Question on Extensibility

[Prashil Wasnik]

Hi,
ParentClass ObjectName = new ChildClass();
What does this exactly mean? What's its significance? Where it can be used?

[Deepak Bala]

You can make a parent reference point to a child object. What this means is that you only have access to methods and variables declared in the parent, but, if any of these non static methods are overridden then the method is called from the child class.

class parent
{
void method(){}
}
class child extends parent
{
void method(){}
}
now if you make a parent reference point to a child then the method in child is called. This is an example of polymorphism.

[bnkiran kumar]

Prashil Wasnik ,this is an example of "is an" relation, which means object of subclass is also an object of super class. This concept is the root for polymorphic behaviour of super class, which used for overriding of nonstatic methods.

class ParentClass
{
int method1()//1
{
// implementation
}
}

class ChildClass extends ParentClass
{

int method1()//2
{

}
public static void main(String arg[])
{
ParentClass a=new ParentClass();
a.method1();//calls 1
ParentClass a=new ChildClass();
a.method1();//calls 2
}
}

Hope it is clear.