Pace 76

co PO WEEKNAPPLIGATION DEVELOPMENT |

SEPTEMBER 11, 1989

O EN
APPLIED INTELLIGENCE. —

OOP Goes Beyond the Commonsense Meaning of 'Object

This is the sec-
ond in a series
of articles on ob-
ject-ortented,
technigues, a
new technology
that's changing
the way pro-
grammers and
tisers deal with
computers.

When we think
of the term "ob-
ject," we have an

z UAMES —
MARYIN NEM intuitive grasp of
what that

means. In our everyday world, objects
have both properties and behaviors: An
oven has shape and size; it can bake and
broil.

The behaviors for responding to the
message depend on the values of the
variables of a particular object.

Object-oriented programming tech
nigues are used to invoke the behaviors
in an object. For example, the verifica-
tion message to an order object would
invoke an order behavior that would
send a message to an inventory object
reguesting the guantity on hand for the
item. The inventory object could, in
turn, send messages to other objects to
obtain the guantity on hand.

The benefit of this approach is in-
creasingly important as systems get
large and complex. Programmers imple-
menting the order object do not need'to

( igning which sends the same check reorder
tke message to all inventory type objects.
The ability of objects to respond to the
same message and each implement it ap
propriately is called polymorphism. It is
one of the aspects of object-oriented pro
gramming that simplifies complex code.
To better understand polymorphism,
let's look at the inventory object. One of
its behaviors checks all inventory for
items that; might need reordering. It is
simple to make a loop that sends the.
check-reorder message to each of the in-
ventory items in the collection. The code
might look like this:
global-check-reorder :
do i < | to end ,
check-reorder —> inventory-item(i)
This has tremendous implications for
program maintainability. As new item

to the specific. For example,
the po object shown in the fig-
ure, the top of the hierarchy might be
the abstract object, "collection." It re-
sponds to a number of messages, such.
as "add an item" and "return an item.
It is a general-purpose programming
construct that might have been
predefined in the system or might be
part of a programmer's library of reus-
able code. : :
'The programmer implementing the in-
ventory object hierarchy might define
categories of parts (such as Type A
part, Type B part, and so on) as a sub-
class of the collection because the inven-
tory is a collection of different types of

The Hieratehical Atratigement of Oljects

Broperties, iiiherited frotn parent objects, becolhe
inore specific for edeh stubordihate part. |

types are added to the inventory and
new procedures for checking reorders
are implemented, the code for the inven-
tory object never needs to be changed.

An object in a program also has prop-
erties and behaviors. For example, an
order form in a business application has

properties such as items and guantity
ordered, as well as behaviors such as
process and verify.

Its easy to see how object-oriented
programming can be used to describe
environments in which a data-modeling
language defines objects and relation:
ships. In fact, object-oriented technology
originated with languages designed for
simulation and is thriving in graphics
applications where the user manipulates
objects on the screen. However, the for-
mal term "object" has meaning beyond
the commonsense meaning of the word.

The programmer's objects can repre-
sent physical entities such as inventory
items, but they can also represent more
abstract entities such as stacks, num-
bers, file-browsers, dispatchers or col-
lections.

Because it contains both data (proper-
ties) and procedures (behaviors), a pro-
gramming object can be used to
"modularize" any programming concept.
Exanples include collections, which are
objects made up of other objects, and
browsers, which are procedures for ex-
amining files.

|The properties of an object can't be
directly accessed from outside the ob-
ject. They are only manipulated by the
behaviors of the object. The behaviors of
an object can only be invoked by send-
ing messages to the object. The imple-
inentation of the properties and
behaviors of an object is completely hid-
žab from the outside. The object's data

and processes are enca; ted i
the object. z a a

To illustrate this, let's look at portions
of a simple customer order-entry appli-
cation. The system has an object—the
order—which has behaviors that allow
Mmanipulation and inspection of it prop-
ere The definition might look like

He

Object class: order; variab!
Customer, item, guantity; behavlora oi
1y, process, back-order, add,

An important distinction needs to be
made between a class of objects and
particular objects, The definition of the
class of object "order" contains defini:
tion of the property variables. Each par-
(icular object, contains Just the values
for the variables of that object. Mes-
SaBes are sent to the particular object.

Ublečiš tecelva and.
tašport to messtgeš.

Raorder
Policy B

ollcy | Policy Policy

OOP lets a company program more complexity into the

elijkea
o
Objects

This is unlike a conventional application
that would need to dispatch the correct
reorder procedure for each inventory
type.

Fine-Tuned Inventory Control
In this example, polymorphism, cou-
' pled with both the inheritance of reor- i
der policies for standard parts and the
ability to customize and override the
standard policies for particular parts, al- |
lows an organization to program more !
complexity in the reordering logic of a |
manufacturing application, thus giving |
it more finely tuned inventory control |
and a significant competitive advantage.
A number of languages are available
for programmers interested in experi-
menting with object-oriented program- |
ming. The oldest and perhaps the purest
is Smalitalk, which has some excellent !
implementations on small machines. The
system enforces object-oriented pro- |
gramming throughout and is an excel-
lent tool for learning pure object-orient- |
ed programming. |
idi Miki j A different approach is to enhance
IE Bar z existing languages with object-oriented
capabilities. This is the approach taken
by C-£-- and Objective-C, which are |
both € extensions, as well as Flavors,
which is a LISP extension. While this
approach provides the familiarity of a

0 Message: ||
Chieck Inventa
uiti

Reordet

Policy Policy

k x K : conventional uage, it has the disad-
logic of a manufacturing application, giving it more Jineliy — vantage oi mot forcika the programer
t l trol idsi i fi tečem iti i to use object-oriented technigues consis-

—— opor: zr

know anything about the internal work.
ings of the inventory object. They sim-
Ply have to know which messages the
inventory object will respond to and
send those messages to it. i

In addition, object-oriented languages
allow information to be organized con-
ceptually, The object definitions are or-
ganized in a class hierarchy, as shown in
the figure. A class above an object is its
superclass, and the one below is a sub-
class. Objects in subelasses can inherit
any or all of the properties and
behaviors of the classes above.

The programmer working with an ob-
ject-oriented language defines classes of
objects ranging from the general down

s) ui

SEZ odlo

(A The , Programmer must unlearn |
e proced programming style that J
is so ingrained in anyone with even a |
little pi experience. Old pro-
gramming habits are hard to break.

Next week T'll look at examples of ob- |

items, The inventory items in each cate-
£ory of parts can themselves be objects,
such as serew or bolt. The inventory ob-

t rograraming appli
ject inherits all the behaviors of the col- REKE ei ; m
pora go pd Category object, so
no further work from the pro- To learn more about i
Sani es Code can add or delete in- these articles, please SNE, a
Each of the Inventory types would Via ia pi |

updated. guarterly, at (800) 2942-1240.

J For information please

ka on semina

zane Peca Of the parti H contact (in the United ate bid Can-

ki lene Check reorder" as it best ada) Tech: er Institute,

mi ded s object, or inherit more 741 10th St, Santa Monica, Calif.

ge ore r behaviors from 90402 (213) 394-8305. m Europe, con-

ek pse b unlexity of diff tact Savant, ? New St, Carnforth,

padel es is completely shielded Lancs, LA5 9BX United. Kingdom
parts of the full application, (0524) 734 505.

rr imo —-