Now instances of various packet objects can be

Now, instances of various packet objects can be created and put into the array:
EtherPacket ep = new ; // extends BasePacket
TokenPacket tp = new ; // extends BasePacket
GPSSPacket gp = new ; // extends EtherPacket
packets[0] = ep;
packets[1] = tp;
packets[2] = gp;
If the data types were, for example, integers, bits, and strings, all of these types could not be
stored into a single array, but with polymorphism , it can be done. In this example, because the
methods were declared as
virtual
, the appropriate subclass methods can be accessed from the
superclass variable, even though the compiler did not know — at compile time — what was going
to be loaded into it.
For example,
packets[1]
packets[1].send();
shall invoke the
send
method associated with the
TokenPacket
class. At run time, the system
correctly binds the method from the appropriate class. This is a typical example of polymorphism
at work, providing capabilities that are far more powerful than what is found in a nonobject-
oriented framework.
11. What is the use of the abstract class?
EXAMPLE: useing abstract class

virtual class A ;
virtual task disp ();
$display ( " This is class A " );
endtask
endclass
class EA extends A ;
task disp ();
$display ( " This is Extended class A " );
endtask
endclass
program main ;
EA my_ea ;
A my_a ;
initial
begin
my_ea = new ();
my_a = my_ea ;
my_ea . disp ();
my_a . disp ();
end
endprogram
RESULT
This is Extended class A
This is Extended class A
EXAMPLE: creating object of virtual class
virtual class A ;
virtual task disp ();
$display ( " This is class A " );
endtask
endclass
program main ;
A my_a ;
initial
begin
my_a = new ();
my_a . disp ();
end

endprogram
RESULT
Abstract class A cannot be instantiated
Virtual keyword is used to express the fact that derived classes must redefine the
properties to fulfill the desired functionality. Thus from the abstract class point of view,
the properties are only specified but not fully defined. The full definition including the
semantics of the properties must be provided by derived classes.
Definition (Abstract Class) A class A is called abstract class if it is only used as a superclass
for other classes. Class A only specifies properties. It is not used to create objects. Derived
classes must define the properties of A.