not 100% wrote by the author.just to share.

1. 1
Chapter 04: Inheritance

2. 2
Objectives
• Learn about the concept of inheritance
• Extend classes
• Override superclass methods
• Understand how constructors are called during
inheritance
• Use superclass constructors that require arguments
• Access superclass methods
• Learn which methods you cannot override

3. 3
Learning About the Concept
of Inheritance
• Inheritance
▫ Mechanism that enables one class to inherit behavior
and attributes of another class
▫ Apply knowledge of general category to more specific
objects

4. 4
Concept of Inheritance

5. 5

6. 6
• Use inheritance to create derived class
▫ Save time
▫ Reduce errors
▫ Reduce amount of new learning required to use new
class

7. 7
• Base class
▫ Used as a basis for inheritance
▫ Also called:
Superclass
Parent class

8. 8
Learning About the Concept
of Inheritance (continued)
• Derived class
▫ Inherits from a base class
▫ Always “is a” case or example of more general base
class
▫ Also called:
Subclass
Child class

9. 9
Extending Classes
• Keyword extends
▫ Achieve inheritance in Java
▫ Example:
public class EmployeeWithTerritory
extends Employee
• Inheritance one-way proposition
▫ Child inherits from parent, not other way around
• Subclasses more specific
• instanceof keyword

10. 10
Extending Classes

11. 11
Quick Quiz
1. ____ is the principle that allows you to apply
your knowledge of a general category to more
specific objects.
2. True or False: A class diagram consists of a
rectangle divided into five sections.
3. You use the keyword ____ to achieve inheritance
in Java.

12. 12
Extra notes

13. Inheritance Relationship
• The inheritance relationship can be viewed as a
relationship between an object category and its
subcategories.
▫ For example...

14. Inheritance Relationship
Transport

15. Inheritance Relationship
Transport
Air Transport

16. Inheritance Relationship
Transport
Air Transport Sea
Transport

17. Inheritance Relationship
Transport
Air Transport Sea
Transport
Land Transport

18. Inheritance Relationship
• The inheritance relationship is sometimes
called the “is-a” relationship.
▫ For example...

19. Inheritance Relationship
Transport
Inheritance Relationship
Sea Transport Land Transport Air Transport
Sea Transport Land Transport Air Transport
object “is a” object “is a” object “is a”
Transport object Transport object Transport object

20. Inheritance Relationship
• Based on the meaning of the “is-a” relationship,
each object will have attributes and behaviour as
defined by its category and all its supercategories.
• An inheritance hierarchy can be built through
generalization or specialization.

21. Inheritance Relationship
Transport
Attributes
speed
speed
getSpeed( )
altitude
Air Transport
Behaviour
getSpeed()
altitude
fly( )
fly()

22. Subclassing
• Java supports class inheritance through
subclassing.
• By declaring a class as a subclass of a base class, an
inheritance relationship between those two classes
is created.
• The Java keyword for this is extends.

23. Subclassing
class Rectangle {
private int width, height;
public Rectangle(int w, int h) {
width = w;
height = h;
}
public int getArea() {
return width * height;
}
}
class Square extends Rectangle {
Base
subclass } class

24. Subclassing
• In the example, the Square class is declared as a
subclass of the Rectangle class
• Each subclass inherits the instance variables and
instance methods of its base class.
• Note that constructors and private methods ARE NOT
INHERITED.
• Besides attributes and methods inherited from its base
class, a subclass can also define its own attributes and
methods.

25. Subclassing
class AudioPlayer {
class AudioPlayer {
private String audioFilename;
private String audioFilename;
public AudioPlayer() {…}
public AudioPlayer() {…}
public void play() {…}
public void play() {…}
private void convert() {…}
private void convert() {…}
}
}
class MP3Player extends AudioPlayer { {
class MP3Player extends AudioPlayer
private boolean encrypted;
private boolean encrypted;
public MP3Player() {…}
public MP3Player() {…}
}}
Attributes: audioFilename
encrypted
: MP3Player Constructor:
MP3Player()
Instance method:
play()

26. Constructors and super()
• Although a subclass inherits private attributes from its
base class, it has no direct access to them. How will the
subclass be able to initialize those attributes in its
constructors?
• Every constructor defined in a subclass must “call” one
of the constructors of its base class. The “call” is
performed through the super() statement.
• The super() statement must be the first statement in
the constructor method.

27. Constructors and super()
class Rectangle {
private int width, height;
public Rectangle(int w, int h) {
width = w;
height = h;
}
public int getArea() {
return width * height;
} }
class Square extends Rectangle { “Calls” the
constructor of the
public Square(int size) { Rectangle class
super(size, size);
}
}

28. Constructors and super()
• Note that if there is no super() statement in
a constructor, the compiler will automatically
insert a super() statement with no
parameters.

29. Constructors and super()
class Rectangle { class Square extends Rectangle
{
private int width, height; public Square(int size) {
setWidth(size);
public Rectangle(int w, int h) {
setHeight(size);
width = w;
}
height = h;
}
} Compiler secretly adds
public void setWidth(int w) { a super()!
width = w;
}
The compiler then displays this error
public void setHeight(int h) {
height = h; message. Why?
} test.java: 16: cannot resolve symbol
symbol : constructor Rectangle ()
} location: class Rectangle
public Square(int size) {
^

30. Benefits of Inheritance
• Reduces redundant code and code modifications
▫ A base class defines attributes and methods which
are inherited by all of its subclasses. This leads to a
reduction of redundant code and code
modifications.
• Makes your program easier to extend
▫ When adding a new subclass, we only need to define
attributes and methods which are specific to objects
of that class.

31. Benefits of Inheritance
• Increases code reuse
▫ Example: The Java library has a JButton class. We
can create a subclass of the JButton class to define
objects which are like JButton objects but with
additional attributes and methods.
▫ This is possible even though we do not have access
to the source code for the JButton class.

32. Method Overriding
• A subclass sometimes inherits instance
methods whose implementation is not
suitable for its instances.
For example …

33. class Circle {
private int radius;
public Circle(int r) {
radius = r;
}
public double getArea( ) {
System.out.print("(Area calculated in Circle::getArea()) ");
return 22.0/7*radius*radius;
} }
class Point extends Circle {
The Point class inherits
private int x, y;
the radius attribute and
public Point(int px, int py) the getArea() method
{ from its base class.
super(0);
x = px;
y = py;
}
}

34. class Application {
public static void main(String[ ] args) {
Circle circle = new Circle(7);
Point pt = new Point(100, 50);
System.out.println(“Area of circle: "+circle.getArea());
System.out.println("Area of point: "+pt.getArea());
}
}
Output:
(Area calculated in Circle::getArea()) Area of circle: 154.0
(Area calculated in Circle::getArea()) Area of point: 0.0
• The area for any Point object is zero. For efficiency,
it is unnecessary to calculate the area; simply return
the value 0.

35. Method Overriding
• Java supports method overriding. Method
overriding means replacing the
implementation of an inherited method with
another implementation.
• To override a method, we simply redefine the
method in the subclass.
• For example …

36. class Circle {
private int radius;
public Circle(int r) {
radius = r;
}
public double getArea( ) {
System.out.print("(Area calculated in Circle::getArea()) ");
return 22.0/7*radius*radius;
}
}
class Point extends Circle {
private int x, y; Method getArea()
overriden
public Point(int px, int py) {
super(0);
x = px; Returns 0
y = py;
}
public double getArea( ) {
System.out.print("(Area calculated in Point::getArea()) ");
return 0;
}
}

37. Method Overriding
class Application {
public static void main(String[ ] args) {
Circle circle = new Circle(7);
Point pt = new Point(100, 50);
System.out.println(“Area of circle: "+circle.getArea());
System.out.println("Area of point: "+pt.getArea());
}
}
Output:
(Area calculated in Circle::getArea()) Area of circle: 154.0
(Area calculated in Point::getArea()) Area of point: 0.0

38. Method Overriding hierarchy:
• Consider the following inheritance
A
D objD = new D();
doThis()
E objE = new E();
B objB = new B();
C D objE.doThis();
doThis() objD.doThis();
objB.doThis();
E F
doThis()
B

39. The super keyword again…
• Consider the following class definitions:
class Rectangle {
…
private int width, height;
public Rectangle(int w, int h) {
width = w;
height = h;
}
public void display( ) {
for (int row=0; row < height; row++) {
for (int col=0; col < width; col++)
System.out.print('*');
System.out.println();
}
}
…
}

40. Rectangle class is
the base class for
this subclass
class LabelledRectangle extends Rectangle {
private String label;
public LabelledRectangle(int w, int h, String str) {
super(w, h);
label = str; Overrides display()
} method inherited from
the base class
public void display( ) {
System.out.println("=================");
for (int row=0; row < height; row++) {
for (int col=0; col < width; col++)
System.out.print('*');
System.out.println();
}
System.out.println(label);
System.out.println("=================");
}
…
}

41. Method Overriding
class Application {
public static void main(String[ ] args) {
Rectangle rect1 = new Rectangle(3, 3);
LabelledRectangle rect2;
rect2 = new LabelledRectangle(2, 3, "Block");
rect1.display();
System.out.println();
rect2.display();
}
}
***
***
***
=================
**
**
**
Block
=================

42. • Observe the body of the display() method in the
LabelledRectangle class…
public void display( ) {
System.out.println("=================");
for (int row=0; row < getHeight(); row++) {
for (int col=0; col < getWidth(); col++)
System.out.print('*');
System.out.println();
}
System.out.println(label);
System.out.println("=================");
}
Is it possible to “call” the Exactly the same as
the code in the
overridden display() method display() method
in the Rectangle class? inherited from the
base class

43. • We can “call” the overridden method by using the
super keyword as follows:
class LabelledRectangle extends Rectangle { Executes the
private String label; overridden method
public LabelledRectangle(int w, int h, String str) {
super(w, h);
label = str;
}
public void display( ) {
System.out.println("=================");
super.display();
System.out.println(label);
System.out.println("=================");
}
…
}