Re: RPG,COBOL or JAVA

Booth wrote:
>Would an example of this, in RPG, be an... say... L-type date
>field?  Should I for example have the ability to extend the
>L-date field's function so that I could have a predefined and
>edited mm/dd/yy field for a display field?   Would that be an
>example of the OO concept of which you speak?

Hmmm, sort of, I think.  But I think your example is
mixing a couple of different concepts here.  In your
example, you have a date field and an edited date
field displayed on the screen.  As I mentioned in my
note to David, you need to distinguish between "IsA"
and "HasA" relationships between your objects.  (Even
before that, you need to identify your objects!)

If one object is some specific instance of another,
that's an "IsA" relationship and inheritance is used
to model the relationship.  But if an object is a
component or attribute of another, that's a "HasA"
relationship.  For example, a dog IS a type of animal
and a dog HAS four legs.  Implemented in an OO language,
class Dog would extend class Animal and contain
references to the legs.

In this case, what is the relationship between a
widget that displays a date and the date field?
IS a date widget a date?  Or does it HAVE a date?

In this specific case, normally a date widget class
would be an extension of another widget class, possibly
a TextField in Java.  A date widget object might
contain a reference to a date field which could be
updated automatically if the user changes that value in
the widget.

An example of inheritance would be if you wanted to
have a date field that could only hold a specific
subset of date values, say dates that represented
the first working day of the month.  Let's say class
Date had a method called setDate.  You could define
a new class called DueDate that extends Date such that
method setDate would be overridden to accept only the
desired dates.  The original class may have say a
dozen different methods.  The extended class might only
define the one method that's different.  All other
methods would be inherited by the sub-class.

As I said in my previous note, experience with OO
design and implementation is an excellent teacher.

Cheers!  Hans

Hans Boldt, ILE RPG Development, IBM Toronto Lab, boldt@ca.ibm.com


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