Inheritance:
o Inheritance is a process of
reusing the features (Data members and member methods) of base class into
derived class.
o A base class is one which
always gives its features to the derived class.
o A derived class is one which
contains its own features as well as the features from its base class.
o Syntax for inheriting the
features from the base class to derived class:
class <DerivedClassName> extends
<BaseClassName>
{
Variable
declaration
Method
definitions
}
o Here
<DerivedClassName> and <BaseClassName> are the names of derived and
base classes respectively.
o Here “extends” is a reserved
keyword which is used for inheriting the base class data members and methods
into derived class and it also improves the functionality of the derived class.
o It is strongly recommended
to create an object of bottom most derived class since it contains the features
of its own as well as its parent class features too.
o When we create an object of
bottom most derived class, JVM always creates the memory space for the data
members of the super class first and later it creates the memory space for the
data members of the derived class.
o Once the base class is
declared as final, it can’t be inherited into derived class.
o Private data members of the
base class will not be inherited into derived class.
o For each and every class in
Java, there is a super class called Object class. This class supplies gc ()
i.e. garbage Collector.
o When the base class is
containing final methods then in derived class the final methods will be
inherited but we can’t change the definitions i.e. we can reuse but can’t
override.
The “super” keyword:
o “super” is a keyword which
is used to differentiate the Base-class
features from the derived-class
features with in a derived-class if
and only if they are similar.
o The keyword “super” plays
important roles at 3 levels. They are:
- At variable level
- At method level
- At constructor level
o
“super” at variable level:
§ Whenever we inherit the data
members from the base class to the derived class then there is a
possibility that the base class members
are similar to the derived class members.
§ In order to distinguish the
base class data members with derived class data members, the base class data
members must be preceded with “super” keyword in the context of derived class.
§ Syntax:
super.baseClassDataMember;
o “super” at method level:
§ Whenever we inherit methods
from the base class to the derived class, there is a possibility that the base
class methods are similar to the derived class methods.
§ In order to differentiate
the base class methods derived class methods, the base class methods must be
preceded with “super” keyword in the context of derived class.
§ Syntax:
super.baseClassMethodName();
o “super” at constructor
level:
§ Whenever we develop any
application using the concept of inheritance, it contains base class, derived
class and intermediate base classes.
§ When we create an object of
the bottom most derived class, it calls object creation of its immediate super
class automatically. An object of the immediate super class is nothing but an
object of its top most super class i.e. an object of the bottom most derived
class is nothing but an object of its immediate super class.
§ Hence, the constructors are
being called from bottom most derived class to top most base class and they are
being executed from top most base class to bottom most derived class.
§ An object of the derived
class can be created with respect to either default or parameterized
constructor.
§ Take the following diagram
to understand the concept of super at constructor level:
o super() :
o This constructor is used to
call the default constructor of the base class from the default constructor or
parameterized constructor of derived class in context of derived class.
o super(…)
§ This constructor is used to
call the parameterized constructor of the base class from the default or
parameterized constructor of the derived class in the context of derived class.
Rule 1:
Whenever we want to call the default constructor of the base class from the
default constructor of the derived class, using super () is optional.
Rule 2:
Whenever we want to call the parameterized constructor of the base class from
the parameterized constructor of the derived class, using super (…) is
mandatory.
Rule 3:
Whenever we want to call the default constructor of the base class from the
parameterized constructor of the derived class, using super () is optional.
Rule 4:
Whenever we want to call the parameterized constructor of the base class from
the default constructor of the derived class, using super (…) is mandatory.
Note: whenever we make a
call for either super () or super (…), they must be the first statement in the
derived class constructors.
Example Program:
class Base
{
int
a,b;
Base()
{
a=2; b=3;
System.out.println("I
am in default constructor of Base class.");
System.out.println("
values of a:" +a);
System.out.println("
values of b:" +b);
}
Base(int
x)
{
this();
a=b=x;
System.out.println("I
am in single parameterized constructor of Base.");
System.out.println("
value of a:" +a);
System.out.println(" value of b:" +b);
}
Base(int x,int y)
{
this(600);
a=x;
b=y;
System.out.println("I am
in double parameterized constructor of Base.");
System.out.println("value
of a" +a);
System.out.println("
value of b" +b);
}
}
class Derived extends Base
{
int
m,n;
Derived()
{
super(400, 500);
m=100;
n=200;
System.out.println("I
am in default constructor of Derived class.");
System.out.println("
value of a"+m);
System.out.println("
value of b"+n);
}
Derived(int x)
{
this();
m=n=x;
System.out.println("I
am in single parameterized constructor of Derived class.");
System.out.println("
value of a"+m);
System.out.println("
value of b"+n);
}
Derived(int x, int y)
{
this(300);
m=x;
n=y;
System.out.println("I am
in double parameterized constructor of Derived class.");
System.out.println(" value of a"+m);
System.out.println(" value of b"+n);
}
}
class SuperDemo
{
public static void
main(String args[])
{
Derived d = new Derived(100,200);
}
}
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