Lecture 14 (inheritance basics)

INHERITANCE BASICS
1. Reusability is achieved by INHERITANCE
2. Java classes Can be Reused by extending a class. Extending
an existing class is nothing but reusing properties of the
existing classes.
3. The class whose properties are extended is known as super
or base or parent class.
4. The class which extends the properties of super class is
known as sub or derived or child class
5. A class can either extends another class or can implement an
interface

A
B
class B extends A { ….. }
A super class
B sub class
A
B
<<class>>
<<class>>
<<class>>
<<interface>>class B implements A { ….. }
A interface
B sub class

Various Forms of Inheritance
A
B
Single Inheritance
A
B
Hierarchical Inheritance
X
A B C
X
A B C
MultiLevel Inheritance
A
B
C
A
B
C
A B
C
Multiple Inheritance
NOT SUPPORTED BY JAVA
A B
C
SUPPORTED BY JAVA

Forms of Inheritance
• Mulitiple Inheritance can be implemented by
implementing multiple interfaces not by extending
multiple classes
Example :
class Z extends A implements C , D
{ …………}
OK
class Z extends A ,B class Z extends A extends B
{ {
OR
} }
A C D
Z
WRONG WRONG

Defining a Subclass
Syntax :
class <subclass name> extends <superclass name>
{
variable declarations;
method declarations;
}
• Extends keyword signifies that properties of the super
class are extended to sub class
• Sub class will not inherit private members of super class

Access Control
Access Modifiers
Access Location
public protected friendly private
Same Class Yes Yes Yes Yes
sub classes in same
package
Yes Yes Yes No
Other Classes in
Same package
Yes Yes Yes No
Subclasses in other
packages
Yes Yes No No
Non-subclasses in
other packages
Yes No No No

1. Whenever a sub class object is created ,super class
constructor is called first.
2. If super class constructor does not have any
constructor of its own OR has an unparametrized
constructor then it is automatically called by Java Run
Time by using call super()
3. If a super class has a parameterized constructor then it
is the responsibility of the sub class constructor to call
the super class constructor by call
super(<parameters required by super class>)
4. Call to super class constructor must be the first
statement in sub class constructor
Inheritance Basics

Inheritance Basics
When super class has a Unparametrized constructor
class A
{
A()
{
System.out.println("This is constructor of class A");
}
} // End of class A
class B extends A
{
B()
{
super();
System.out.println("This is constructor of class B");
}
} // End of class B
Optional
Cont…..

class inhtest
{
public static void main(String args[])
{
B b1 = new B();
}
}
OUTPUT
This is constructor of class A
This is constructor of class B

class A
{
A()
{
System.out.println("This is class A");
}
}
class B extends A
{
B()
{
System.out.println("This is class B");
}
}
class inherit1
{
{
B b1 = new B();
}
}
/*
E:Java>java inherit1
This is class A
This is class B
E:Java>
*/
File Name is xyz.java

class A
{
private A()
{
}
}
class B extends A
{
B()
{
}
}
class inherit2
{
{
B b1 = new B();
}
}
/*
E:Java>javac xyz1.java
xyz1.java:12: A() has private access
in A
{
^
1 error

class A
{
private A()
{
}
A()
{
}
}
class B extends A
{
B()
{
}
}
class inherit2
{
{
B b1 = new B();
} }
/*
E:Java>javac xyz2.java
xyz2.java:7: A() is already defined in
A
A()
^
xyz2.java:16: A() has private access
in A
{
^
2 errors
*/

When Super class has a parametrized constructor.
class A
{
private int a;
A( int a)
{
this.a =a;
System.out.println("This is constructor of class A");
}
}
class B extends A
{
private int b;
private double c;
B(int b,double c)
{
this.b=b;
this.c=c;
System.out.println("This is constructor of class B");
}
}
D:javabin>javac
inhtest.java
inhtest.java:15: cannot find
symbol
symbol : constructor A()
location: class A
{
^
1 errors
B b1 = new B(10,8.6);

class A
{
private int a;
A( int a)
{
this.a =a;
System.out.println("This is
constructor of class A");
} }
class B extends A
{
private int b;
private double c;
B(int a,int b,double c)
{
super(a);
this.b=b;
this.c=c;
System.out.println("This is
constructor of class B");
} }
B b1 = new B(8,10,8.6);
OUTPUT

class A
{
private int a;
protected String name;
A(int a, String n)
{
this.a = a;
this.name = n;
}
void print()
{
System.out.println("a="+a);
}
}
class B extends A
{
int b;
double c;
B(int a,String n,int b,double c)
{
super(a,n);
this.b=b;
this.c =c;
}
void show()
{
//System.out.println("a="+a);
print();
System.out.println("name="+name);
System.out.println("b="+b);
System.out.println("c="+c);
}
}
a is private in
class A
Call to print()
from super
class A Accessing name field from
super class (super.name)

class xyz3
{
{
B b1 = new B(10,"OOP",8,10.56);
b1.show();
}
}
E:Java>java xyz3
a=10
name=OOP
b=8
c=10.56

USE OF super KEYWORD
• Can be used to call super class constrctor
super();
super(<parameter-list>);
• Can refer to super class instance
variables/Methods
super.<super class instance variable/Method>

class A
{
private int a;
A( int a)
{
this.a =a;
System.out.println("This is constructor
of class A");
}
void print()
{
}
void display()
{
System.out.println("hello This is Display
in A");
}
} // End of class A
class B extends A
{
private int b;
private double c;
{
super(a);
this.b=b;
this.c=c;
of class B");
}
void show()
{
print();
}
} // End of class B

class inhtest1
{
{
B b1 = new B(10,8,4.5);
b1.show();
}
}
/* OutPUt
D:javabin>java inhtest1
a=10
b=8
c=4.5
*/

class A
{
private int a;
A( int a)
{
this.a =a;
of class A");
}
void show()
{
}
void display()
{
in A");
}
}
class B extends A
{
private int b;
private double c;
{
super(a);
this.b=b;
this.c=c;
of class B");
}
void show()
{
super.show();
display();
}
}

class inhtest1
{
{
B b1 = new B(10,8,4.5);
b1.show();
}
}
/* OutPut
a=10
b=8
c=4.5
hello This is Display in A
*/

class A
{
int a;
A( int a)
{ this.a =a; }
void show()
{
}
void display()
{
in A");
}
}
class B extends A
{
int b;
double c;
{
super(a);
this.b=b;
this.c=c;
}
void show()
{
//super.show();
}
}

class inhtest2
{
{
B b1 = new B(10,20,8.4);
b1.show();
}
}
/*
a=10
b=20
c=8.4
*/

class A
{
int a;
A( int a)
{ this.a =a; }
}
class B extends A
{
// super class variable a hides here
int a;
int b;
double c;
{
super(100);
this.a = a;
this.b=b;
this.c=c;
}
void show()
{
System.out.println("Super class a="+super.a);
}
}

class inhtest2
{
{
B b1 = new B(10,20,8.4);
b1.show();
}
}
/* Out Put
Super class a=100
a=10
b=20
c=8.4
*/

Lecture 14 (inheritance basics)

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