17 April 2012

Overriding Java Explanation with example


The benefit of overriding is: ability to define a behavior that's specific to the sub class type. Which means a subclass can implement a parent calss method based on its requirement.

In object oriented terms, overriding means to override the functionality of any existing method.


Consider the following program :

 The classes Vehicle, Plane, OverridingTest are in same package



public class Vehicle {
public void tyre(){
System.out.println("Vehicles have tyres");
}

}



public class Plane extends Vehicle {
public void tyre(){
System.out.println("Plane have tyres");
}
}




public class OverridingTest {
public static void main(String[] arg){
Vehicle a = new Vehicle();
Vehicle b = new Plane();
a.tyre();  // runs the method in Vehicle class
b.tyre();  // runs the method in Plane class
}

}


Output:


Vehicles have tyres
Plane have tyres

In the above example you can see that the even though b is a type of Vehicle it runs the tyre method in the Plane class. The reason for this is : In compile time the check is made on the reference type. However in the runtime JVM figures out the object type and would run the method that belongs to that particular object.

Therefore in the above example, the program will compile properly since Vehicle class has the method tyre. Then at the runtime it runs the method specific for that object.




Consider the Following Program:


public class Vehicle {
public void tyre(){
System.out.println("Vehicles have tyres");
}

}



public class Plane extends Vehicle {
public void tyre(){
System.out.println("Plane have tyres");
}
            public void fly(){
System.out.println("Plane can fly");
}
}




public class OverridingTest {
public static void main(String[] arg){
Vehicle a = new Vehicle();
Vehicle b = new Plane();
a.tyre();  // runs the method in Vehicle class
b.tyre();  // runs the method in Plane class
                b.fly();
}

}


Output:

Exception in thread "main" java.lang.Error: Unresolved compilation problem: 
The method fly() is undefined for the type Vehicle

at OverridingTest.main(OverridingTest.java:8)



This program will throw a compile time error since b's reference type Vehicle doesn't have a method by the name of fly.



Rules for method overriding:

  • The argument list should be exactly the same as that of the overridden method.
  • The return type should be the same or a subtype of the return type declared in the original overridden method in the super class.
  • The access level cannot be more restrictive than the overridden method's access level. For example: if the super class method is declared public then the overridding method in the sub class cannot be either private or public. However the access level can be less restrictive than the overridden method's access level.
  • Instance methods can be overridden only if they are inherited by the subclass.
  • A method declared final cannot be overridden.
  • A method declared static cannot be overridden but can be re-declared.
  • If a method cannot be inherited then it cannot be overridden.
  • A subclass within the same package as the instance's superclass can override any superclass method that is not declared private or final.
  • A subclass in a different package can only override the non-final methods declared public or protected.
  • An overriding method can throw any uncheck exceptions, regardless of whether the overridden method throws exceptions or not. However the overriding method should not throw checked exceptions that are new or broader than the ones declared by the overridden method. The overriding method can throw narrower or fewer exceptions than the overridden method.
  • Constructors cannot be overridden.


Using the super keyword:

When invoking a superclass version of an overridden method the super keyword is used.




public class Vehicle {
public void tyre(){
System.out.println("Vehicles have tyres");
}

}



public class Plane extends Vehicle {
public void tyre(){

                super.tyre(); // invokes the super class method
// it will call base class here it is Vehicle class and execute
the tyre method

System.out.println("Plane have tyres");
}
}




public class OverridingTest {
public static void main(String[] arg){
Vehicle b = new Plane();
b.tyre(); 
}

}


Output:


Vehicles have tyres
Plane have tyres









1 comments:

Naviya Nair said...

I have read your blog its very attractive and impressive. I like it your blog.

Java Online Training Java EE Online Training Java EE Online Training Java 8 online training Core Java 8 online training

Java Online Training from India Java Online Training from India Core Java Training Online Core Java Training Online Java Training InstitutesJava Training Institutes

Post a Comment

Related Posts with Thumbnails

Popular Posts

 
;