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Inheritance - CIS 1068 Program Design and Abstraction

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Inheritance - CIS 1068 Program Design and Abstraction Zhen Jiang CIS Dept. Temple University 1050 Wachman Hall, Main Campus Email: zhen.jiang_at_temple.edu * * – PowerPoint PPT presentation

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Title: Inheritance - CIS 1068 Program Design and Abstraction

1
Inheritance - CIS 1068 Program Design and
Abstraction

Zhen Jiang
CIS Dept.
Temple University
1050 Wachman Hall, Main Campus
Email zhen.jiang_at_temple.edu

2
Table of Contents

Introduction to inheritance
Inheritance
Syntax
More!
Overridding
OOP (object-oriented programming)
Polymorphism
Modifiers
Abstract class and interfaces
GUI

3
Introduction

Write an Employee class with methods that return
values for the following properties of employees
at a particular company
Work week 40 hours
Annual salary 40,000
Paid time off 2 weeks
Leave of absence form Yellow form

// A class to represent employees
public class Employee
public int getHours()
return 40 // works 40 hours /
week
public double getSalary()
return 40000.0 // 40,000.00 / year
public int getVacationDays()
return 10 // 2 weeks' paid
vacation
public String getVacationForm()
return "yellow" // use the yellow
form

Write a Secretary class with methods that return
values for the following properties of
secretaries at a particular company
Work week 40 hours
Annual salary 40,000
Paid time off 2 weeks
Leave of absence form Yellow form
Add a method takeDictation that takes a string as
a parameter and prints out the string prefixed by
"Taking dictation of text ".

// A class to represent secretaries
public class Secretary
public int getHours()
return 40 // works 40 hours /
week
public double getSalary()
return 40000.0 // 40,000.00 / year
public int getVacationDays()
return 10 // 2 weeks' paid
vacation
public String getVacationForm()
return "yellow" // use the yellow
form
public void takeDictation(String text)

// A class to represent employees
public class Employee
public int getHours()
return 40
public double getSalary()
return 40000.0
public int getVacationDays()
return 10
public String getVacationForm()
return "yellow"

// A class to represent secretaries
public class Secretary
public int getHours()
return 40
public double getSalary()
return 40000.0
public int getVacationDays()
return 10
public String getVacationForm()
return "yellow"
public void takeDictation(String text)

Compare these two!
8
Inheritance Is-a relationship

is-a relationship A hierarchical connection
where one category can be treated as a
specialized version of another.
Examples
Every secretary is an employee.
Every square is a rectangle.
Every dog is a mammal.

code reuse The practice of writing program code
once and using it in many contexts.
We'd like to be able to say the following
// A class to represent secretaries
public class Secretary
ltcopy all the contents from Employee classgt
public void takeDictation(String text)
System.out.println("Taking dictation of
text "
text)
That way we would be reusing the Employee code.

inheritance A way to specify a relationship
between two classes where one class inherits the
state and behavior of another.
The child class (also called subclass) inherits
from the parent class (also called superclass).
The subclass receives a copy of every field and
method from the superclass.

11
Syntax

Creating a subclass, general syntax
public class ltsubclass namegt extends ltsuperclass
namegt
Example
public class Secretary extends Employee
....
By extending Employee, each Secretary object
automatically has a getHours, getSalary,
getVacationDays, and getVacationForm method.

Improved Secretary class

// A class to represent secretaries
public class Secretary extends Employee
public void takeDictation(String text)
System.out.println("Taking dictation of
text "
text)

13
Writing even more classes

Write a Marketer class that represents marketers
who have the same properties as general
employees, but instead of making only a paltry
40,000, marketers make 50,000!
Can we still leverage the Employee class or do we
have to re-write everything, because one method
(getSalary) is different?
If only Marketer could write a new version of the
getSalary method, but inherit everything else

14
Overriding methods

override To write a new version of a method in a
subclass to replace the superclass's version.
To override a superclass method, just write a new
version of it in the subclass. This will replace
the inherited version.

15
Marketer class

// A class to represent marketers
public class Marketer extends Employee
public void advertise()
System.out.println("Act now while
supplies last!")
public double getSalary()
return 50000.0 // 50,000.00 / year

16
OOP Based in reality or too convenient?

At many companies, all new employees attend a
common orientation to learn general rules (e.g.,
what forms to fill out when).
Each person receives a big manual of these rules.
Each employee also attends a subdivision-specific
orientation to learn rules specific to their
subdivision (e.g., marketing department).
Everyone receives a smaller manual of these rules.

The smaller manual adds some rules and also
changes (read overrides) some rules from the
large manual (e.g., "use the pink form instead of
the yellow form")?

Why not just have a 22-page manual for lawyers,
21-page manual for secretaries, 23-page manual
for marketers, etc?

maintenance If a common rule changes, only the
common manual needs to be updated.

The 20 page manual manner is useful to be able to
specify general rules that will apply to many
groups.
Locality
A person can look at the manual for lawyers and
quickly discover all rules that are specific to
lawyers.
It is also useful to specify a smaller set of
rules for such a particular group, including
being able to replace rules from the overall set
(overriding, e.g., "use the pink form instead of
the yellow form").

21
//base classpublic class PubA public int x
1 public void setX() x2 public int
getX() return x
//attribute method inheritancepublic class
PubB extends PubA public void setX()x3
overriding
2 3
PubA a new PubA() PubB b new PubB()
a.setX() b.setX() System.out.println(a.getX())
System.out.println(b.getX())
22
Why inheritance?

Need for Jbutton, FlowLayout, JTextField, etc
http//www.cis.temple.edu/jiang/Extend_action.pdf
Need for a customized response from the computer
for all GUI actions
http//www.cis.temple.edu/jiang/MultiEventSource.
pdf

23
Constructor for superclass
24

public class Employee
private double salary
public Employee(double initialSalary)
salary initialSalary
public int getHours()
return 40 // 40 hours per week
public double getSalary()
return salary
public int getVacationDays()
return 10 // 2 weeks' paid
vacation
public String getVacationForm()
return "yellow" // use the yellow
form

Use the super() method to call the superclasss
constructor
public class Marketer extends Employee
// inherits double salary
public Marketer(double initialSalary)
//construct superclass
super(initialSalary)
- For every constructor of a subclass, the call
to super() must be the first statement in the
subclasss constructor.
- Make sure to give the same number of arguments
as there are parameters in the definition of the
superclasss constructor.

Question If a method is declared private, does
a subclass inherit it?
Actually, yes. Subclasses inherit everything
that they dont override.
If a method is declared private, can a subclass
call it?
NO! Only code inside the same class can call a
private method.
What if you want a subclass to be able to use it?
Use the protected access level

public class Employee
private double salary 40000.00
public int getHours()
return 40 // works 40 hours /
week
public double getSalary()
return salary
public int getVacationDays()
return 10 // 2 weeks' paid
vacation
public String getVacationForm()
return "yellow" // use the yellow
form

Subclasses cannot see salary directly!
public class CEO extends Employee
public void giveMyselfRaise()
salary 1000000.00 // Compile-time
Error!
public static void main(String args)
CEO c new CEO()
// This is fine, no error here
// Access to salary field is indirect
// Were accessing the public getSalary()
method
System.out.println(My salary is
c.getSalary())

public class Employee
protected double salary 40000.00
public int getHours()
return 40 // works 40 hours /
week
public double getSalary()
return salary
public int getVacationDays()
return 10 // 2 weeks' paid
vacation
public String getVacationForm()
return "yellow" // use the yellow
form

Subclasses can see protected variables and
methods just fine.
public class CEO extends Employee
public void giveMyselfRaise()
salary 1000000.00 // No longer an
error
public static void main(String args)
CEO c new CEO()
// This is fine, no error here
// Access to salary field is indirect
// Were accessing the public getSalary()
method
System.out.println(My salary is
c.getSalary())

31
What would happen if ....

public class Employee
private double salary 40000.00
public int getHours()
return 40 // works 40 hours /
week
public double getSalary()
return salary
public void addToSalary(double raise)
salary raise
public int getVacationDays()
return 10 // 2 weeks' paid
vacation

public class CEO extends Employee
public void giveMyselfRaise()
addToSalary(1000000.00)
CEO still has its own copy of the salary field,
and this code will change the value of it
appropriately.
The fact that salary is private simply means that
CEO can't access it directly. It can still call
public (or protected) superclass methods that can
access it.

33
//base class public class ProA private int x
1 protected void setX(int a) xa
protected int getX() return x
//sub class public class ProB extends ProA
public int getB() setX(2) // your next step
is to return x // but return x does not work
// due to the private modifier, so return
getX()
1 2
ProA a new ProA() ProB b new ProB()
System.out.println(a.getX())
System.out.println(b.getB())
34
Polymorphism

A reference variable of type T can refer to an
object of any subclass of T.
Employee Laura new Lawyer()
Employee Mark new Marketer()
polymorphism The ability for the same code to be
used with several different types of objects and
behave differently depending on the type of
object used.

Employee Laura new Lawyer()
System.out.println(Laura.getSalary()) //
40000.0
System.out.println(Laura.getVacationForm()) //
"pink"
You can call any method from Employee on the
person variable, but not any method specific to
Lawyer (such as sue).
Once a method is called on that object, it
behaves in its normal, overridden way (as a
Lawyer, not as a normal Employee).

36
public class Marketer extends Employee
public void advertise()
System.out.println("Act now while supplies
last!") public double getSalary()
return 50000.0 // 50,000.00 /
year
public class Secretary extends Employee
public void takeDictation(String text)
System.out.println("Taking dictation of text "
text)

public class EmployeeMain
public static void main(String args)
Secretary laura new Secretary()
Marketer mark new Marketer()
printInfo(laura)
printInfo(mark)
public static void printInfo(Employee empl)
System.out.println("salary "
empl.getSalary())
System.out.println("days "
empl.getVacationDays())
System.out.println("form "
empl.getVacationForm())
System.out.println()
Output
salary 40000.0
vacation days 10

Polymorphism and parameters
37

public class EmployeeMain2
public static void main(String args)
Employee employees new Secretary(),
new Marketer()
for (int i 0 i lt employees.length
i)
System.out.println("salary "
employeesi.getSalary())
System.out.println("vacation days "
employeesi.getVac
ationDays())
System.out.println()
Output
salary 40000.0
vacation days 10
salary 50000.0
vacation days 10

Polymorphism and arrays
38
Exercises

public class Foo
public void method1()
System.out.println("foo 1")
public void method2()
System.out.println("foo 2")
public String toString()
return "foo"
public class Bar extends Foo
public void method2()

public class Baz extends Foo
public void method1()
System.out.println("baz 1")
public String toString()
return "baz"
public class Mumble extends Baz
public void method2()
System.out.println("mumble 2")

Assume that the following four classes have been
declared

What would be the output of the following client
code?
Foo pity new Baz(), new Bar(),
new Mumble(), new Foo()
for (int i 0 i lt pity.length i)
System.out.println(pityi)
pityi.method1()
pityi.method2()
System.out.println()

40
(No Transcript)
41

The code produces the following output
baz
baz 1
foo 2
foo
foo 1
bar 2
baz
baz 1
mumble 2
foo
foo 1
foo 2

Kind of override under the standard of superclass!

43
Variable ShadowingSomething to avoid!!

Polymorphism applies to methods in Java
But not to fields!
public class A
int x 1
int method() return 1
public class B extends A
int x 2
int method() return 2

A a1 new A() A a2 new B() System.out.printl
n(a1.method()) // prints 1 System.out.println(a2.
method()) // prints 2 System.out.println(a1.x)
// prints 1 System.out.println(a2.x) // prints
1 still! // Not like method, which prefers to
its own.
44

Variable Shadowing
When a class extends another class and defines a
field with the same name, each object of the
subclass contains two fields with that name.
The supclasss version of the field is said to
shadow the subclasss version, making the
subclasss version invisible within that class.
This is called variable shadowing.

45
public class A protected int x 1 protected
void setX(int a) xa protected int getX()
return x public class B extends A
protected int x 3 public int getX() return
x public int getB() return x
1 3 3 1 3
A a new A() B b new B()
System.out.println(a.getX())
System.out.println(b.getX())
System.out.println(b.getB())
System.out.println(a.x) System.out.println(b.x)

The difference is shadow!
A a new A() A b new B()
System.out.println(a.getX())
System.out.println(b.getX())
//System.out.println(b.getB())
System.out.println(a.x) System.out.println(b.x)

1 3 1 1
46

http//www.cis.temple.edu/jiang/1068_modifier_inh
eritance.pdf

Exercises
Slide 39
PolymorphismDemo
www.cis.temple.edu/jiang/PolymorphismDemo.pdf
www.cis.temple.edu/jiang/Person.pdf
www.cis.temple.edu/jiang/Student.pdf
www.cis.temple.edu/jiang/Undergradaute.pdf

48
Modifiers

Public
A class, method, constructor, interface etc
declared public can be accessed from any other
class. Therefore fields, methods, blocks declared
inside a public class can be accessed from any
class belonging to the Java Universe.
Because of class inheritance, all public methods
and variables of a class are inherited by its
subclasses.

Private
Methods, Variables and Constructors that are
declared private can only be accessed within the
declared class itself.
Private access modifier is the most restrictive
access level. Class and interfaces cannot be
private.
Variables that are declared private can be
accessed outside the class only if public
accessor methods are present in the class.

Protected
Variables, methods and constructors which are
declared protected in a superclass can be
accessed only by the subclasses.
Protected access gives the subclass a chance to
use the helper method or variable, while
preventing a nonrelated class from trying to use
it.

Default (like public)
Default access modifier means we do not
explicitly declare an access modifier for a
class, field, method etc.
A variable or method declared without any access
control modifier is available to any other class
in the same package.
For example
public class Logger
String format
String getFormat() return this.format
public void setFormat(String f) format
f

52
Abstract and Interfaces

What have you learnt from the above exercises on
extends?
Is it good to block the use of subclass method
that is not declared in super class?
Good, because methods can be in template. In the
security control, no leakage!
Is it good to have the direct access of
attribute?
Better not, if it is not in your control. See how
complicate it is in the variable shadowing.

Suppose weve defined an Employee class, and we
dont want someone to come along and muck it up
E.g., we dont want a CEO subclass that gives
itself a raise
The final keyword indicates that some definition
(of a class, method, or field) cannot be changed
or overridden by a subclass.

54
final Example

// A class to represent employees
public class Employee
public int getHours()
return 40 // works 40 hours /
week
public final double getSalary()
return 40000.0 // 40,000.00 / year
public int getVacationDays()
return 10 // 2 weeks' paid
vacation
public String getVacationForm()
return "yellow" // use the yellow
form
No subclass is allowed to change the definition
of getSalary()!

55
final fields, methods, and classes

The final keyword can be applied to fields (as
weve seen before)
// no code may change the value of salary,
//including a subclasss code
public final double salary 40000.0
Or to methods
// subclasses cannot override the getSalary
method
public final double getSalary() return salary
Or even to classes
// the Employee class cannot be extended
// It cant have any subclasses at all!
public final class Employee ...

56
Opposite of final

The final keyword prevents subclasses from
changing (overriding) code
Sometimes, you want to do the opposite
Force another programmer or piece of code
to finish parts of a class.

57
Example Employee salary

Lets say you want every subclass of Employee to
have a salary, but you want the subclass to
decide what the salary should be.
We can define an abstract getSalary() method
public abstract double getSalary()
Note no method definition!
Abstract method declarations dont provide
definitions, just signatures.
They are there to force subclasses to provide the
definitions.

58
Abstract Rules (1)

If a class has an abstract method, or it inherits
an abstract method that it doesnt override, then
the class must be declared abstract.
public abstract class Employee
public abstract double getSalary()
// you can mix abstract and non-abstract
methods
// in an abstract class
public int getHours() // Note not
abstract!
return 40

59
Abstract Rules (2)

If a class is abstract, it cant have a
constructor.
? No Employee object can be constructed
? But you can declare Employee references.
public abstract class Employee
public abstract double getSalary()
public static void main(String args)
Employee e // NO ERROR reference is fine
e new Employee() // ERROR! No constructor

60
Extending an abstract class

public class Lawyer extends Employee
// since Employee declares an abstract
getSalary,
// Lawyer must define getSalary by overriding
it
// or else Lawyer must be an abstract class
public double getSalary()
return 45000.0
public static void main(String args)
Employee e // Fine, no problem
e new Lawyer() // Also fine (polymorphism)
e new Employee() // ERROR! No
constructor!

61
Abstract classes whats the point?

If you cant construct objects for a class,
whats the point of the class? How can we use
it?
Short Answer polymorphism.
We can use references of type Employee as a place
to store Lawyers, Secretaries, CEOs, etc.
Because getSalary() is declared in Employee,
e.getSalary() is legal syntax, even though
getSalary() is not defined in Employee.

P636
http//docs.oracle.com/javase/tutorial/java/IandI/
abstract.html

63
Exercise

Create an abstract ClosedShape class with an
abstract getArea() method
Write non-abstract subclasses Rectangle and
Circle
Write main methods for each that construct an
object and print its area. Do not use attribute
to store area information.

64
public abstract class Closedshapepublic
abstract double getArea()
public class shapeApp public static void
main(String args) Rectangle r new
Rectangle(2, 3) Circle c new Circle(1.5)
System.out.println(r.getArea())
System.out.println(c.getArea())
public class Circle extends Closedshapeprivate
double radiuspublic Circle (double r)
radius r public double getArea ()return
Math.PIradiusradius public double
getCircumference()return radius
6.07.0685834705770345
public class Rectangle extends Closedshapeprivat
e int length0private int width0public
Rectangle(int l, int w) lengthl
widthw public double getArea ()return
lengthwidth
65
Going full abstract

What if our abstract class had no non-abstract
methods?

public abstract class Employee public
abstract double getSalary() public abstract
int getHours() public abstract String
getVacationForm() Each subclass would have
different definitions. They share only the names
of their methods. Java has an alternative way to
do this interfaces
66
Interfaces

Let's say you have the following two related
classes

public class Scientist public void
discover() System.out.println(Eureka! I have
found it!) public void publish()
System.out.println(My research is better than
yours.)
public class Engineer public void
discover() System.out.println(Cool, what did
I just do?) public void publish()
System.out.println(I don't know how this
happened, but it works.)

Neither of their methods do the same thing.

67
Code Reuse

But they're still similar they both discover
and publish. Can we get code reuse?
interface Researcher
void discover()
void publish()
Now we can create Researcher references

68
Using Interface Objects

public static void researchCycle(Researcher r)
r.discover()
r.publish()
public static void main(String args)
Researcher researcher1 new Scientist()
Researcher researcher2 new Engineer()
// Interfaces have no constructors
// They can only be used as types for
references
researcher2 new Researcher() // ERROR!
researchCycle(researcher1)
researchCycle(researcher2)

69
Using Interfaces

Interfaces are a way of specifying what objects
are capable of, without saying how.
Interface variables can execute any of the
methods listed in the interface, but the behavior
depends on the class of the object
That is, interface variables are polymorphic.
There are no constructors for interfaces. They
are not classes, and no objects of that run-time
type are created. They are compile-time types
for references.

70
Implementing Interfaces

public class Scientist implements Researcher
public void discover()
System.out.println(Eureka! I have found
it!)
public void publish()
System.out.println(My research is better
than yours.)

public class Engineer implements Researcher
public void discover()
System.out.println(Whoa, what did I just
do?)
public void publish()
System.out.println(I don't know how this
happened, but it
works.)

72
Exercise

Create an interface Measurable class with an
non-abstract getArea() method
Write subclasses Rectangle and Circle to
implement the above interface
Write main methods for each that construct an
object and print its area. Do not use attribute
to store area information.

73
public interface Measurablepublic abstract
double getArea()
public class shapeApp public static void
main(String args) Rectangle r new
Rectangle(2, 3) Circle c new Circle(1.5)
System.out.println(r.getArea())
System.out.println(c.getArea())
public class Circle implements Measurableprivate
double radiuspublic Circle (double r)
radius r public double getArea ()return
Math.PIradiusradius public double
getCircumference()return radius
6.07.0685834705770345
public class Rectangle implements
Measurableprivate int length0private int
width0public Rectangle(int l, int w)
lengthl widthw public double getArea
()return lengthwidth
74
public interface Measurablepublic abstract
double getArea()
public class shapeApp public static void
main(String args)// Rectangle r new
Rectangle(2, 3)// Circle c new
Circle(1.5)// System.out.println(r.getArea())
// System.out.println(c.getArea())
Measurable r new Rectangle(2, 3)
//polymorphism display(r) Circle c
new Circle(1.5) Measurable m c
display(m) // (m.getCircumference()) not
allowed Circle t (Circle) m
System.out.println(t.getCircumference())
public class Circle implements Measurableprivate
double radiuspublic Circle (double r)
radius r public double getArea ()return
Math.PIradiusradius public double
getCircumference()return radius
public class Rectangle implements
Measurableprivate int length0private int
width0public Rectangle(int l, int w)
lengthl widthw public double getArea
()return lengthwidth
6.07.0685834705770345 1.5
75
Comparable interface