COP 3003 Object-Oriented
Programming - Inheritance
Dr. Janusz Zalewski, Fall 2013
Prepared by Dr Dahai Guo
Outline
• Introduction to Inheritance in OOP
• Superclasses and Subclasses
• Relationship between Superclasses and
Subclasses
• Constructors in Subclasses
• Object Class
Introduction to Inheritance in OOP
• Inheritance is one of the primary features
of OOP
• It is a form of software reuse.
– Superclass vs. Subclass
– “is-a” relationship.
– A subclass can be created by absorbing an
existing superclass’s members and
embellishing them with new or modified
capability.
Superclasses and Subclasses (1/3)
• A subclass possesses all attributes and
methods of its superclass. Additionally, it
has its own attributes and methods.
– Note the subclass’ own methods can be new
or modified versions of superclass’ methods.
• The “is-a” relationship between a subclass
and superclass can be direct or indirect.
Superclasses and Subclasses (2/3)
CommunityMember
Employee
Faculty
Administrator
Student
Staff
Teacher
Alumnus
Superclasses and Subclasses (3/3)
Shape
TwoDimensionalShape
Circle
Square
Triangle
ThreeDimensionalShape
Sphere
Cube
Tetrahedron
Relationship between Superclasses and
Subclasses (1/18)
• Rules:
– A subclass can only have one direct superclass.
– All members of the superclass become members of
the subclass.
– But the superclass’ private members are NOT directly
accessible in the subclass’ methods. They have to be
accessed through the superclass’ public methods.
– The first statement of the subclass’ constructor is
invoking one of the direct superclass’ constructor.
Relationship between Superclasses and
Subclasses (2/18)
• Example:
– CommissionEmploye:
salary is only based on sales
– BasePlusCommissionEmployee:
salary = commission + baseSalary
Relationship between Superclasses and
Subclasses (3/18)
extends
CommissionEmployee
BasePlusCommissionEmployee
firstName
baseSalary
lastName
firstName
socialSecurityNumber
lastName
grossSales
socialSecurityNumber
commissionRate
grossSales
commissionRate
Relationship between Superclasses and
Subclasses (4/18)
1. public class CommissionEmployee
2.
extends Object {
3.
private String firstName;
4.
private String lastName;
5.
private String socialSecurityNumber;
6.
private double grossSales;
7.
private double commissionRate;
8. }
Every class in Java extends Object implicitly or explicitly.
Relationship between Superclasses and
Subclasses (5/18)
1. public class BasePlusCommissionEmployee
2.
extends CommissionEmployee
3. {
4.
private double baseSalary;
5.
private String firstName;
6.
private String lastName;
7.
private String socialSecurityNumber;
8.
private double grossSales;
9.
private double commissionRate;
10. }
Relationship between Superclasses and
Subclasses (6/18)
1. // in CommissionEmployee
2. public CommissionEmployee(String first,
3.
String last, String ssn, double sales,
4.
double rate) {
5.
firstName=first;
6.
lastName=last;
7.
socialSecurityNumber=ssn;
8.
setGrossSales(sales);
9.
setCommissionRate(rate);
10. }
Relationship between Superclasses and
Subclasses (7/18)
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
// in BasePlusCommissionEmployee
// BasePlusCommissionEmployee inherits CommissionEmployee
public BasePlusCommissionEmployee(String first,
String last, String ssn, double sales,
double rate, double base) {
firstName=first;
lastName=last;
Anything wrong?
socialSecurityNumber=ssn;
setGrossSales(sales);
setCommissionRate(rate);
setBaseSalary(base);
}
Relationship between Superclasses and
Subclasses (8/18)
• Rules: (cont)
– A subclass can only have one direct superclass.
– All members of the superclass become members of
the subclass.
– But the superclass’ private members are not directly
accessible in the subclass’ methods. They have to be
accessed through the superclass’ public methods.
– The first statement of the subclass’ constructor is
invoking one of the direct superclass’ constructor.
Relationship between Superclasses and
Subclasses (9/18)
1. // in BasePlusCommissionEmployee
2. public BasePlusCommissionEmployee(String first,
3.
String last, String ssn, double sales,
4.
double rate, double base){
5.
firstName=first;
6.
lastName=last;
7.
socialSecurityNumber=ssn;
8.
setGrossSales(sales);
9.
setCommissionRate(rate);
super(first, last, ssn, sales, rate);
10.
setBaseSalary(base);
// calls the superclass’
11. }
// constructor
// must be the first statement
Relationship between Superclasses and
Subclasses (10/18)
• Rules: (cont)
– A superclass’ method can be redefined in the
subclass.
Relationship between Superclasses and
Subclasses (11/18)
• Sometimes, it is desirable to directly
access superclass’ instance variables.
• Access modifier protected makes it
possible.
Relationship between Superclasses and
Subclasses (12/18)
• Rules: (cont)
– A superclass’s protected members can be
accessed by
• Members of that superclass
• Members of its subclass (direct or indirect)
• Members of other classes in the same package.
Relationship between Superclasses and
Subclasses (13/18)
• // in BasePlusCommissionEmployee
• // and commissionRate and grossSales
• // are private in the superclass.
1.public double earnings(){
2. return baseSalary+
3.
getCommissionRate()*getGrossSales();
4.}
It will be great if we can call the superclass’s method earnings.
Relationship between Superclasses and
Subclasses (14/18)
• Rules:
– The redefined methods in the superclass can
be accessed in its subclasses by preceding
the method name with “super.”
Relationship between Superclasses and
Subclasses (15/18)
• // in BasePlusCommissionEmployee
• // and commissionRate and grossSales
• // are private in the superclass.
1.public double earnings(){
2. return baseSalary+
3.
getCommissionRate()*getGrossSales();
super.earnings();
4.}
Relationship between Superclasses and
Subclasses (16/18)
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
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13.
14.
// in CommissionEmployee
public String toString(){
return String.format(“%s: %s %s\n%s: %s\n%s: %.2f\n%s %.2f”,
“commission employee”, firstName, lastName,
“ssn”, socialSecurity,
“gross sales”, grossSales,
“commission rate”, commissionRate);
}
// in BasePlusCommissionEmployee
pulbic String toString(){
return String.format(“%s %s\n%s: %.2f”,
“base-salaried”, super.toString(),
“base salary”, baseSalary);
}
Relationship between Superclasses and
Subclasses (17/18)
• Rules:
– A subclass can only have one direct superclass.
– All members of the superclass become members of
the subclass.
– But the superclass’ private members are not directly
accessible in the subclass’ methods. They have to be
accessed through the superclass’ public methods.
– The first statement of the subclass’ constructor is
invoking one of the direct superclass’ constructor.
Relationship between Superclasses and
Subclasses (18/18)
• Rules: (cont)
– A superclass’ method can be redefined in the
subclass.
– A superclass’s protected members can be accessed
by
• Members of that superclass
• Members of its subclass (direct or indirect)
• Members of other classes in the same package.
– The redefined methods in the superclass can be
accessed in its subclasses by preceding the method
name with “super.”
Object Class
• All classes in Java inherit directly or indirectly from
the Object class (package java.lang), so its 11
methods are inherited by all other classes.
–
–
–
–
–
–
–
–
–
clone()
equals()
finalize()
getClass()
hashCode()
wait() - three versions
notify()
notifyAll()
toString()