Inheritance in Java is an Object oriented or OOPS
concepts, which allows to emulate real world Inheritance behavior, Inheritance
allows code reuse in Object oriented programming language e.g. Java. Along with
Abstraction,
Polymorphism
and Encapsulation,
Inheritance forms basis of Object oriented programming. Inheritance is
implemented using extends keyword in Java and When one
Class extends another Class it inherit all non
private members including fields and methods. Inheritance in Java can be
best understand in terms of Parent and Child class,
also known as Super class and Sub class in Java programming language. The class
which extends another class becomes Child of the class it extends and
inherit all its functionality which is not private or package-private given
where Child class is created. Inheritance is the
most easy way to reuse already written and tested code but not always best way
to do so, which we will see in this article. There are lot of example of
Inheritance in Object oriented world e.g. Child inherit properties from parent,
Dog inherit properties of Animal etc. In this Java and OOPS
tutorial we will learn What is Inheritance in Java, How to use
Inheritance in Java, Simple Example of
Java Inheritance and some important points about Inheritance in Java.
What
is Inheritance in Java
Inheritance in Java is way to define relationship between two classes.Inheritance defines Parent and Child relationship between two classes. Similar
to real world where a Child inherit his parents Surname and other
qualities, In Java programming language, Child class
inherit its Parent’s property and code. In Java
programming, terms Super class and Sub Class is used to
represent Parent and Child class,
while in other object oriented language like C++, terms base and derived class
is used to denote Inheritance. Inheritance is also simplest but not
optimal way to reuse code and When one class use Inheritance to extend another
class it get access to all non private
code written in Super class. In Context of Inheritance in Java, following are
legal :
1) Super class reference variable can point to Sub Class Object e.g.
SuperClass parent = new
SubClass();
is legal at compile time because of IS-A relationship between Super class
and Sub Class. In Java Sub Class IS-A Super class like Mango IS-A Fruit.
Similarly you can pass Sub class object in method arguments where Super
class is expected, return Sub Class instance where return
type of method is Super Class etc.
On the other hand if an you want to store object of Sub class, which is
stored in super class reference variable, back on Sub class reference variable
you need to use casting in Java, as shown below :
SubClass child = (SubClass) parent; //since parent variable pointing to SubClass object
This code will throw ClassCastException if parent
reference variable doesn't point to a SubClass object.
Important points about Inheritance in Java
let’s see some worth noting points about Inheritance concepts in Java programming language. This tips are
very helpful while using Inheritance in Java program.
1) As I said earlier Inheritance in Java is supported using extends and implements keyword, extends keyword is
used to inherit from another Java
Class and allow to reuse functionality of Parent class. While implements keyword is
used to implement Interface in Java. Implementing an interface in Java doesn't actually meant for code reuse but
provides Type hierarchy support. You can also use extends keyword
when one interface extends another interface
in Java.
2)If you do not want to allow Inheritance for your class than you can make
it final. final classes can not be extended in Java and
any attempt to inherit final
class will result in compile time error.
2)Constructor in Java are not inherited by Sub Class. In face
Constructors are chained, first statement in constructor is always a call to
another constructor, either implicitly or explicitly. If you don't call any
other constructor compiler will insert super(), which
calls no argument constructor of super class in Java. this
keyword represent current instance of class and super keyword
represent instance of super class in Java.
3) Inheritance in Java represents IS-A relationship. If you see
IS-A relationship between your domain Objects and Classes than consider using Inheritance e.g. if
you have a class called ProgrammingLanguage than Java IS-A
ProgrammingLanguage and should inherit from ProgrammingLanguage class.
4) Private members of Super class is not visible to Sub class even after
using Inheritance in Java. Private members include any private
field or method in Java.
5) Java has a special access modifier known as protected which is
meant to support Inheritance in Java. Any protected member including protected method and
field are only accessible in Child class or Sub class outside the package
on which they are declared.
6)One of the risk of Inheritance in Java is that derived class can alter behavior of base class by overriding
methods, which can compromise variants of base class e.g. if a malicious
class overrides String's equals
method in Java to change the comparison logic, you may get different behavior when String reference variable points to that class. to prevent such
malicious overriding, you can make your class final to disallow inheritance.
But beware making a class final severely implements its client's ability to
reuse code. It make more sense from security perspective and that's one of the
reason Why
String is final in Java.
7) Use @Override annotation while overriding
super class's method in subclass. This will ensure a compile time check on
whether overriding method actually overrides super class method or not. Its
common mistake to overload
a method instead of overriding
it mostly when super class method accept Object type, common examples are
equals method, compareTo method and compare()
method in Java.
When to use Inheritance in Java
Many programmer says favor
composition over Inheritance which is true but there are cases where
Inheritance is a natural choice, Even in Java API there are many places where
inheritances is used e.g. In Java
collection framework most of concrete collection classes inherit from there
Abstract counterpart e.g. HashSet
extends AbstractSet , LinkedHashSet extends HashSet, ArrayList
extends AbstractList etc. My general policy to decide
whether to use Inheritance or not is to check "IS-A"
relationship. For example all above example of Inheritance satisfy IS-A rule
e.g. HashSet IS-A Set. Similarly if you have class called Fruit and want
to create another class called Mango, its best to use
inheritance and Mango should extend Fruit because Mango is a Fruit. By
extending Fruit class it gets common state and behavior of Fruit object.
Conversely if you find HAS-A relationship between two classes than use Composition e.g. Car HAS-A Seat, So Car class
should be composed with a Seat and Seat should not
extend Car here. Another general rule of Inheritance is that
if you are creating a class which adds more feature into existing class, you
can extend it to reuse all of its code. That’s the reason of using Runnable
interface over Thread class for creating Thread
in Java.
How to
use Inheritance in Java
You can use Inheritance in Java by using two keywords, extends and implements. extends keyword is
used when one Class inherit from other Class or one interface extend another
interface. On the other hand implements keyword is
used when
a class implement an interface which is also a form of Abstraction
in Java. Interestingly, Inheritance facilitate Polymorphism in Java. By
using Inheritance Sub class gets all property of Super class, except
private, and can represent Super class
i.e. you can store sub class instance in a Super class reference variable,
which is a form of Polymorphism
in Java. All flexibility which is provided by interface based design is
achieved using polymorphism. Common example of this Factory
design pattern in Java where return type of Factory method should be base
interface, which allows Factory method to return any implementation of base
interface, which is actually created using Inheritance in Java. In next section
we will an example of Inheritance, which will show How to code for inheritance.
Inheritance Example in Java
Here is a simple example of Inheritance in Java where we have a class
called Server which represent any Server has common
functionality e.g. uptime(), start() and stop(). Since
every Server has own way of starting and stopping, it can override
those method but any Server class can reuse common code which
is applicable to all type of Server e.g. uptime.
/**
*
* Java program to demonstrate Inheritance in Java programming language.
* Inheritance is used to reuse
code and Java programming language allows you
* to either extend class or
implements Interface. In this Program we have a
* Super Class Server and a Sub
Class Tomcat, which is a Server.
* Tomcat inherit start() and
stop() method of Server Super Class.
*
* @author Javin
*/
public class
Inheritance{
public static void
main(String args[]) {
//Super class reference variable
can hold Sub Class instance
Server server
= new Tomcat();
//we need to cast to get actual
Server instance back in reference variable.
Tomcat tomcat = (Tomcat) server;
tomcat.start(); //starting Server
System.out.println( "Uptime of Server in nano: " + server.uptime());
tomcat.stop();
}
}
class Server{
private long uptime;
public void start(){
uptime = System.nanoTime();
}
public void stop(){
uptime = 0;
}
public long uptime(){
return
uptime;
}
}
class Tomcat extends Server{
@Override
public void start(){
super.start();
//Tomcat Server
specific task
System.out.println("Tomcat Server started");
}
@Override
public void stop(){
super.stop(); //you can call
super class method using super keyword
System.out.println("Tomcat Server Stopped");
}
}
Output:
Tomcat Server started
Uptime of Server : 105898370823666
Tomcat Server Stopped
That’s all on What is Inheritance
in Java, When to use Inheritance
and How to use Inheritance in Java programming language. If you are
coming from C++ background than only surprise for you is that Java
does not support multiple Inheritance, other than that Java Inheritance is similar
to Inheritance in any other programming language e.g. C++.
Related Java programming tutorial from Javarevisited Blog
Source:http://javarevisited.blogspot.com/2012/10/what-is-inheritance-in-java-and-oops-programming.html
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