Inheritance in Java
Corejava InheritanceNotes
Table of contents
Defination:
Inheritance is one of the key features of Object-oriented programming. It is the mechanism in java which allows us to create a new class (derived class) from an existing class (base class).
The derived class inherits the features (attributes and methods) from the base class and can have additional features of its own.
Example:
class A{ void m1(){ System.out.print ("m1"); } } class B extends A{ void m2(){ System.out.print ("m2"); } } class Test{ public static void main(String args){ A aob= new A(); B aob = new B(); aob.m1(); bob.m2();
bob.m1(); // inherited from A
}
}
super keyword
Used to invoke parent class constructor from the child class, to pass parameters from the child constructor to parent constructor.
ex:
class A
int x;
A(int x)
{
this.x=x;
}
void area() {
int asq = x * x;
System.out.println(asq);
}
}
class B extends A
int y,z;
B(int i, int j, int k ) {
super(i); //invokes A's constructor and (i is passed to x) y = j; z = k;
}
area() {
int ar = y * z;
System.out.println(ar);
} } In the above, ex. B can be extended only one class.
It cannot be extended by more than one class, ex:
class B extends A, C //not allowed so
Multiple inheritance can be achieved by Interfaces.
Interfaces in Java
Interface contains only method declarations(empty methods).
These methods are implemented by other classes
Interface declaration
interface Iface1{
m1();
m2();
m3();
}
Interface implementation
interface Iface1{ m1(); m2(); m3(); } class A implements Iface1{ m1(){ ....; ....; } m2() { ....; ....; } m3() { ....; ....; }
public static void main(String args{}){ A aobj = new A(); aobj .m1();
aobj .m2();
aobj .m3();
}
} ex2: implementing multiple interfaces
interface Iface1
{
v1; m1(); } interface Iface2{ v2;
m2();
}class A implements Iface1,Iface2 {
m1(){
....; ....;
}
m2(){ ....; ....;
} }
example2
interface Shape{ void area();
} class square implements Shape { area(){.... }
}
class Rect implements Shape { area(){.... }
}
class Circle implements Shape { area(){.... }
}
-- in the above ex. area() is common in all classes(Square, Rect, Circle).
Uses of Interfaces:
To achieve commonality, i.e. to maintain some standard among different classes, interfaces are used.
Rule for implementing interfaces
- All Methods must be defined in the implementation class.
- If not defined all methods, then that class becomes abstract. (means not complete) this class can't be instantiated. So this class must be extended by some other class which implements all remaining methods of interface
- Then this child class called as Concrete Class (this class can be instantiated).
- Abstract classes extended by some other classes
Abstract class in Java
- Abstract class contains some methods undefined.
- Abstract class can't be instantiated.
ex: abstract class A { m1(); // method declaration. m2(){ .......; .......; } } ---m1() // not defined just it is declared.
-- extended class will provides definition for method declarations.
ex:class B extends A
{
m1(); // A's Method defined
{
.......;
}
m3() // B's own method.
{
.......;
}
}
Concrete class
The class which defines all the methods of its parents is called concrete class. Concrete class can be instantiated. In the above example, class B can be concrete class.
ex:
class B extends A{
m1() // A's Method defined { .......; }
m3() // B's own method.{ .......;
}
public static void main(String args{}){ B b= new B(); b.m1(); b.m2(); b.m3();
}
Overriding in Java Inheritace
Base class method is redefined in the derived class.
Rules:
- Both methods should have same signature i.e., both method name, parameters list should be same.
- return type can be different.
- method which is invoked depends on the object created.
ex:
class Animal{
eat() {
s.o.p("eats something"); }
class Horse extends Animal {
eat(){
s.o.p("oates"); }
}
} class OverrideDemo{ public static void main(String args[]) {
Animal a= new Animal();// o/p -->something
Animal b= new Horse();// o/p -->oates
a.eat();
b.eat();
}} program: public class Animal{
void eat() {
System.out.println("eats something");
}
}
class Horse extends Animal {
void eat(){
System.out.println("eats oates");
}
}
class OverrideDemo{
public static void main(String args[])
{
Animal a= new Animal();//-----> o/p -->eats something
Animal b= new Horse();//-----> o/p -->eats oates
a.eat();
b.eat();
}
}
Dynamic Binding Vs static binding in Java
static binding
Reference points to its own type of object.
Rect rt = new Rect();
dynamic binding
It is super type of reference, points to any of its child object
ex:
Shape sp;
sp = new Square(); //or
sp = new Rect(); //or
sp = new Circle();
sp = rt // assining Rect ref to its super class ref.
example interface Shape{
area();
}
class square implements Shape {
area(){
....;
}
}
class Rect implements Shape
{
area(){
....;
}
}
class Circle implements Shape { area(){
.... ;
}
}
class Test{
Shape sp;
Square sq= new Square();
sq.area();
Rect rt = new Rect();//static binding occurs at compile time[compiler]
rt.area();
Circle cr= new Circle();
cr.area();
sp=rt; sp.area(); //Dynamic Binding occurs at runtime[jvm]
}
} example program
interface Shape{
public float area();
}
class Square implements Shape {
int side;
Square(int side)
{
this.side=side;
}
public float area(){
return (side*side);
}
}
class Rect implements Shape
{
int side; int width;
Rect(int side, int width)
{
this.side=side;
this.width=width;
}
public float area()
{
return (side*width);
}
}
class Circle implements Shape
{
public final float pi=3.14f;
int r;
Circle(int r)
{
this.r=r;
}
public float area(){
return (2*pi*r*r);
}
}
class Test1{
public static void main(String args[]){ float area = 0.0f;
Square sq = new Square(4);
area = sq.area();
System.out.println("Square Area="+area); Rect rt = new Rect(5,8);
area = rt.area();
System.out.println("RectangleArea="+area); Circle cr = new Circle(5);
area = cr.area();
System.out.println("Circle Area="+area); Shape sp; sp = sq;
area = sp.area(); //Dynamic Binding occurs at runtime[jvm]
System.out.println("Area="+area);
}
}
Types of interitance in Java
Extends Vs implements
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