Base Class, Sub Class Confusion

[Preety Narashimhan]

Hi all,
I have a problem here regd the base and sub class,
Assumen the subclass extends the base class.
SubClass objSC = new SubClass();
BaseClass objBC = objSC;
Now here the object of the BaseClass refers to the SubClass?
So all methods of the SubClass can be referred to by objBC?

[Barry Gaunt]

Now here the object of the BaseClass refers to the SubClass?

Yes.

So all methods of the SubClass can be referred to by objBC?

No. Through objBC you can only call the methods of BaseClass. However, if SubClass overrides a method of BaseClass it will be SubClass's version of the method that will be executed by the call.
-Barry
[ October 02, 2002: Message edited by: Barry Gaunt ]
[ October 02, 2002: Message edited by: Barry Gaunt ]

[Preety Narashimhan]

Tx Barry,
But i am unable to understand, when the parent object (now referring to the subclass) cannot access any other methods of the SubClass( by which i mean those methods which are not overridden by the SubClass)...after all it has teh refernece of the SubClass ?

[Barry Gaunt]

The BaseClass class does not necessarily have all the methods that the SubClass class has. So if you are looking at an instance of the SubClass "through" the BaseClass' view how can you call a method which it does not have?
-Barry
[ October 03, 2002: Message edited by: Barry Gaunt ]

[Preety Narashimhan]

Agreed a base calss cannot access methods it does not ahve but a baseclass object referring to a subclass .. that shud b able to since it now no longer points to the base class but to the subclass rt?

[Barry Gaunt]

Sorry but I can only suggest that you read up on the subject by downloading a book from here.
-Barry

[Ilja Preuss]

Originally posted by Preety Narashimhan:
Agreed a base calss cannot access methods it does not ahve but a baseclass object referring to a subclass .. that shud b able to since it now no longer points to the base class but to the subclass rt?

Java is a statically typed language, that is, all method calls are checked versus validity at compile time. But the compiler doesn't know what object the BaseClass reference is pointing to, therefore it can only allow you to call BaseClass' methods.
What you are describing is possible in dynamically typed languages like Smalltalk and Ruby. In those languages you get an error at runtime when you call a method on an object that doesn't understand the message.