in java all function are virtual.only the function marked 
with the final is not virtual.
means to say that if there is any method in super class and 
you implemented this method in subclasses too.
then you can invoke on  any subcalsses instances refer to 
as a superclasses.and the method behaviour will change 
according to subcallses instances .
class Amit
{
public void sleep()
{
System.out.println("amit slleping");
}
public static void main(String []args)
{
Amit a = new Amit();
a.sleep();
a = new Subclass1();
a.sleep();
a= new Subclass2();
a.sleep();
}
}

class Subclass1 extends Amit
{
public void sleep()
{
Sysetm.out.println("subclass sleeping");
}
}

class Subclass1 extends Amit
{
public void sleep()
{
System.out.println("subclass 2 sleeping");
}
}
so the output will be 
amit sleeping
Subclass1 sleeping
Subclass2 sleeping

so the eat function behave virtualy for the differnt 
instance .so this is called the virtual function .so don't 
worry every function in java are virtual not the final 
function.