ObjectOriented Programming: Inheritance

1
Object-Oriented ProgrammingInheritance

• Chapter 9

2
Introduction

• Besides composition, another form of reuse is
  inheritance.
• With inheritance, an object can inherit behavior
  from another object, thus reusing its code.
• The class inheriting is called a subclass (or
  derived class).
• The class inherited from is called a superclass
  (or base class).

3
Subclass

• A subclass usually adds to its superclass, thus
  creating a more specialized object.
• A subclass may be the superclass for its own
  subclass, thus creating a class hierarchy.
• The immediate superclass for a subclass is called
  its direct superclass.
• A superclass above the direct superclass is
  called an indirect superclass.
• In Java, the Object class is a direct or indirect
  superclass to all other classes.

4
Multiple Inheritance

• Java, unlike C, does not support multiple
  inheritance (Java allows only one direct
  superclass).
• Java uses interfaces to provide some of this
  capability.
• With an interface, multiple classes can support
  some of the same behavior.

5
Inheritance

• Many projects deal with closely-related cases.
• Coding for these minor differences in a large
  project can obscure the overall effort.
• With inheritance, a superclass can define the
  general behavior, then each subclass can
  specialize this behavior.

6
Inheritance Relationships

• Inheritance is known as an is-a relationship.
• Composition is a has-a relationship.
• A car is a vehicle.
• A car has a steering wheel.

7
Protected Members

• If a member is given the protected access
  modifier, then subclasses of that class may
  access it, as well as classes in the same package

8
Overriding Methods

• A subclass may override a method of a superclass
  by declaring a method with the same signature.
• When overriding a method, its access modifier
  cannot be changed to be more restrictive (for
  example, public to private).
• This is so the subclass can respond to the same
  messages as the superclass.

9
Examples

• The next several examples illustrate inheritance.
• A commissioned employee is created.
• A basecommission employee is created without
  inheriting from the commissioned employee class.
• A basecommission employee is created by
  inheriting from commissioned employee, but cannot
  access private fields.
• The commissioned employee is changed to have
  protected variables so that a basecommission
  employee can access them.
• The commissioned employee fields are made private
  again but given accessors for the subclass
  basecommission employee to use.

10
// Fig. 9.4 CommissionEmployee.java //
CommissionEmployee class represents a commission
employee. public class CommissionEmployee
extends Object // extends Object not required
here private String firstName private
String lastName private String
socialSecurityNumber private double
grossSales // gross weekly sales private
double commissionRate // commission percentage
// five-argument constructor //
constructors are NOT inherited but are implicitly
or explicitly called. // if no constructor,
the default constructor calls the superclass
constructor public CommissionEmployee( String
first, String last, String ssn, double
sales, double rate ) // implicit call
to Object default constructor occurs here
firstName first lastName last
socialSecurityNumber ssn setGrossSales(
sales ) // validate and store gross sales
setCommissionRate( rate ) // validate and store
commission rate // end five-argument
CommissionEmployee constructor
11
// set first name public void setFirstName(
String first ) firstName first
// end method setFirstName // return first
name public String getFirstName()
return firstName // end method
getFirstName // set last name public void
setLastName( String last ) lastName
last // end method setLastName //
return last name public String getLastName()
return lastName // end method
getLastName
12
// set social security number public void
setSocialSecurityNumber( String ssn )
socialSecurityNumber ssn // should validate
// end method setSocialSecurityNumber //
return social security number public String
getSocialSecurityNumber() return
socialSecurityNumber // end method
getSocialSecurityNumber // set commission
employee's gross sales amount public void
setGrossSales( double sales )
grossSales ( sales lt 0.0 ) ? 0.0 sales
// end method setGrossSales // return
commission employee's gross sales amount
public double getGrossSales() return
grossSales // end method getGrossSales
13
// set commission employee's rate public
void setCommissionRate( double rate )
commissionRate ( rate gt 0.0 rate lt 1.0 ) ?
rate 0.0 // end method setCommissionRate
// return commission employee's rate public
double getCommissionRate() return
commissionRate // end method
getCommissionRate // calculate commission
employee's pay public double earnings()
return commissionRate grossSales //
end method earnings // return String
representation of CommissionEmployee object
public String toString() return
String.format( "s s s\ns s\ns .2f\ns
.2f", "commission employee",
firstName, lastName, "social security
number", socialSecurityNumber, "gross
sales", grossSales, "commission rate",
commissionRate ) // end method toString
// end class CommissionEmployee
14

• // Fig. 9.5 CommissionEmployeeTest.java.
  Testing class CommissionEmployee.
• public class CommissionEmployeeTest
• public static void main( String args )
• // instantiate CommissionEmployee object
• CommissionEmployee employee new
  CommissionEmployee(
• "Sue", "Jones", "222-22-2222", 10000,
  .06 )
• // get commission employee data
• System.out.println( "Employee information
  obtained by get methods \n" )
• System.out.printf( "s s\n", "First name
  is", employee.getFirstName() )
• System.out.printf( "s s\n", "Last name
  is", employee.getLastName() )
• System.out.printf( "s s\n", "Social
  security number is",
• 
  employee.getSocialSecurityNumber() )
• System.out.printf( "s .2f\n", "Gross
  sales is", employee.getGrossSales() )
• System.out.printf( "s .2f\n", "Commission
  rate is", employee.getCommissionRate() )

15

• // Fig. 9.6 BasePlusCommissionEmployee.java
• // BasePlusCommissionEmployee class represents an
  employee that receives
• // a base salary in addition to commission.
• public class BasePlusCommissionEmployee //
  extends Object by default
• private String firstName
• private String lastName
• private String socialSecurityNumber
• private double grossSales // gross weekly
  sales
• private double commissionRate // commission
  percentage
• private double baseSalary // base salary per
  week
• // six-argument constructor
• public BasePlusCommissionEmployee( String
  first, String last,
• String ssn, double sales, double rate,
  double salary )
• // implicit call to Object constructor
  occurs here
• firstName first

same as commissioned employee
new to this class
16

• // set first name
• public void setFirstName( String first )
• firstName first
• // end method setFirstName
• // return first name
• public String getFirstName()
• return firstName
• // end method getFirstName
• // set last name
• public void setLastName( String last )
• lastName last
• // end method setLastName
• // return last name

17

• // set social security number
• public void setSocialSecurityNumber( String
  ssn )
• socialSecurityNumber ssn // should
  validate
• // end method setSocialSecurityNumber
• // return social security number
• public String getSocialSecurityNumber()
• return socialSecurityNumber
• // end method getSocialSecurityNumber
• // set gross sales amount
• public void setGrossSales( double sales )
• grossSales ( sales lt 0.0 ) ? 0.0 sales
• // end method setGrossSales
• // return gross sales amount

18

• // set commission rate
• public void setCommissionRate( double rate )
• commissionRate ( rate gt 0.0 rate lt 1.0
  ) ? rate 0.0
• // end method setCommissionRate
• // return commission rate
• public double getCommissionRate()
• return commissionRate
• // end method getCommissionRate
• // set base salary
• public void setBaseSalary( double salary )
• baseSalary ( salary lt 0.0 ) ? 0.0
  salary
• // end method setBaseSalary
• // return base salary

19

• // calculate earnings
• public double earnings()
• return baseSalary ( commissionRate
  grossSales )
• // end method earnings
• // return String representation of
  BasePlusCommissionEmployee
• public String toString()
• return String.format(
• "s s s\ns s\ns .2f\ns
  .2f\ns .2f",
• "base-salaried commission employee",
  firstName, lastName,
• "social security number",
  socialSecurityNumber,
• "gross sales", grossSales, "commission
  rate", commissionRate,
• "base salary", baseSalary )
• // end method toString
• // end class BasePlusCommissionEmployee

20

• // Fig. 9.7 BasePlusCommissionEmployeeTest.java
• // Testing class BasePlusCommissionEmployee
• public class BasePlusCommissionEmployeeTest
• public static void main( String args )
• // instantiate BasePlusCommissionEmployee
  object
• BasePlusCommissionEmployee employee
• new BasePlusCommissionEmployee( "Bob",
  "Lewis", "333-33-3333", 5000, .04, 300 )
• // get base-salaried commission employee
  data
• System.out.println( "Employee information
  obtained by get methods \n" )
• System.out.printf( "s s\n", "First name
  is", employee.getFirstName() )
• System.out.printf( "s s\n", "Last name
  is", employee.getLastName() )
• System.out.printf( "s s\n", "Social
  security number is",
• employee.getSocialSecurityNumber() )
• System.out.printf( "s .2f\n", "Gross
  sales is", employee.getGrossSales() )
• System.out.printf( "s .2f\n", "Commission
  rate is",

21

• // Fig. 9.8 BasePlusCommissionEmployee2.java
• // BasePlusCommissionEmployee2 inherits from
  class CommissionEmployee.
• public class BasePlusCommissionEmployee2 extends
  CommissionEmployee
• private double baseSalary // base salary per
  week
• // six-argument constructor
• public BasePlusCommissionEmployee2( String
  first, String last,
• String ssn, double sales, double rate,
  double salary )
• // explicit call to superclass
  CommissionEmployee constructor
• super( first, last, ssn, sales, rate )
• setBaseSalary( salary ) // validate and
  store base salary
• // end six-argument BasePlusCommissionEmploye
  e2 constructor
• // set base salary
• public void setBaseSalary( double salary )

inheritance
just need to declare new field
22

• // return base salary
• public double getBaseSalary()
• return baseSalary
• // end method getBaseSalary
• // calculate earnings
• public double earnings()
• // not allowed commissionRate and
  grossSales private in superclass
• return baseSalary ( commissionRate
  grossSales )
• // end method earnings
• // return String representation of
  BasePlusCommissionEmployee2
• public String toString()
• // not allowed attempts to access private
  superclass members
• return String.format(

23

• // Fig. 9.9 CommissionEmployee2.java
• // CommissionEmployee2 class represents a
  commission employee.
• public class CommissionEmployee2
• protected String firstName
• protected String lastName
• protected String socialSecurityNumber
• protected double grossSales // gross weekly
  sales
• protected double commissionRate // commission
  percentage
• // five-argument constructor
• public CommissionEmployee2( String first,
  String last, String ssn,
• double sales, double rate )
• // implicit call to Object constructor
  occurs here
• firstName first
• lastName last

use protected fields
24

• // set first name
• public void setFirstName( String first )
• firstName first
• // end method setFirstName
• // return first name
• public String getFirstName()
• return firstName
• // end method getFirstName
• // set last name
• public void setLastName( String last )
• lastName last
• // end method setLastName
• // return last name

25

• // set social security number
• public void setSocialSecurityNumber( String
  ssn )
• socialSecurityNumber ssn // should
  validate
• // end method setSocialSecurityNumber
• // return social security number
• public String getSocialSecurityNumber()
• return socialSecurityNumber
• // end method getSocialSecurityNumber
• // set gross sales amount
• public void setGrossSales( double sales )
• grossSales ( sales lt 0.0 ) ? 0.0 sales
• // end method setGrossSales
• // return gross sales amount

26

• // set commission rate
• public void setCommissionRate( double rate )
• commissionRate ( rate gt 0.0 rate lt 1.0
  ) ? rate 0.0
• // end method setCommissionRate
• // return commission rate
• public double getCommissionRate()
• return commissionRate
• // end method getCommissionRate
• // calculate earnings
• public double earnings()
• return commissionRate grossSales
• // end method earnings
• // return String representation of
  CommissionEmployee2 object

27

• // Fig. 9.10 BasePlusCommissionEmployee3.java
• // BasePlusCommissionEmployee3 inherits from
  CommissionEmployee2 and has
• // access to CommissionEmployee2's protected
  members.
• public class BasePlusCommissionEmployee3 extends
  CommissionEmployee2
• private double baseSalary // base salary per
  week
• // six-argument constructor
• public BasePlusCommissionEmployee3( String
  first, String last,
• String ssn, double sales, double rate,
  double salary )
• super( first, last, ssn, sales, rate )
• setBaseSalary( salary ) // validate and
  store base salary
• // end six-argument BasePlusCommissionEmploye
  e3 constructor
• // set base salary
• public void setBaseSalary( double salary )

inheritance
just need to declare new field, can
access protected members of superclass
explicit constructor call base class doesnt have
default constructor to call implicitly
28

• // return base salary
• public double getBaseSalary()
• return baseSalary
• // end method getBaseSalary
• // calculate earnings
• public double earnings()
• return baseSalary ( commissionRate
  grossSales )
• // end method earnings
• // return String representation of
  BasePlusCommissionEmployee3
• public String toString()
• return String.format(
• "s s s\ns s\ns .2f\ns
  .2f\ns .2f",
• "base-salaried commission employee",
  firstName, lastName,
• "social security number",
  socialSecurityNumber,

notice how much less code there is due to
inheritance
29

• // Fig. 9.11 BasePlusCommissionEmployeeTest3.java
  
• // Testing class BasePlusCommissionEmployee3.
• public class BasePlusCommissionEmployeeTest3
• public static void main( String args )
• // instantiate BasePlusCommissionEmployee3
  object
• BasePlusCommissionEmployee3
  basePlusCommissionEmployee
• new BasePlusCommissionEmployee3(
• "Bob", "Lewis", "333-33-3333", 5000,
  .04, 300 )
• // get base-salaried commission employee
  data
• System.out.println( "Employee information
  obtained by get methods \n" )
• System.out.printf( "s s\n", "First name
  is",
• basePlusCommissionEmployee.getFirstName()
  )
• System.out.printf( "s s\n", "Last name
  is",
• basePlusCommissionEmployee.getLastName()
  )
• System.out.printf( "s s\n", "Social
  security number is",

30

• basePlusCommissionEmployee.setBaseSalary(
  1000 ) // set base salary
• System.out.printf( "\ns\n\ns\n",
• "Updated employee information obtained
  by toString",
• basePlusCommissionEmployee.toString() )
• // end main
• // end class BasePlusCommissionEmployeeTest3

31
Protected Instance Variables

• Our textbook recommends using private superclass
  variables rather than protected.
• This forces the subclass to use the get and set
  methods of the superclass.
• This is better since any validation or
  formatting/conversion is not bypassed.
• The next example takes this approach.

32

• // Fig. 9.12 CommissionEmployee3.java
• // CommissionEmployee3 class represents a
  commission employee.
• public class CommissionEmployee3
• private String firstName
• private String lastName
• private String socialSecurityNumber
• private double grossSales // gross weekly
  sales
• private double commissionRate // commission
  percentage
• // five-argument constructor
• public CommissionEmployee3( String first,
  String last, String ssn,
• double sales, double rate )
• // implicit call to Object constructor
  occurs here
• firstName first
• lastName last
• socialSecurityNumber ssn

use private fields
33

• // set first name
• public void setFirstName( String first )
• firstName first
• // end method setFirstName
• // return first name
• public String getFirstName()
• return firstName
• // end method getFirstName
• // set last name
• public void setLastName( String last )
• lastName last
• // end method setLastName
• // return last name

34

• // set social security number
• public void setSocialSecurityNumber( String
  ssn )
• socialSecurityNumber ssn // should
  validate
• // end method setSocialSecurityNumber
• // return social security number
• public String getSocialSecurityNumber()
• return socialSecurityNumber
• // end method getSocialSecurityNumber
• // set gross sales amount
• public void setGrossSales( double sales )
• grossSales ( sales lt 0.0 ) ? 0.0 sales
• // end method setGrossSales
• // return gross sales amount

35

• // set commission rate
• public void setCommissionRate( double rate )
• commissionRate ( rate gt 0.0 rate lt 1.0
  ) ? rate 0.0
• // end method setCommissionRate
• // return commission rate
• public double getCommissionRate()
• return commissionRate
• // end method getCommissionRate
• // calculate earnings
• public double earnings()
• return getCommissionRate()
  getGrossSales()
• // end method earnings
• // return String representation of
  CommissionEmployee3 object

36

• // Fig. 9.13 BasePlusCommissionEmployee4.java
• // BasePlusCommissionEmployee4 class inherits
  from CommissionEmployee3 and
• // accesses CommissionEmployee3's private data
  via CommissionEmployee3's
• // public methods.
• public class BasePlusCommissionEmployee4 extends
  CommissionEmployee3
• private double baseSalary // base salary per
  week
• // six-argument constructor
• public BasePlusCommissionEmployee4( String
  first, String last,
• String ssn, double sales, double rate,
  double salary )
• super( first, last, ssn, sales, rate )
• setBaseSalary( salary ) // validate and
  store base salary
• // end six-argument BasePlusCommissionEmploye
  e4 constructor
• // set base salary
• public void setBaseSalary( double salary )

inheritance
just need to declare new field, can
access private members of superclass
with get/set methods
37

• // return base salary
• public double getBaseSalary()
• return baseSalary
• // end method getBaseSalary
• // calculate earnings
• public double earnings()
• return getBaseSalary() super.earnings()
• // end method earnings
• // return String representation of
  BasePlusCommissionEmployee4
• public String toString()
• return String.format( "s s\ns .2f",
  "base-salaried",
• super.toString(), "base salary",
  getBaseSalary() )
• // end method toString

38

• // Fig. 9.14 BasePlusCommissionEmployeeTest4.java
  
• // Testing class BasePlusCommissionEmployee4.
• public class BasePlusCommissionEmployeeTest4
• public static void main( String args )
• // instantiate BasePlusCommissionEmployee4
  object
• BasePlusCommissionEmployee4 employee
• new BasePlusCommissionEmployee4(
• "Bob", "Lewis", "333-33-3333", 5000,
  .04, 300 )
• // get base-salaried commission employee
  data
• System.out.println(
• "Employee information obtained by get
  methods \n" )
• System.out.printf( "s s\n", "First name
  is",
• employee.getFirstName() )
• System.out.printf( "s s\n", "Last name
  is",
• employee.getLastName() )

39

• System.out.printf( "s .2f\n", "Commission
  rate is",
• employee.getCommissionRate() )
• System.out.printf( "s .2f\n", "Base
  salary is",
• employee.getBaseSalary() )
• employee.setBaseSalary( 1000 ) // set base
  salary
• System.out.printf( "\ns\n\ns\n",
• "Updated employee information obtained
  by toString",
• employee.toString() )
• // end main
• // end class BasePlusCommissionEmployeeTest4

40

• // Fig. 9.15 CommissionEmployee4.java,
  demonstrates constructor calls
• // CommissionEmployee4 class represents a
  commission employee.
• public class CommissionEmployee4
• private String firstName
• private String lastName
• private String socialSecurityNumber
• private double grossSales // gross weekly
  sales
• private double commissionRate // commission
  percentage
• // five-argument constructor
• public CommissionEmployee4( String first,
  String last, String ssn,
• double sales, double rate )
• // implicit call to Object constructor
  occurs here
• firstName first
• lastName last

prints message when constructor is called
41

• // set first name
• public void setFirstName( String first )
• firstName first
• // end method setFirstName
• // return first name
• public String getFirstName()
• return firstName
• // end method getFirstName
• // set last name
• public void setLastName( String last )
• lastName last
• // end method setLastName
• // return last name

42

• // set social security number
• public void setSocialSecurityNumber( String
  ssn )
• socialSecurityNumber ssn // should
  validate
• // end method setSocialSecurityNumber
• // return social security number
• public String getSocialSecurityNumber()
• return socialSecurityNumber
• // end method getSocialSecurityNumber
• // set gross sales amount
• public void setGrossSales( double sales )
• grossSales ( sales lt 0.0 ) ? 0.0 sales
• // end method setGrossSales
• // return gross sales amount

43

• // set commission rate
• public void setCommissionRate( double rate )
• commissionRate ( rate gt 0.0 rate lt 1.0
  ) ? rate 0.0
• // end method setCommissionRate
• // return commission rate
• public double getCommissionRate()
• return commissionRate
• // end method getCommissionRate
• // calculate earnings
• public double earnings()
• return getCommissionRate()
  getGrossSales()
• // end method earnings
• // return String representation of
  CommissionEmployee4 object

44

• // Fig. 9.16 BasePlusCommissionEmployee5.java,
  demonstrates constructor calls
• // BasePlusCommissionEmployee5 class declaration.
• public class BasePlusCommissionEmployee5 extends
  CommissionEmployee4
• private double baseSalary // base salary per
  week
• // six-argument constructor
• public BasePlusCommissionEmployee5( String
  first, String last,
• String ssn, double sales, double rate,
  double salary )
• super( first, last, ssn, sales, rate )
• setBaseSalary( salary ) // validate and
  store base salary
• System.out.printf(
• "\nBasePlusCommissionEmployee5
  constructor\ns\n", this )
• // end six-argument BasePlusCommissionEmploye
  e5 constructor
• // set base salary

prints message when constructor is called
45

• // return base salary
• public double getBaseSalary()
• return baseSalary
• // end method getBaseSalary
• // calculate earnings
• public double earnings()
• return getBaseSalary() super.earnings()
• // end method earnings
• // return String representation of
  BasePlusCommissionEmployee5
• public String toString()
• return String.format( "s s\ns .2f",
  "base-salaried",
• super.toString(), "base salary",
  getBaseSalary() )
• // end method toString
• // end class BasePlusCommissionEmployee5

46

• // Fig. 9.17 ConstructorTest.java, demonstrates
  constructor calls
• // Display order in which superclass and subclass
  constructors are called.
• public class ConstructorTest
• public static void main( String args )
• CommissionEmployee4 employee1 new
  CommissionEmployee4(
• "Bob", "Lewis", "333-33-3333", 5000, .04
  )
• System.out.println()
• BasePlusCommissionEmployee5 employee2
• new BasePlusCommissionEmployee5(
• "Lisa", "Jones", "555-55-5555", 2000,
  .06, 800 )
• System.out.println()
• BasePlusCommissionEmployee5 employee3
• new BasePlusCommissionEmployee5(
• "Mark", "Sands", "888-88-8888", 8000,
  .15, 2000 )

47

• CommissionEmployee4 constructor
• commission employee Bob Lewis
• social security number 333-33-3333
• gross sales 5000.00
• commission rate 0.04
• CommissionEmployee4 constructor
• base-salaried commission employee Lisa Jones
• social security number 555-55-5555
• gross sales 2000.00
• commission rate 0.06
• base salary 0.00
• CommissionEmployee5 constructor
• base-salaried commission employee Lisa Jones
• social security number 555-55-5555
• gross sales 2000.00
• commission rate 0.06

• CommissionEmployee4 constructor
• base-salaried commission employee Mark Sands
• social security number 888-88-8888
• gross sales 8000.00
• commission rate 0.15
• base salary 0.00
• CommissionEmployee5 constructor
• base-salaried commission employee Lisa Jones
• social security number 888-88-8888
• gross sales 8000.00
• commission rate 0.15
• base salary 2000.00

48
Inheritance

• Inheritance may be performed without access to
  the superclass source file.
• Java simply requires access to the .class file.
• This is good for independent software vendors
  (ISVs) who do not wish to expose their
  sourcecode.
• Having the sourcecode is usually best so you can
  inspect and evaluate it.

49
Inheritance

• Reading subclass declarations can be confusing
  because the sourcecode being called exists in the
  superclass.
• Documentation can be difficult to do for this
  reason as well.

50
Object Class Methods
51
Graphics

• Displaying labels can be accomplished with class
  JLabel.
• JLabel.BorderLayout contains constants for 5
  regions of the frame NORTH, SOUTH, EAST, WEST,
  CENTER
• Class ImageIcon lets an image be displayed on a
  label.
• Method setText changes text on a label.

52

• // Fig 9.19 LabelDemo.java
• // Demonstrates the use of labels.
• import java.awt.BorderLayout
• import javax.swing.ImageIcon
• import javax.swing.JLabel
• import javax.swing.JFrame
• public class LabelDemo
• public static void main( String args )
• // Create a label with plain text
• JLabel northLabel new JLabel( "North" )
• // create an icon from an image so we can
  put it on a JLabel
• ImageIcon labelIcon new ImageIcon(
  "GUItip.gif" )
• // create a label with an Icon instead of
  text
• JLabel centerLabel new JLabel( labelIcon
  )

53

• // create a frame to hold the labels
• JFrame application new JFrame()
• application.setDefaultCloseOperation(
  JFrame.EXIT_ON_CLOSE )
• // add the labels to the frame the second
  argument specifies
• // where on the frame to add the label
• application.add( northLabel,
  BorderLayout.NORTH )
• application.add( centerLabel,
  BorderLayout.CENTER )
• application.add( southLabel,
  BorderLayout.SOUTH )
• application.setSize( 300, 300 ) // set the
  size of the frame
• application.setVisible( true ) // show the
  frame
• // end main
• // end class LabelDemo

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