5., 3.),
new Rectangle(4.,6.) };
for (int i=0; i< o.length; i++)
System.out.println("The two objects have same area?"+
equalArea(geo2,o[i]));

Q.: Can you guess/predict the expected output?
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Inheritance, polymorphism and abstract classes
 Expected
–
–
–
–
–
–
output:
Geo1
Geo2
=======================
false
false
false
false
false
true
false
false
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Digression on equalArea() problem
 The
task to compare the area of a circle and the area
of a rectangle could be solved without specifying
abstract class GeometricObject and erasing
the getArea()/getPerimeter() methods
from the super class.
 BUT:
such a solution should lose the advantages
provided when using polymorphism
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Java predefined abstract classes
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Java library: Calendar - GregorianCalendar
java.util.Calendar is an abstract base class
for extracting detailed calendar info: y, m, d, h, m, s
 Subclasses of Calendar can implement specific
calendar systems:

– Gregorian calendar,
– Lunar calendar,
– Jewish calendar
 Java
supports subclass
java.util.GregorianCalendar
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Open file ProgCalendar.java
Comments:
 Calendar cl1 = new GregorianCalendar();

– cl1 declared type is Calendar
– cl1 actual type is GregorianCalendar
– Polymorphism in action: in other words a ref var of a super class
type can point to an object of its sub class type.

Task: Modify the ProgCalendar.java source file
with String dayNameOfWeek(int dayOfWeek)
method to return strings “Sunday”, “Monday”, …
“Saturday” depending on calling integer argument value 1,
2, ...7
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Interfaces
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Interfaces
 Interface
is class-like construct that contains only
constants and abstract methods.
 Interface is similar to abstract class, but its intent
is to specify common behavior for objects that
belong even to different inheritance hierarchies.
 Reminder1: Abstract class contain regular
methods and abstract methods and polymorphic
approach is within one only specific inheritance
hierarchy.
 Reminder 2: the instead of introduction slide 6.
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Interfaces

To distinguish an interface from a class, Java uses
interface reserved word:
public interface Edible {
public abstract String howToEat();
}
Each interface to save in separate file like Edible.java.
 You cannot create an instance of interface with new
 You can use interface as data type for ref var, or as a result
of casting

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Interface example

Given: Edible interface and two separate independent
inheritance hierarchies:
===================================================================================================
Animal
↑
↑
Tiger Chicken(Edible)
Fruit(Edible)
↑
↑
Apple Orange
===================================================================================================
Animal – regular base class
 Fruit – base class implements Edible interface
 Chicken – child class implements Edible interface
 Tiger – regular child class
 Apple, Orange – regular child classes

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Open files Edible.java & ProgInterfaceEdible.java
using Edible interface to specify which
object is edible, in other words has implemented
method howToEat().
 Task:
 Browse
the source text
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Open files Edible.java & ProgInterfaceEdible.java
 Chicken
extends Animal, implements Edible to
specify that chickens are edible, implements
method howToEat()
 Tiger
extends Animal, does not implement Edible
i.e. tigers are not edible, no need to implement
method howToEat()
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Open files Edible.java & ProgInterfaceEdible.java
 Fruit
implements Edible, and does not
implement howToEat() method.
 Therefore Fruit is to be modified as abstract
class and
 concrete subclasses of Fruit must implement
howToEat() method, as it is done with Apple
class and Orange class
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Open files Edible.java & ProgInterfaceEdible.java
 Run
the application
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Java supported interfaces
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Java supported interfaces: Comparable
Problem: we need a method to return the larger of two
objects of the same type:
 Two students, Two dates, Two circles, Two rectangles
 It is must the two objects to be comparable
 Comparability is considered a common behavior
 Java provides Comparable interface for this purpose
package java.lang;
public interface Comparable {
public int compareTo(Object o);
{

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Java supported interfaces: Comparable

Many Java predefined classes (String, Date) implement
Comparable interface to define an order for the objects. String, Date
classes provide compareTo() method. Their definition is like this:
//------------------------------------public class String extends Object
implements Comparable {
// class body
}
//------------------------------------Public class Date extends Object
implements Comparable {
// class body
}
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Java supported interfaces: Comparable
 Given
String s;
Date d;
//------------------------------(s instanceof String)
is
true
(s instanceof Object)
is
true
(s instanceof Comparable)
is
true
//------------------------------(d instanceof Date)
is
true
(d instanceof Object)
is
true
(d instanceof Comparable)
is
true
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User Defined class implement interfaces: Comparable
 Reminder
: class Rectangle
 We cannot compare instances of Rectangle,
because Rectangle does not implement
Comparable interface
 How to proceed?
– Re edit Rectangle class to implement Comparable
OR
– Create new class ComparableRectangle to extend
Rectangle and to implement Comparable
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User Defined class implement interfaces: Comparable
 Solution
scheme for the second option:
GeometricObject
↑
Rectangle
Comparable
↑
↑
ComparableRectangle - - - - -
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User Defined class implement interfaces: Comparable
Open source file ProgComparableRectangle.java
 Source text skeleton for ComparableRectangle

public class ComparableRectangle
extends Rectangle
implements Comparable {
public ComparableRectangle(…) { …}
public int compareTo(Object o) {…}
}
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User Defined class implement interfaces: Comparable

How to use the compareTo() method
ComparableRectangle r1 = new ComparableRectangle(4.,5.);
ComparableRectangle r2 = new ComparableRectangle(4.,6.);
ComparableRectangle r3 = new ComparableRectangle(5.,4.);
System.out.println(r1.compareTo(r2));
System.out.println(r1.compareTo(r3));
System.out.println(r2.compareTo(r1));
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Java supported interfaces: ActionListener
 Run
the application
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Java supported interfaces: Cloneable
 Run
the application
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Conclusion
Abstract
class contain
– regular methods and
– abstract methods and
polymorphic
approach is within one
only specific inheritance hierarchy.
GeometricObject, Circe, Rectangle
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
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Conclusion
 Interface
contains only abstract methods.
 Interface is similar to abstract class, but its
intent is to specify common behavior for
objects that belong even to different
inheritance hierarchies
 Animal, Tiger, Chicken (Edible interface)
 Fruit (Edible interface), Apple, Orange
 Interface is a back door to implement multiple
inheritance in Java
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
Thank You
for
Your attention!
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