‫Chapter3 inheritance

Object Oriented Programming
Chapter 3: Inheritance
Prepared by: Mahmoud Rafeek Alfarra
2016

‫تعاىل‬ ‫اهلل‬ ‫قال‬:
(‫َلَى‬‫ع‬ ‫ُوا‬‫ف‬َ‫ر‬ ْ‫س‬ َ‫أ‬ َ‫ن‬‫ِي‬‫ذ‬َّ‫ل‬‫ا‬ َ‫ي‬ِ‫د‬‫ِبَا‬‫ع‬ ‫يَا‬ ْ‫ل‬ُ‫ق‬
َّ‫ِن‬‫إ‬ ِ‫ه‬َّ‫ل‬‫ال‬ ِ‫ة‬َ‫م‬ْ‫ح‬َّ‫ر‬ ‫ِن‬‫م‬ ‫ُوا‬‫ط‬َ‫ن‬ْ‫ق‬َ‫ت‬ ‫َا‬‫ل‬ ْ‫م‬ِ‫ه‬ِ‫س‬ُ‫ف‬‫َن‬‫أ‬
َ‫و‬ُ‫ه‬ ُ‫ه‬َّ‫ِن‬‫إ‬ ‫ِيعًا‬‫م‬َ‫ج‬ َ‫ب‬‫ُو‬‫ن‬ُّ‫لذ‬ ‫ا‬ ُ‫ر‬ِ‫ْف‬‫غ‬َ‫ي‬ َ‫ه‬َّ‫الل‬
ُ‫م‬‫ِي‬‫ح‬َّ‫ر‬‫ال‬ ُ‫ر‬‫ُو‬‫ف‬َ‫غ‬ْ‫ل‬‫ا‬)

Outlines
◉ Motivation
◉ What is Inheritance ?
◉ Types of Inheritance
◉ Superclasses and Subclasses
◉ Defining a Subclass
◉ "is-a" and the "has-a" relationship
Note: I have prepared this material Based on (Liang: “Introduction to Programming using Java”, 10’th edition, 2015)
◉ Are superclass’s Constructor Inherited?
◉ Overriding Methods in the Subclass
◉ Full Example
◉ Important Notes

Lecture
Let’s think on Inheritance
1

o Suppose you will define classes to model circles, rectangles, and
triangles.
o These classes have many common features. What is the best way
to design these classes so to avoid redundancy?
The answer is to use inheritance.
Motivation

What is Inheritance ?
o Inheritance is a form of software reuse in which a new class is
created by absorbing an existing class's members and
embellishing them with new or modified capabilities.
P_Properties
P_ methods
Parent Class
C_Properties
C_methods
Child Class
This class has its
properties, methods
and that of its parent.

Types of Inheritance
Single Inheritance
When a Derived Class to
inherit properties and
behavior from a single Base
Class, it is called as single
inheritance.
Multi Level Inheritance
A derived class is created
from another derived class
is called Multi Level
Inheritance.
Hierarchical Inheritance
More than one derived class
are created from a single
base class, is called
Hierarchical Inheritance

Hybrid Inheritance
Any combination of above
three inheritance (single,
hierarchical and multi level) is
called as hybrid inheritance.
Multiple Inheritance
Multiple inheritances allows
programmers to create classes
that combine aspects of
multiple classes and their
corresponding hierarchies.

Superclasses and
Subclasses
GeometricObject
-color: String
-filled: boolean
-dateCreated: java.util.Date
+GeometricObject()
+GeometricObject(color: String,
filled: boolean)
+getColor(): String
+setColor(color: String): void
+isFilled(): boolean
+setFilled(filled: boolean): void
+getDateCreated(): java.util.Date
+toString(): String
The color of the object (default: white).
Indicates whether the object is filled with a color (default: false).
The date when the object was created.
Creates a GeometricObject.
Creates a GeometricObject with the specified color and filled
values.
Returns the color.
Sets a new color.
Returns the filled property.
Sets a new filled property.
Returns the dateCreated.
Returns a string representation of this object.
Circle
-radius: double
+Circle()
+Circle(radius: double)
+Circle(radius: double, color: String,
filled: boolean)
+getRadius(): double
+setRadius(radius: double): void
+getArea(): double
+getPerimeter(): double
+getDiameter(): double
+printCircle(): void
Rectangle
-width: double
-height: double
+Rectangle()
+Rectangle(width: double, height: double)
+Rectangle(width: double, height: double
color: String, filled: boolean)
+getWidth(): double
+setWidth(width: double): void
+getHeight(): double
+setHeight(height: double): void
+getArea(): double
+getPerimeter(): double

o In Java, as in other object-oriented programming languages,
classes can be derived from other classes.
o The derived class (the class that is derived from another class)
is called a subclass.
o The class from which its derived is called the superclass.
Superclasses and Subclasses

o A subclass normally adds its own fields and methods.
o Therefore, a subclass is more specific than its superclass.
o Typically, the subclass exhibits the behaviors of its superclass
and additional behaviors that are specific to the subclass.
Superclasses and Subclasses

Direct & Indirect Superclasses
properties3
methods3 SubClass
properties1
methods1
Indirect
SuperClass
properties2
methods2 Direct
SuperClass
o The direct superclass is the superclass
from which the subclass explicitly
inherits.
o An indirect superclass is any class above
the direct superclass in the class
hierarchy.

o A subclass inherits from a superclass. You can also:
 Add new properties
 Add new methods
 Override the methods of the superclass.
Defining a Subclass

Defining a Subclass
public class Faculty extends Employee {
//…
}
SubClass SuperClass

Defining a Subclass
class Vehicle {
protected String model;
protected float price;
public Vehicle(String model, float price){
this.model = model;
this.price = price;
}
public String print(){
return "Model: "+model+"t Price: "+price;
}
public void setModel(String model){
this.model = model;
}
public String getModel(){
return model;
} // set and get of price
}
Vehicle
Class SuperClass
Car Class SubClass

Defining a Subclass
class Car extends Vehicle{
private int passengers;
public Car(String model, float price,int passengers){
super( model, price);
this.passengers = passengers;
}
public String print(){
return "Data is:n"+super.print()+
" # of passengers: "+passengers;
}
}

Defining a Subclass
public class VehicleProjectInheritance {
public static void main(String[] args) {
Car c = new Car("Honda", 455.0f, 4);
System.out.println(c.print());
}
}

"is-a" and the "has-a" relationship
o "Is-a" represents inheritance.
o In an "is-a" relationship, an object of a subclass can also be
treated as an object of its superclass.
o “Has–a” represents the encapsulation, i.e: The relation between
object and its members.

"is-a" and the "has-a" relationship
Vehicle
Class SuperClass
Car Class SubClass
Honda is a car &
Honda is a vehicle
Object of sub is also an object of super

THANKS!
Any questions?
You can find me at:
Fb/mahmoudRAlfarra
Staff.cst.ps/mfarra
Youtube.com/mralfarra1
@mralfarra

‫تعاىل‬ ‫اهلل‬ ‫قال‬:
(‫باده‬‫ع‬ ‫عن‬ ‫بة‬‫التو‬ ‫بل‬‫يق‬ ‫لذي‬‫ا‬ ‫هو‬‫و‬
‫السيئات‬ ‫عن‬ ‫ويعفو‬)

Lecture
Let’s focus on Superclass’s Constructor
2

Are superclass’s Constructor Inherited?
o No. They are not inherited.
o They are invoked explicitly or implicitly.
o Explicitly using the super keyword.
o A constructor is used to construct an instance of a class.
Unlike properties and methods, a superclass's constructors are
not inherited in the subclass.

Superclass’s Constructor Is Always Invoked
o A constructor may invoke an overloaded constructor or its
superclass’s constructor. If none of them is invoked explicitly,
the compiler puts super() as the first statement in the
constructor. For example,
public A(double d) {
// some statements
}
is equivalent to
public A(double d) {
super();
// some statements
}
public A() {
}
is equivalent to
public A() {
super();
}

Using the Keyword super
o The keyword super refers to the superclass of the class in
which super appears. This keyword can be used in two ways:
1. To call a superclass constructor
2. To call a superclass method

Constructor Chaining
o Constructing an instance of a class invokes all the superclasses’
constructors along the inheritance chain.
o This is known as constructor chaining.

public class Faculty extends Employee {
new Faculty();
}
public Faculty() {
System.out.println("(4) Faculty's no-arg constructor is invoked");
}
}
class Employee extends Person {
public Employee() {
this("(2) Invoke Employee’s overloaded constructor");
System.out.println("(3) Employee's no-arg constructor is invoked");
}
public Employee(String s) {
System.out.println(s);
}
}
class Person {
public Person() {
System.out.println("(1) Person's no-arg constructor is invoked");
}
}
Trace Execution

Overriding Methods in the Subclass
o A subclass inherits methods from a superclass. Sometimes it is
necessary for the subclass to modify the implementation of a
method defined in the superclass.
o This is referred to as method overriding.
public class Circle extends GeometricObject {
// Other methods are omitted
/** Override the toString method defined in GeometricObject */
public String toString() {
return super.toString() + "nradius is " + radius; }
}

Overriding vs. Overloading
public class Test {
A a = new A();
a.p(10);
a.p(10.0);
}
}
class B {
public void p(double i) {
System.out.println(i * 2);
}
}
class A extends B {
// This method overrides the method in B
System.out.println(i);
}
}
public class Test {
A a = new A();
a.p(10);
a.p(10.0);
}
}
class B {
System.out.println(i * 2);
}
}
class A extends B {
// This method overloads the method in B
public void p(int i) {
System.out.println(i);
}
}

Overriding vs. Overloading
o The example above show the differences between overriding
and overloading.
o In (a), the method p(double i) in class A overrides the same
method in class B.
o In (b), the class A has two overloaded methods: p(double i) and
p(int i).
o The method p(double i) is inherited from B.

Full Example
Student
Post
Graduate
Graduate
• Have part time hours • Have a field training

Full Example
o Constructing an instance of a class invokes all the superclasses’
constructors along the inheritance chain.
o This is known as constructor chaining.

Full Example
Methods of a subclass cannot directly access private members of their
superclass.
With inheritance, the common instance variables and methods of all the
classes in the hierarchy are declared in a superclass.
Use the protected access modifier when a superclass should
provide a method only to its subclasses and other classes in
the same package, but not to other clients.

Full Example
A subclass is more specific than its superclass and represents a smaller
group of objects.
A superclass's protected members have an intermediate level
of protection between public and private access. They can be
accessed by members of the superclass, by members of its
subclasses and by members of other classes in the same
package.
A compilation error occurs if a subclass constructor calls one
of its superclass constructors with arguments that do not match
the superclass constructor declarations.

Full Example
In Java, the class hierarchy begins with class Object (in package
java.lang), which every class in Java directly or indirectly extends.
Invoking a superclass constructor’s name in a subclass causes a syntax
error.
Java requires that the statement that uses the keyword super appear first
in the constructor.
If a class is designed to be extended, it is better to provide a no-arg
constructor to avoid programming errors.

Practices
Practice 1
Using charts, What is
Inheritance ?
Practice 2
Compare between the
and which of them
supported in java.
Practice 3
By UML class, explain
the concepts of
Superclasses and
Subclasses.
Practice 4
Give 3 examples to
explain Direct & Indirect
Superclasses.
Practice 5
Diffrenciate between
"is-a" and the "has-
a" relationship.
Practice 6
Explain the
Constructor Chaining
using code.

Practices
Practice 7
True or false?
 You can override a
private method
defined in a
superclass.
 You can override a
static method
defined in a
superclass.
 A subclass is a
subset of a
superclass.
Practice 8
Write simple code:
 What keyword do
you use to define a
subclass?
 How do you
explicitly invoke a
superclass’s
constructor from a
subclass?
 How do you invoke
an overridden
superclass method
from a subclass?
Practice 9 (In groups)
Define The
GeometricObject,
Circle and Rectangle,
based on the
GeometricObject is the
superclass for Circle
Rectangle .

‫Chapter3 inheritance

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