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Programming With Java
ICS 201
Chapter 8
Abstract Class
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Programming With Java
ICS 201
Abstract Class
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An abstract class is a class that has some
methods without complete definitions (Abstract
Method).
Sometimes, it does NOT make sense to create
objects from specific classes. In such case,
these classes should be created as abstract
An abstract class can only be used to derive
other classes; you cannot create objects from
an abstract class
An abstract class must have at least one
abstract method
8-2
Programming With Java
ICS 201
Abstract Method
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An abstract method serves as a
a method that will be fully
descendent class. An abstract
complete method heading with
the modifier abstract.
placeholder for
defined in a
method has a
the addition of
It has no method body but does end with a
semicolon in place of a method body.
public abstract long getSerNumber();
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An abstract method cannot be private
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An abstract class can have any number of
abstract and/or fully defined methods
Programming With Java
ICS 201
Abstract Classes (Cont’d)
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Attempting to instantiate an object of an
abstract class is a compilation error.
Example:
abstract class Shape {
…
}
Shape sh = new Shape();
Programming With Java
ICS 201
Abstract Classes (Cont’d)
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Failure to implement a superclass’s abstract
methods in a subclass is a compilation error
unless the subclass is also declared abstract.
Example:
abstract class Shape {
public abstract void drawHere();
}
class Rectangle extends Shape {
// The method drawHere() is NOT implemented!
}
Programming With Java
ICS 201
UML Inheritance Diagrams for Abstract Classes
Programming With Java
ICS 201
Creating Abstract Superclass:
Employee class
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abstract superclass Employee
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earnings is declared abstract
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No implementation can be given for
earnings in the Employee abstract class
An array of Employee variables will
store references to subclass objects

earnings method calls from these
variables will call the appropriate version
of the earnings method
Programming With Java
ICS 201
The Employee Class Hierarchy
Programming With Java
ICS 201
Abstract Class Employee: Outline
// Employee abstract superclass.
public abstract class Employee {
private String firstName;
private String lastName;
private String socialSecurityNumber;
{
// three-argument constructor
public Employee(String first, String last, String ssn )
firstName = first;
lastName = last;
socialSecurityNumber = ssn;
} // end three-argument Employee constructor
Programming With Java
ICS 201
Abstract Class Employee: Outline (Cont’d)
public void setFirstName(String first) { firstName = first;
public String getFirstName()
{
return firstName;
}
public void setLastName( String last ) { lastName = last;
public String getLastName()
{
return lastName;
}
public void setSocialSecurityNumber( String ssn ) {
socialSecurityNumber = ssn;
}
public String getSocialSecurityNumber()
return socialSecurityNumber;
{
}
public abstract double earnings(); // no implementation
here
} // end abstract class Employee
}
}
Programming With Java
ICS 201
Extending Concrete Classes
public class SalariedEmployee extends Employee {
private double weeklySalary;
…
// calculate earnings; override abstract method earnings in Employee
public double earnings() {
return getWeeklySalary();
} // end method earnings
…
}
Programming With Java
ICS 201
Extending Concrete Classes
(Cont’d)
public class CommissionEmployee extends Employee {
private double grossSales; // gross weekly sales
private double commissionRate; // commission percentage…
// calculate earnings; override abstract method earnings in Employee
public double earnings()
{
return getCommissionRate() * getGrossSales();
} // end method earnings
…
}
Programming With Java
ICS 201
Extending Concrete Classes
(Cont’d)
public class HourlyEmployee extends Employee {
private double wage; // wage per hour
private double hours; // hours worked for week
// calculate earnings; override abstract method earnings in Employee
public double earnings() {
if ( getHours() <= 40 ) // no overtime
return getWage() * getHours();
else
return 40 * getWage() + ( getHours() - 40 ) * getWage() * 1.5;
} // end method earnings
Implementing an Abstract Method
Define the speak() method as abstract in the superclass Animal.
Programming With Java
ICS 201
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public abstract class Animal {
protected String kind; // Cow, pig,
cat, etc.
public Animal() { }
public String toString() {
return "I am a " + kind + " and I
go " + speak();
}
public abstract String speak();/
• Implement
Abstract
method speak() differently in each subclass.
}
public class Cat extends Animal {public class Cow extends Animal {
public Cat() {
public Cow() {
kind = "cat";
kind = "cow";
}
}
public String speak() {
public String speak() {
return "meow";
return "moo";
}
}
}
}