JavaHTP6e_10.ppt

 2005 Pearson Education, Inc. All rights reserved.
1
10
Object-Oriented
Programming:
Polymorphism

 2005 Pearson Education Inc. All rights reserved.
2
One Ring to rule them all, One Ring to find them, One Ring to
bring them all and in the darkness bind them.
— John Ronald Reuel Tolkien
General propositions do not decide concrete cases.
— Oliver Wendell Holmes
A philosopher of imposing stature doesn’t think in a vacuum.
Even his most abstract ideas are, to some extent, conditioned by
what is or is not known in the time when he lives.
— Alfred North Whitehead
Why art thou cast down, O my soul?
— Psalms 42:5

3
OBJECTIVES
In this chapter you will learn:
 The concept of polymorphism.
 To use overridden methods to effect
polymorphism.
 To distinguish between abstract and concrete
classes.
 To declare abstract methods to create abstract
classes.
 How polymorphism makes systems extensible
and maintainable.
 To determine an object's type at execution time.
 To declare and implement interfaces.

4
• 10.1 Introduction
• 10.2 Polymorphism Examples
• 10.3 Demonstrating Polymorphic Behavior
• 10.4 Abstract Classes and Methods
• 10.5 Case Study: Payroll System Using
Polymorphism
• 10.5.1 Creating Abstract Superclass
Employee
• 10.5.2 Creating Concrete Subclass
SalariedEmployee
HourlyEmployee
CommissionEmployee
• 10.5.5 Creating Indirect Concrete Subclass
BasePlusCommissionEmployee
• 10.5.6 Demonstrating Polymorphic Processing,
Operator instanceof and Downcasting
• 10.5.7 Summary of the Allowed Assignments
Between Superclass and Subclass Variables
• 10.6 final Methods and Classes

5
• 10.7 Case Study: Creating and Using Interfaces
• 10.7.1 Developing a Payable Hierarchy
• 10.7.2 Declaring Interface Payable
• 10.7.3 Creating Class Invoice
• 10.7.4 Modifying Class Employee to Implement
Interface Payable
• 10.7.5 Modifying Class SalariedEmployee for
Use in the Payable Hierarchy
• 10.7.6 Using Interface Payable to Process
Invoices and Employees
Polymorphically
• 10.7.7 Declaring Constants with Interfaces
• 10.7.8 Common Interfaces of the Java API
• 10.8 (Optional) GUI and Graphics Case Study: Drawing
with Polymorphism
• 10.9 (Optional) Software Engineering Case Study:
Incorporating Inheritance into the ATM System
• 10.10 Wrap-Up

6
10.1 Introduction
• Polymorphism
– Enables “programming in the general”
– The same invocation can produce “many forms” of results
• Interfaces
– Implemented by classes to assign common functionality to
possibly unrelated classes

7
10.2 Polymorphism Examples
• Polymorphism
– When a program invokes a method through a superclass
variable, the correct subclass version of the method is
called, based on the type of the reference stored in the
superclass variable
– The same method name and signature can cause different
actions to occur, depending on the type of object on which
the method is invoked
– Facilitates adding new classes to a system with minimal
modifications to the system’s code

8
Software Engineering Observation 10.1
Polymorphism enables programmers to deal in
generalities and let the execution-time environment
handle the specifics. Programmers can command
objects to behave in manners appropriate to those
objects, without knowing the types of the objects
(as long as the objects belong to the same
inheritance hierarchy).

9
Polymorphism promotes extensibility: Software
that invokes polymorphic behavior is
independent of the object types to which
messages are sent. New object types that can
respond to existing method calls can be
incorporated into a system without requiring
modification of the base system. Only client code
that instantiates new objects must be modified to
accommodate new types.

10
10.3 Demonstrating Polymorphic
Behavior
• A superclass reference can be aimed at a subclass
object
– This is possible because a subclass object is a superclass
object as well
– When invoking a method from that reference, the type of
the actual referenced object, not the type of the reference,
determines which method is called
• A subclass reference can be aimed at a superclass
object only if the object is downcasted

 2005 Pearson Education,
Inc. All rights reserved.
11
Outline
PolymorphismTest
.java
(1 of 2)
1 // Fig. 10.1: PolymorphismTest.java
2 // Assigning superclass and subclass references to superclass and
3 // subclass variables.
4
5 public class PolymorphismTest
6 {
7 public static void main( String args[] )
8 {
9 // assign superclass reference to superclass variable
10 CommissionEmployee3 commissionEmployee = new CommissionEmployee3(
11 "Sue", "Jones", "222-22-2222", 10000, .06 );
12
13 // assign subclass reference to subclass variable
14 BasePlusCommissionEmployee4 basePlusCommissionEmployee =
15 new BasePlusCommissionEmployee4(
16 "Bob", "Lewis", "333-33-3333", 5000, .04, 300 );
17
18 // invoke toString on superclass object using superclass variable
19 System.out.printf( "%s %s:nn%snn",
20 "Call CommissionEmployee3's toString with superclass reference ",
21 "to superclass object", commissionEmployee.toString() );
22
23 // invoke toString on subclass object using subclass variable
24 System.out.printf( "%s %s:nn%snn",
25 "Call BasePlusCommissionEmployee4's toString with subclass",
26 "reference to subclass object",
27 basePlusCommissionEmployee.toString() );
28
Typical reference assignments

12
Outline
PolymorphismTest
.java
(2 of 2)
29 // invoke toString on subclass object using superclass variable
30 CommissionEmployee3 commissionEmployee2 =
31 basePlusCommissionEmployee;
32 System.out.printf( "%s %s:nn%sn",
33 "Call BasePlusCommissionEmployee4's toString with superclass",
34 "reference to subclass object", commissionEmployee2.toString() );
35 } // end main
36 } // end class PolymorphismTest
Call CommissionEmployee3's toString with superclass reference to superclass
object:
commission employee: Sue Jones
social security number: 222-22-2222
gross sales: 10000.00
commission rate: 0.06
Call BasePlusCommissionEmployee4's toString with subclass reference to
subclass object:
base-salaried commission employee: Bob Lewis
base salary: 300.00
Call BasePlusCommissionEmployee4's toString with superclass reference to
subclass object:
base salary: 300.00
Assign a reference to a
basePlusCommissionEmployee object
to a CommissionEmployee3 variable
Polymorphically call
basePlusCommissionEmployee’s
toString method

13
10.4 Abstract Classes and Methods
• Abstract classes
– Classes that are too general to create real objects
– Used only as abstract superclasses for concrete subclasses
and to declare reference variables
– Many inheritance hierarchies have abstract superclasses
occupying the top few levels
– Keyword abstract
• Use to declare a class abstract
• Also use to declare a method abstract
– Abstract classes normally contain one or more abstract
methods
– All concrete subclasses must override all inherited
abstract methods

14
10.4 Abstract Classes and Methods
(Cont.)
•Iterator class
– Traverses all the objects in a collection, such as an array
– Often used in polymorphic programming to traverse a
collection that contains references to objects from various
levels of a hierarchy

15
An abstract class declares common attributes and
behaviors of the various classes in a class
hierarchy. An abstract class typically contains one
or more abstract methods that subclasses must
override if the subclasses are to be concrete. The
instance variables and concrete methods of an
abstract class are subject to the normal rules of
inheritance.

16
Common Programming Error 10.1
Attempting to instantiate an object of an abstract
class is a compilation error.

17
Failure to implement a superclass’s abstract
methods in a subclass is a compilation error
unless the subclass is also declared abstract.

18
Fig. 10.2 | Employee hierarchy UML class diagram.

19
A subclass can inherit “interface” or
“implementation” from a superclass. Hierarchies
designed for implementation inheritance
tend to have their functionality high in the
hierarchy—each new subclass inherits one or
more methods that were implemented in a
superclass, and the subclass uses the superclass
implementations. (cont…)

20
Hierarchies designed for interface
inheritance tend to have their functionality
lower in the hierarchy—a superclass specifies one
or more abstract methods that must be declared
for each concrete class in the hierarchy, and the
individual subclasses override these methods to
provide subclass-specific implementations.

21
10.5.1 Creating Abstract Superclass
Employee
•abstract superclass Employee
– earnings is declared abstract
• 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

22
Fig. 10.3 | Polymorphic interface for the Employee hierarchy classes.

23
Outline
Employee.java
(1 of 3)
1 // Fig. 10.4: Employee.java
2 // Employee abstract superclass.
3
4 public abstract class Employee
5 {
6 private String firstName;
7 private String lastName;
8 private String socialSecurityNumber;
9
10 // three-argument constructor
11 public Employee( String first, String last, String ssn )
12 {
13 firstName = first;
14 lastName = last;
15 socialSecurityNumber = ssn;
16 } // end three-argument Employee constructor
17
Declare abstract class Employee
Attributes common to all employees

24
Outline
Employee.java
(2 of 3)
18 // set first name
19 public void setFirstName( String first )
20 {
22 } // end method setFirstName
23
24 // return first name
25 public String getFirstName()
26 {
27 return firstName;
28 } // end method getFirstName
29
30 // set last name
31 public void setLastName( String last )
32 {
33 lastName = last;
34 } // end method setLastName
35
36 // return last name
37 public String getLastName()
38 {
39 return lastName;
40 } // end method getLastName
41

25
Outline
Employee.java
(3 of 3)
42 // set social security number
43 public void setSocialSecurityNumber( String ssn )
44 {
45 socialSecurityNumber = ssn; // should validate
46 } // end method setSocialSecurityNumber
47
48 // return social security number
49 public String getSocialSecurityNumber()
50 {
51 return socialSecurityNumber;
52 } // end method getSocialSecurityNumber
53
54 // return String representation of Employee object
55 public String toString()
56 {
57 return String.format( "%s %snsocial security number: %s",
58 getFirstName(), getLastName(), getSocialSecurityNumber() );
59 } // end method toString
60
61 // abstract method overridden by subclasses
62 public abstract double earnings(); // no implementation here
63 } // end abstract class Employee
abstract method earnings
has no implementation

26
Outline
SalariedEmployee
.java
(1 of 2)
1 // Fig. 10.5: SalariedEmployee.java
2 // SalariedEmployee class extends Employee.
3
4 public class SalariedEmployee extends Employee
5 {
6 private double weeklySalary;
7
8 // four-argument constructor
9 public SalariedEmployee( String first, String last, String ssn,
10 double salary )
11 {
12 super( first, last, ssn ); // pass to Employee constructor
13 setWeeklySalary( salary ); // validate and store salary
14 } // end four-argument SalariedEmployee constructor
15
16 // set salary
17 public void setWeeklySalary( double salary )
18 {
19 weeklySalary = salary < 0.0 ? 0.0 : salary;
20 } // end method setWeeklySalary
21
Class SalariedEmployee
extends class Employee
Call superclass constructor
Validate and set weekly salary value
Call setWeeklySalary method

27
Outline
SalariedEmployee
.java
(2 of 2)
22 // return salary
23 public double getWeeklySalary()
24 {
25 return weeklySalary;
26 } // end method getWeeklySalary
27
28 // calculate earnings; override abstract method earnings in Employee
29 public double earnings()
30 {
31 return getWeeklySalary();
32 } // end method earnings
33
34 // return String representation of SalariedEmployee object
36 {
37 return String.format( "salaried employee: %sn%s: $%,.2f",
38 super.toString(), "weekly salary", getWeeklySalary() );
40 } // end class SalariedEmployee
Override earnings method so
SalariedEmployee can be concrete
Override toString method
Call superclass’s version of toString

28
Outline
HourlyEmployee
.java
(1 of 2)
1 // Fig. 10.6: HourlyEmployee.java
2 // HourlyEmployee class extends Employee.
3
4 public class HourlyEmployee extends Employee
5 {
6 private double wage; // wage per hour
7 private double hours; // hours worked for week
8
9 // five-argument constructor
10 public HourlyEmployee( String first, String last, String ssn,
11 double hourlyWage, double hoursWorked )
12 {
13 super( first, last, ssn );
14 setWage( hourlyWage ); // validate hourly wage
15 setHours( hoursWorked ); // validate hours worked
16 } // end five-argument HourlyEmployee constructor
17
18 // set wage
19 public void setWage( double hourlyWage )
20 {
21 wage = ( hourlyWage < 0.0 ) ? 0.0 : hourlyWage;
22 } // end method setWage
23
24 // return wage
25 public double getWage()
26 {
27 return wage;
28 } // end method getWage
29
Class HourlyEmployee
Validate and set hourly wage value

29
Outline
HourlyEmployee
.java
(2 of 2)
30 // set hours worked
31 public void setHours( double hoursWorked )
32 {
33 hours = ( ( hoursWorked >= 0.0 ) && ( hoursWorked <= 168.0 ) ) ?
34 hoursWorked : 0.0;
35 } // end method setHours
36
37 // return hours worked
38 public double getHours()
39 {
40 return hours;
41 } // end method getHours
42
45 {
46 if ( getHours() <= 40 ) // no overtime
47 return getWage() * getHours();
48 else
49 return 40 * getWage() + ( gethours() - 40 ) * getWage() * 1.5;
51
52 // return String representation of HourlyEmployee object
54 {
55 return String.format( "hourly employee: %sn%s: $%,.2f; %s: %,.2f",
56 super.toString(), "hourly wage", getWage(),
57 "hours worked", getHours() );
59 } // end class HourlyEmployee
Validate and set hours worked value
HourlyEmployee can be concrete
Call superclass’s toString method

30
Outline
CommissionEmployee
.java
(1 of 3)
1 // Fig. 10.7: CommissionEmployee.java
2 // CommissionEmployee class extends Employee.
3
4 public class CommissionEmployee extends Employee
5 {
6 private double grossSales; // gross weekly sales
7 private double commissionRate; // commission percentage
8
9 // five-argument constructor
10 public CommissionEmployee( String first, String last, String ssn,
11 double sales, double rate )
12 {
13 super( first, last, ssn );
14 setGrossSales( sales );
15 setCommissionRate( rate );
16 } // end five-argument CommissionEmployee constructor
17
18 // set commission rate
19 public void setCommissionRate( double rate )
20 {
21 commissionRate = ( rate > 0.0 && rate < 1.0 ) ? rate : 0.0;
22 } // end method setCommissionRate
23
Class CommissionEmployee
Validate and set commission rate value

31
Outline
CommissionEmployee
.java
(2 of 3)
24 // return commission rate
25 public double getCommissionRate()
26 {
27 return commissionRate;
28 } // end method getCommissionRate
29
30 // set gross sales amount
31 public void setGrossSales( double sales )
32 {
33 grossSales = ( sales < 0.0 ) ? 0.0 : sales;
34 } // end method setGrossSales
35
36 // return gross sales amount
37 public double getGrossSales()
38 {
39 return grossSales;
40 } // end method getGrossSales
41
Validate and set the gross sales value

32
Outline
CommissionEmployee
.java
(3 of 3)
44 {
45 return getCommissionRate() * getGrossSales();
47
48 // return String representation of CommissionEmployee object
50 {
51 return String.format( "%s: %sn%s: $%,.2f; %s: %.2f",
52 "commission employee", super.toString(),
53 "gross sales", getGrossSales(),
54 "commission rate", getCommissionRate() );
56 } // end class CommissionEmployee
CommissionEmployee can be concrete

33
Outline
BasePlusCommission
Employee.java
(1 of 2)
1 // Fig. 10.8: BasePlusCommissionEmployee.java
2 // BasePlusCommissionEmployee class extends CommissionEmployee.
3
4 public class BasePlusCommissionEmployee extends CommissionEmployee
5 {
6 private double baseSalary; // base salary per week
7
8 // six-argument constructor
9 public BasePlusCommissionEmployee( String first, String last,
10 String ssn, double sales, double rate, double salary )
11 {
12 super( first, last, ssn, sales, rate );
13 setBaseSalary( salary ); // validate and store base salary
14 } // end six-argument BasePlusCommissionEmployee constructor
15
16 // set base salary
17 public void setBaseSalary( double salary )
18 {
19 baseSalary = ( salary < 0.0 ) ? 0.0 : salary; // non-negative
20 } // end method setBaseSalary
21
Class BasePlusCommissionEmployee
extends class CommissionEmployee
Validate and set base salary value

34
Outline
BasePlusCommission
Employee.java
(2 of 2)
22 // return base salary
23 public double getBaseSalary()
24 {
25 return baseSalary;
26 } // end method getBaseSalary
27
28 // calculate earnings; override method earnings in CommissionEmployee
30 {
31 return getBaseSalary() + super.earnings();
33
34 // return String representation of BasePlusCommissionEmployee object
36 {
37 return String.format( "%s %s; %s: $%,.2f",
38 "base-salaried", super.toString(),
39 "base salary", getBaseSalary() );
41 } // end class BasePlusCommissionEmployee
Override earnings method
Call superclass’s earnings method

35
Outline
PayrollSystemTest
.java
(1 of 5)
1 // Fig. 10.9: PayrollSystemTest.java
2 // Employee hierarchy test program.
3
4 public class PayrollSystemTest
5 {
7 {
8 // create subclass objects
9 SalariedEmployee salariedEmployee =
10 new SalariedEmployee( "John", "Smith", "111-11-1111", 800.00 );
11 HourlyEmployee hourlyEmployee =
12 new HourlyEmployee( "Karen", "Price", "222-22-2222", 16.75, 40 );
13 CommissionEmployee commissionEmployee =
14 new CommissionEmployee(
15 "Sue", "Jones", "333-33-3333", 10000, .06 );
16 BasePlusCommissionEmployee basePlusCommissionEmployee =
17 new BasePlusCommissionEmployee(
18 "Bob", "Lewis", "444-44-4444", 5000, .04, 300 );
19
20 System.out.println( "Employees processed individually:n" );
21

36
Outline
PayrollSystemTest
.java
(2 of 5)
22 System.out.printf( "%sn%s: $%,.2fnn",
23 salariedEmployee, "earned", salariedEmployee.earnings() );
25 hourlyEmployee, "earned", hourlyEmployee.earnings() );
27 commissionEmployee, "earned", commissionEmployee.earnings() );
29 basePlusCommissionEmployee,
30 "earned", basePlusCommissionEmployee.earnings() );
31
32 // create four-element Employee array
33 Employee employees[] = new Employee[ 4 ];
34
35 // initialize array with Employees
36 employees[ 0 ] = salariedEmployee;
37 employees[ 1 ] = hourlyEmployee;
38 employees[ 2 ] = commissionEmployee;
39 employees[ 3 ] = basePlusCommissionEmployee;
40
41 System.out.println( "Employees processed polymorphically:n" );
42
43 // generically process each element in array employees
44 for ( Employee currentEmployee : employees )
45 {
46 System.out.println( currentEmployee ); // invokes toString
47
Assigning subclass objects to
supercalss variables
Implicitly and polymorphically call toString

37
Outline
PayrollSystemTest
.java
(3 of 5)
48 // determine whether element is a BasePlusCommissionEmployee
49 if ( currentEmployee instanceof BasePlusCommissionEmployee )
50 {
51 // downcast Employee reference to
52 // BasePlusCommissionEmployee reference
53 BasePlusCommissionEmployee employee =
54 ( BasePlusCommissionEmployee ) currentEmployee;
55
56 double oldBaseSalary = employee.getBaseSalary();
57 employee.setBaseSalary( 1.10 * oldBaseSalary );
58 System.out.printf(
59 "new base salary with 10%% increase is: $%,.2fn",
60 employee.getBaseSalary() );
61 } // end if
62
63 System.out.printf(
64 "earned $%,.2fnn", currentEmployee.earnings() );
65 } // end for
66
67 // get type name of each object in employees array
68 for ( int j = 0; j < employees.length; j++ )
69 System.out.printf( "Employee %d is a %sn", j,
70 employees[ j ].getClass().getName() );
71 } // end main
72 } // end class PayrollSystemTest
If the currentEmployee variable points to a
BasePlusCommissionEmployee object
Downcast currentEmployee to a
BasePlusCommissionEmployee
reference
Give BasePlusCommissionEmployees
a 10% base salary bonus
Polymorphically call
earnings method
Call getClass and getName methods to display
each Employee subclass object’s class name

38
Outline
PayrollSystemTest
.java
(4 of 5)
Employees processed individually:
salaried employee: John Smith
weekly salary: $800.00
earned: $800.00
hourly employee: Karen Price
hourly wage: $16.75; hours worked: 40.00
earned: $670.00
gross sales: $10,000.00; commission rate: 0.06
earned: $600.00
gross sales: $5,000.00; commission rate: 0.04; base salary: $300.00
earned: $500.00

39
Outline
PayrollSystemTest
.java
(5 of 5)
Employees processed polymorphically:
earned $800.00
hourly employee: Karen Price
hourly wage: $16.75; hours worked: 40.00
earned $670.00
gross sales: $10,000.00; commission rate: 0.06
earned $600.00
gross sales: $5,000.00; commission rate: 0.04; base salary: $300.00
new base salary with 10% increase is: $330.00
earned $530.00
Employee 0 is a SalariedEmployee
Employee 1 is a HourlyEmployee
Employee 2 is a CommissionEmployee
Employee 3 is a BasePlusCommissionEmployee
Same results as when the employees
were processed individually
Base salary is increased by 10%
Each employee’s type is displayed

40
10.5.6 Demonstrating Polymorphic
Processing, Operator instanceof and
Downcasting
• Dynamic binding
– Also known as late binding
– Calls to overridden methods are resolved at execution time,
based on the type of object referenced
•instanceof operator
– Determines whether an object is an instance of a certain
type

41
Assigning a superclass variable to a subclass
variable (without an explicit cast) is a compilation
error.

42
If at execution time the reference of a subclass
object has been assigned to a variable of one of its
direct or indirect superclasses, it is acceptable to
cast the reference stored in that superclass
variable back to a reference of the subclass type.
Before performing such a cast, use the
instanceof operator to ensure that the object is
indeed an object of an appropriate subclass type.

43
When downcasting an object, a
ClassCastException occurs, if at
execution time the object does not
have an is-a relationship with the
type specified in the cast operator.
An object can be cast only to its own
type or to the type of one of its
superclasses.

44
10.5.6 Demonstrating Polymorphic
Processing, Operator instanceof and
Downcasting (Cont.)
• Downcasting
– Convert a reference to a superclass to a reference to a
subclass
– Allowed only if the object has an is-a relationship with the
subclass
•getClass method
– Inherited from Object
– Returns an object of type Class
•getName method of class Class
– Returns the class’s name

45
10.5.7 Summary of the Allowed
Assignments Between Superclass and
Subclass Variables
• Superclass and subclass assignment rules
– Assigning a superclass reference to a superclass variable is
straightforward
– Assigning a subclass reference to a subclass variable is
straightforward
– Assigning a subclass reference to a superclass variable is
safe because of the is-a relationship
• Referring to subclass-only members through superclass
variables is a compilation error
– Assigning a superclass reference to a subclass variable is a
compilation error
• Downcasting can get around this error

46
10.6 final Methods and Classes
•final methods
– Cannot be overridden in a subclass
– private and static methods are implicitly final
– final methods are resolved at compile time, this is known
as static binding
• Compilers can optimize by inlining the code
•final classes
– Cannot be extended by a subclass
– All methods in a final class are implicitly final

47
Performance Tip 10.1
The compiler can decide to inline a final method
call and will do so for small, simple final
methods. Inlining does not violate encapsulation
or information hiding, but does improve
performance because it eliminates the overhead of
making a method call.

48
Attempting to declare a subclass of a final class
is a compilation error.

49
In the Java API, the vast majority of classes are
not declared final. This enables inheritance
and polymorphism—the fundamental
capabilities of object-oriented programming.
However, in some cases, it is important to
declare classes final—typically for security
reasons.

50
10.7 Case Study: Creating and Using
Interfaces
• Interfaces
– Keyword interface
– Contains only constants and abstract methods
• All fields are implicitly public, static and final
• All methods are implicitly public abstract methods
– Classes can implement interfaces
• The class must declare each method in the interface using the
same signature or the class must be declared abstract
– Typically used when disparate classes need to share
common methods and constants
– Normally declared in their own files with the same names
as the interfaces and with the .java file-name extension

51
Good Programming Practice 10.1
According to Chapter 9 of the Java Language
Specification, it is proper style to declare an
interface’s methods without keywords public and
abstract because they are redundant in interface
method declarations. Similarly, constants should
be declared without keywords public, static
and final because they, too, are redundant.

52
Failing to implement any method of an interface
in a concrete class that implements the interface
results in a syntax error indicating that the class
must be declared abstract.

53
10.7.1 Developing a Payable Hierarchy
•Payable interface
– Contains method getPaymentAmount
– Is implemented by the Invoice and Employee classes
• UML representation of interfaces
– Interfaces are distinguished from classes by placing the
word “interface” in guillemets (« and ») above the
interface name
– The relationship between a class and an interface is known
as realization
• A class “realizes” the method of an interface

54
Good Programming Practice 10.2
When declaring a method in an interface, choose
a method name that describes the method’s
purpose in a general manner, because the method
may be implemented by a broad range of
unrelated classes.

55
Fig. 10.10 | Payable interface hierarchy UML class diagram.

56
Outline
Payable.java
1 // Fig. 10.11: Payable.java
2 // Payable interface declaration.
3
4 public interface Payable
5 {
6 double getPaymentAmount(); // calculate payment; no implementation
7 } // end interface Payable
Declare interface Payable
Declare getPaymentAmount method which is
implicitly public and abstract

57
Outline
Invoice.java
(1 of 3)
1 // Fig. 10.12: Invoice.java
2 // Invoice class implements Payable.
3
4 public class Invoice implements Payable
5 {
6 private String partNumber;
7 private String partDescription;
8 private int quantity;
9 private double pricePerItem;
10
12 public Invoice( String part, String description, int count,
13 double price )
14 {
15 partNumber = part;
16 partDescription = description;
17 setQuantity( count ); // validate and store quantity
18 setPricePerItem( price ); // validate and store price per item
19 } // end four-argument Invoice constructor
20
21 // set part number
22 public void setPartNumber( String part )
23 {
24 partNumber = part;
25 } // end method setPartNumber
26
Class Invoice implements
interface Payable

58
Outline
Invoice.java
(2 of 3)
27 // get part number
28 public String getPartNumber()
29 {
30 return partNumber;
31 } // end method getPartNumber
32
33 // set description
34 public void setPartDescription( String description )
35 {
36 partDescription = description;
37 } // end method setPartDescription
38
39 // get description
40 public String getPartDescription()
41 {
42 return partDescription;
43 } // end method getPartDescription
44
45 // set quantity
46 public void setQuantity( int count )
47 {
48 quantity = ( count < 0 ) ? 0 : count; // quantity cannot be negative
49 } // end method setQuantity
50
51 // get quantity
52 public int getQuantity()
53 {
54 return quantity;
55 } // end method getQuantity
56

59
Outline
Invoice.java
(3 of 3)
57 // set price per item
58 public void setPricePerItem( double price )
59 {
60 pricePerItem = ( price < 0.0 ) ? 0.0 : price; // validate price
61 } // end method setPricePerItem
62
63 // get price per item
64 public double getPricePerItem()
65 {
66 return pricePerItem;
67 } // end method getPricePerItem
68
69 // return String representation of Invoice object
71 {
72 return String.format( "%s: n%s: %s (%s) n%s: %d n%s: $%,.2f",
73 "invoice", "part number", getPartNumber(), getPartDescription(),
74 "quantity", getQuantity(), "price per item", getPricePerItem() );
76
77 // method required to carry out contract with interface Payable
78 public double getPaymentAmount()
79 {
80 return getQuantity() * getPricePerItem(); // calculate total cost
81 } // end method getPaymentAmount
82 } // end class Invoice Declare getPaymentAmount to fulfill
contract with interface Payable

60
10.7.3 Creating Class Invoice
• A class can implement as many interfaces as it
needs
– Use a comma-separated list of interface names after
keyword implements
• Example: public class ClassName extends
SuperclassName implements FirstInterface,
SecondInterface, …

61
Outline
Employee.java
(1 of 3)
1 // Fig. 10.13: Employee.java
2 // Employee abstract superclass implements Payable.
3
4 public abstract class Employee implements Payable
5 {
6 private String firstName;
7 private String lastName;
8 private String socialSecurityNumber;
9
10 // three-argument constructor
11 public Employee( String first, String last, String ssn )
12 {
14 lastName = last;
15 socialSecurityNumber = ssn;
16 } // end three-argument Employee constructor
17
Class Employee implements
interface Payable

62
Outline
Employee.java
(2 of 3)
18 // set first name
19 public void setFirstName( String first )
20 {
22 } // end method setFirstName
23
24 // return first name
25 public String getFirstName()
26 {
27 return firstName;
28 } // end method getFirstName
29
30 // set last name
31 public void setLastName( String last )
32 {
33 lastName = last;
34 } // end method setLastName
35
36 // return last name
37 public String getLastName()
38 {
39 return lastName;
40 } // end method getLastName
41

63
Outline
Employee.java
(3 of 3)
42 // set social security number
43 public void setSocialSecurityNumber( String ssn )
44 {
45 socialSecurityNumber = ssn; // should validate
46 } // end method setSocialSecurityNumber
47
48 // return social security number
49 public String getSocialSecurityNumber()
50 {
51 return socialSecurityNumber;
52 } // end method getSocialSecurityNumber
53
54 // return String representation of Employee object
56 {
57 return String.format( "%s %snsocial security number: %s",
58 getFirstName(), getLastName(), getSocialSecurityNumber() );
60
61 // Note: We do not implement Payable method getPaymentAmount here so
62 // this class must be declared abstract to avoid a compilation error.
63 } // end abstract class Employee
getPaymentAmount method is
not implemented here

64
10.7.5 Modifying Class
SalariedEmployee for Use in the
Payable Hierarchy
• Objects of any subclasses of the class that
implements the interface can also be thought of as
objects of the interface
– A reference to a subclass object can be assigned to an
interface variable if the superclass implements that
interface

65
Inheritance and interfaces are similar in their
implementation of the “is-a” relationship. An
object of a class that implements an interface
may be thought of as an object of that interface
type. An object of any subclasses of a class that
implements an interface also can be thought of
as an object of the interface type.

66
Outline
SalariedEmployee
.java
(1 of 2)
1 // Fig. 10.14: SalariedEmployee.java
2 // SalariedEmployee class extends Employee, which implements Payable.
3
4 public class SalariedEmployee extends Employee
5 {
6 private double weeklySalary;
7
9 public SalariedEmployee( String first, String last, String ssn,
10 double salary )
11 {
12 super( first, last, ssn ); // pass to Employee constructor
13 setWeeklySalary( salary ); // validate and store salary
14 } // end four-argument SalariedEmployee constructor
15
16 // set salary
17 public void setWeeklySalary( double salary )
18 {
19 weeklySalary = salary < 0.0 ? 0.0 : salary;
20 } // end method setWeeklySalary
21
Class SalariedEmployee extends class Employee
(which implements interface Payable)

67
Outline
SalariedEmployee
.java
(2 of 2)
22 // return salary
23 public double getWeeklySalary()
24 {
25 return weeklySalary;
26 } // end method getWeeklySalary
27
28 // calculate earnings; implement interface Payable method that was
29 // abstract in superclass Employee
30 public double getPaymentAmount()
31 {
32 return getWeeklySalary();
33 } // end method getPaymentAmount
34
35 // return String representation of SalariedEmployee object
37 {
38 return String.format( "salaried employee: %sn%s: $%,.2f",
39 super.toString(), "weekly salary", getWeeklySalary() );
41 } // end class SalariedEmployee
Declare getPaymentAmount method
instead of earnings method

68
The “is-a” relationship that exists between
superclasses and subclasses, and between
interfaces and the classes that implement them,
holds when passing an object to a method.
When a method parameter receives a variable
of a superclass or interface type, the method
processes the object received as an argument
polymorphically.

69
Using a superclass reference, we can
polymorphically invoke any method specified
in the superclass declaration (and in class
Object). Using an interface reference, we can
polymorphically invoke any method specified
in the interface declaration (and in class
Object).

70
Outline
PayableInterface
Test.java
(1 of 2)
1 // Fig. 10.15: PayableInterfaceTest.java
2 // Tests interface Payable.
3
4 public class PayableInterfaceTest
5 {
7 {
8 // create four-element Payable array
9 Payable payableObjects[] = new Payable[ 4 ];
10
11 // populate array with objects that implement Payable
12 payableObjects[ 0 ] = new Invoice( "01234", "seat", 2, 375.00 );
13 payableObjects[ 1 ] = new Invoice( "56789", "tire", 4, 79.95 );
14 payableObjects[ 2 ] =
15 new SalariedEmployee( "John", "Smith", "111-11-1111", 800.00 );
16 payableObjects[ 3 ] =
17 new SalariedEmployee( "Lisa", "Barnes", "888-88-8888", 1200.00 );
18
19 System.out.println(
20 "Invoices and Employees processed polymorphically:n" );
21
Declare array of Payable variables
Assigning references to
Invoice objects to
Payable variables
Assigning references to
SalariedEmployee
objects to Payable variables

71
Outline
PayableInterface
Test.java
(2 of 2)
22 // generically process each element in array payableObjects
23 for ( Payable currentPayable : payableObjects )
24 {
25 // output currentPayable and its appropriate payment amount
26 System.out.printf( "%s n%s: $%,.2fnn",
27 currentPayable.toString(),
28 "payment due", currentPayable.getPaymentAmount() );
29 } // end for
30 } // end main
31 } // end class PayableInterfaceTest
Invoices and Employees processed polymorphically:
invoice:
part number: 01234 (seat)
quantity: 2
price per item: $375.00
payment due: $750.00
invoice:
part number: 56789 (tire)
quantity: 4
price per item: $79.95
salaried employee: Lisa Barnes
weekly salary: $1,200.00
payment due: $1,200.00
Call toString and getPaymentAmount
methods polymorphically

72
All methods of class Object can be called by using
a reference of an interface type. A reference refers
to an object, and all objects inherit the methods of
class Object.

73
10.7.7 Declaring Constants with
Interfaces
• Interfaces can be used to declare constants used
in many class declarations
– These constants are implicitly public, static and
final
– Using a static import declaration allows clients to use
these constants with just their names

74
As of J2SE 5.0, it is considered a better
programming practice to create sets of
constants as enumerations with keyword
enum. See Section 6.10 for an introduction to
enum and Section 8.9 for additional enum
details.

75
Fig. 10.16 | Common interfaces of the Java API.
(Part 1 of 2)
Interface Description
Comparable As you learned in Chapter 2, Java contains several comparison operators (e.g.,
<, <=, >, >=, ==, !=) that allow you to compare primitive values. However,
these operators cannot be used to compare the contents of objects. Interface
Comparable is used to allow objects of a class that implements the interface
to be compared to one another. The interface contains one method,
compareTo, that compares the object that calls the method to the object
passed as an argument to the method. Classes must implement compareTo
such that it returns a value indicating whether the object on which it is invoked
is less than (negative integer return value), equal to (0 return value) or greater
than (positive integer return value) the object passed as an argument, using any
criteria specified by the programmer. For example, if class Employee
implements Comparable, its compareTo method could compare
Employee objects by their earnings amounts. Interface Comparable is
commonly used for ordering objects in a collection such as an array. We use
Comparable in Chapter 18, Generics, and Chapter 19, Collections.
Serializable A tagging interface used only to identify classes whose objects can be written
to (i.e., serialized) or read from (i.e., deserialized) some type of storage (e.g.,
file on disk, database field) or transmitted across a network. We use
Serializable in Chapter 14, Files and Streams, and Chapter 24,
Networking.

76
Fig. 10.16 | Common interfaces of the Java API.
(Part 2 of 2)
Interface Description
Runnable Implemented by any class for which objects of that class should be able to execute
in parallel using a technique called multithreading (discussed in Chapter 23,
Multithreading). The interface contains one method, run, which describes the
behavior of an object when executed.
GUI event-listener
interfaces
You work with Graphical User Interfaces (GUIs) every day. For example, in your
Web browser, you might type in a text field the address of a Web site to visit, or
you might click a button to return to the previous site you visited. When you type a
Web site address or click a button in the Web browser, the browser must respond to
your interaction and perform the desired task for you. Your interaction is known as
an event, and the code that the browser uses to respond to an event is known as an
event handler. In Chapter 11, GUI Components: Part 1, and Chapter 22, GUI
Components: Part 2, you will learn how to build Java GUIs and how to build event
handlers to respond to user interactions. The event handlers are declared in classes
that implement an appropriate event-listener interface. Each event listener interface
specifies one or more methods that must be implemented to respond to user
interactions.
SwingConstants Contains a set of constants used in GUI programming to position GUI elements on
the screen. We explore GUI programming in Chapters 11 and 22.

77
Fig. 10.17 | MyShape hierarchy.

78
Fig. 10.18 | MyShape hierarchy with MyBoundedShape.

79
Fig. 10.19 | Attributes and operations of classes BalanceInquiry,
Withdrawal and Deposit.

80
10.9 (Optional) Software Engineering
Case Study: Incorporating Inheritance
into the ATM System
• UML model for inheritance
– The generalization relationship
• The superclass is a generalization of the subclasses
• The subclasses are specializations of the superclass
•Transaction superclass
– Contains the methods and fields BalanceInquiry,
Withdrawal and Deposit have in common
• execute method
• accountNumber field

81
Fig. 10. 20 | Class diagram modeling generalization of superclass
Transaction and subclasses BalanceInquiry, Withdrawal and
Deposit. Note that abstract class names (e.g., Transaction) and
method names (e.g., execute in class Transaction) appear in
italics.

82
Fig. 10.21 | Class diagram of the ATM system (incorporating
inheritance). Note that abstract class names (e.g., Transaction)
appear in italics.

83
A complete class diagram shows all the
associations among classes and all the attributes
and operations for each class. When the number
of class attributes, methods and associations is
substantial (as in Fig. 10.21 and Fig. 10.22), a good
practice that promotes readability is to divide this
information between two class diagrams—one
focusing on associations and the other on
attributes and methods.

84
10.9 (Optional) Software Engineering
Case Study: Incorporating Inheritance
into the ATM System (Cont.)
• Incorporating inheritance into the ATM system
design
– If class A is a generalization of class B, then class B extends
class A
– If class A is an abstract class and class B is a subclass of
class A, then class B must implement the abstract methods
of class A if class B is to be a concrete class

85
Fig. 10.22 | Class diagram with attributes and operations (incorporating
inheritance). Note that abstract class names (e.g., Transaction) and method
names (e.g., execute in class Transaction) appear in italic

86
Outline
Withdrawal.java
1 // Class Withdrawal represents an ATM withdrawal transaction
2 public class Withdrawal extends Transaction
3 {
4 } // end class Withdrawal Subclass Withdrawal extends
superclass Transaction

87
Outline
Withdrawal.java
1 // Withdrawal.java
2 // Generated using the class diagrams in Fig. 10.21 and Fig. 10.22
3 public class Withdrawal extends Transaction
4 {
5 // attributes
6 private double amount; // amount to withdraw
7 private Keypad keypad; // reference to keypad
8 private CashDispenser cashDispenser; // reference to cash dispenser
9
10 // no-argument constructor
11 public Withdrawal()
12 {
13 } // end no-argument Withdrawal constructor
14
15 // method overriding execute
16 public void execute()
17 {
18 } // end method execute
19 } // end class Withdrawal
Subclass Withdrawal extends
superclass Transaction

88
Several UML modeling tools convert UML-based
designs into Java code and can speed the
implementation process considerably. For more
information on these tools, refer to the Internet
and Web Resources listed at the end of
Section 2.9.

89
Outline
Transaction.java
(1 of 2)
1 // Abstract class Transaction represents an ATM transaction
2 public abstract class Transaction
3 {
4 // attributes
5 private int accountNumber; // indicates account involved
6 private Screen screen; // ATM’s screen
7 private BankDatabase bankDatabase; // account info database
8
9 // no-argument constructor invoked by subclasses using super()
10 public Transaction()
11 {
12 } // end no-argument Transaction constructor
13
14 // return account number
15 public int getAccountNumber()
16 {
17 } // end method getAccountNumber
18
Declare abstract superclass Transaction

90
Outline
Transaction.java
(2 of 2)
19 // return reference to screen
20 public Screen getScreen()
21 {
22 } // end method getScreen
23
24 // return reference to bank database
25 public BankDatabase getBankDatabase()
26 {
27 } // end method getBankDatabase
28
29 // abstract method overridden by subclasses
30 public abstract void execute();
31 } // end class Transaction
Declare abstract method execute

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