RE: Instantiation question

["Joe Pluta" <joepluta@xxxxxxxxxxxxxxxxx>]

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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