Java inheritance in object oriented programming

A detailed ppt on inheritance in java

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University Of Ha’il 1
Chapter 1
Inheritance

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Introduction to Inheritance
Inheritance is one of the main techniques of object-
oriented programming (OOP)
Using this technique, further classes can be created
from existing ones; those classes are said to inherit
the methods and instance variables of the class they
inherited
The original class is called the base class
The new class is called a derived class
Advantage: Reusing existing code
7-2
Base Class
Derived Class

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Inheritance
Animal
Dog FoxCat
3

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Inheritance
Land Vehicle
Truck CarBus
4

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Inheritance
Land Vehicle
Car
Toyota
TruckBus
Yaris Corolla Camry
5

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6
An Example Inheritance Hierarchy
Data and behavior associated with super-classes
are accessible to those subclasses

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7
The is-a Rule
Always check generalizations to ensure
they obey the is-a rule
“A checking account is an account”
“A village is a municipality”
“A cat is an animal”

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University Of Ha’il 8
Derived Class (subclass)
A derived class, also called a subclass (or child class), is
defined by starting with another already defined class,
called a base class or superclass or parent class, and
adding (and/or changing) methods, instance variables,
and static variables
The derived class inherits:

all the public methods,
all the public and private instance variables, and
all the public and private static variables from the base class.
The derived class can add more instance variables,
static variables, and/or methods.

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University Of Ha’il 9
Derived Classes
The extends clause in a class declaration establishes an
inheritance relationship between two classes. It has the
following syntax:
class DerivedClass extends BaseClass
{
// body of the class
}
Base Class
Derived Class

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Parent and Child Classes
A base class is often called the parent class
A derived class is then called a child class
These relationships are often extended such that
a class that is a parent of a parent . . . of another
class is called an ancestor class
If class A is an ancestor of class B, then class B can be
called a descendent of class A
7-10

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University Of Ha’il 11
Inheritance and Variables
Variable Hiding
If a variable of a class has a same name (type maybe
different) as a superclass variable, in that case class
variable hides the superclass variable.
You can access a hidden variable by using super keyword
super.variable

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University Of Ha’il 12
Example (Inheritance and variables)
class C1 {
static int x;
}
class C2 extends C1 {
static String x;
}
class Cc {
public static void main(String[] args)
{
C1 p = new C1();
p.x = 55;
System.out.println("p.x=" +
p.x);
C2 q = new C2();
q.x = "This is a String";
System.out.println( "q.x=" +
q.x);
}
}
Output:
p.x=55
q.x=This is a String

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University Of Ha’il 13
Example (Inheritance and variables)
class M100 {
int x = 100;
}
class M200 extends M100 {
String x = " Welcome “ ;
void display()
{
System.out.println( "x=" + x);
System.out.println( "super.x="
+super.x);
}
}
class SuperKeyword {
public static void main(String[] args)
{
M200 m200 = new M200();
m200.display();
}
}
Output:
x= Welcome
super.x=100

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Example
Inherit1.java
Inherit2.java
University Of Ha’il 14

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Dr.Mwaffaq Abu Alhija University Of Ha’il 15
Homework
Write an application that illustrates how to access a hidden
variable. Class G declares a static variable x. Class H
extends G and declares an instance variable x. A display()
method in H displays both of these variables.

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Inheriting Methods
Override method:
Supply a different implementation of a method that
exists in the superclass
Must have same signature (same name and same
parameter types)
Inherit method:
Don't supply a new implementation of a method that
exists in superclass
Superclass method can be applied to the subclass
objects

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Inheriting Methods (Cont’d)
Add method:
Supply a new method that doesn't exist in
the superclass
New method can be applied only to subclass
objects

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18
Overloading vs Overriding
Comparison between overloading and overriding
Overriding deals with two
methods, one in a parent
class and one in a child class
with the same signature
Overriding lets you define a
similar operation in different
ways for different object types
Overloading deals with
multiple methods in the same
class with the same name but
different signatures.
Overloading lets you define a
similar operation in different
ways for different data

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University Of Ha’il 19
Method Overriding
The access permission of an overridden method can be
changed from private in the base class to public in the
derived class.
However, the access permission of an overridden
method can not be changed from public in the base
class to a more restricted access permission in the
derived class.

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University Of Ha’il 20
Method Overriding
Given the following method header in a base case:
private void doSomething()
The following method header is valid in a derived class:
public void doSomething()
However, the opposite is not valid
Given the following method header in a base case:
public void doSomething()
The following method header is not valid in a derived
class:
private void doSomething()

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University Of Ha’il 21
Example (Method Overriding)
class A1
{
void hello()
{
System.out.println( "Hello from A1" );
}
}
class B1 extends A1
{
void hello()
{
System.out.println( "Hello from B1" );
}
}
class C1 extends B1
{
void hello()
{
System.out.println( "Hello from C1");
}
}
class MethodOverriding
{
public static void main(String[] arg)
{
C1 obj = new C1();
obj.hello();
A1 a = new C1() ;
a.hello();
}
}
Output:
Hello from C1
Hello from C1