RE: Instantiation question

["Stone, Brad V (TC)" <bvstone@xxxxxxxxxxxxxx>]

Ok, so just to make sure I understand.

Animal animal = new Cat();

Methods fron the class Animal are available to animal.
Unique methods in the Cat class are not available to animal.
If a method exists in both Animal and Cat, the methods in Cat will be used.

Animal contains methods isMammal, hasFur, isPoisonous
Cat contains methods hasFur, breed

I can do animal.isMamal(), aninmal.hasFur(), and animal.isPoisonous()
I cannot do animal.breed()
If I used animal.hasFur(), the method from the Cat class will be used, not
the method from the Animal class.

Is this right?

Brad

> -----Original Message-----
> From: Joe Pluta [mailto:joepluta@plutabrothers.com]
> Sent: Wednesday, February 07, 2001 10:28 AM
> To: JAVA400-L@midrange.com
> Subject: RE: Instantiation question
> 
> 
> In Java, every variable is identified by its type and its 
> name.  Non-primitive variables are in effect pointers to 
> objects.  I call them handles.  The type tells the compiler 
> what characteristics to apply to the object that will 
> eventually reside in the handle.
> 
> Why is this important?  Because you can do the following 
> (provided Dog is a subclass of Animal):
> 
> Animal animal = new Dog();
> 
> At this point, you can only call the methods associated with 
> Animal, not those unique to Dog.  This technique is used when 
> you want to deal with objects of different types at a higher 
> level of abstraction.  Also, this allows the use of 
> polymorphism by causing different methods to be called based 
> on the actual type of the object stored in the handle.  This 
> is because, if a class overrides a method of its superclass, 
> when the method is called, the overriding method is called 
> regardless of the type of the handle.  (Whew!)
> 
> Specifically, let's say that class Animal has a method called 
> "isMammal()" that returns a boolean value of false.  In this 
> case, if Dog overrides isMammal and returns true, then 
> "animal.isMammal()" will return true, not false.  This is the 
> most powerful aspect of polymorphism.
> 
> Another side effect (specific to Java's singly-rooted object 
> hierarchy) is that, carried to its extreme, any object can be 
> stored in a handle of type Object.  This allows, for example, 
> heterogenous sets like Vectors to contain any type of object.
> 
> 
> ---------- Original Message ----------------------------------
> From: "Stone, Brad V (TC)" <bvstone@taylorcorp.com>
> Reply-To: JAVA400-L@midrange.com
> Date: Wed, 7 Feb 2001 09:57:00 -0600 
> 
> >I'll let others answer you question, but I'd like to expand on your
> question.
> 
> Why do you need to say:
> 
> Student show = new Student();
> 
> Why not just:
> 
> show = new Student();
> 
> Is there a case where you would want to do something like:
> 
> Student show = new Principal();
> or
> Principal show = new Student();
> 
> Maybe the answer to your original question will answer mine.  :)  I've
> always thought that this type of declaration was a bit redundant.
> 
> Brad
> 
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