programming-for-dummies

Encapsulation Isolates Data and Subprograms 235
might include a Move subprogram that can change the value of the
object’s X and Y coordinates (to determine where to display the cartoon
monster on-screen).
✦ Private data and subprograms are what an object uses to do something
useful, so the object doesn’t need to allow other parts of the program to
access this information.
The Monster object might contain a private subprogram that calculates
exactly how the Monster object moves. Because other parts of the program
don’t need to know exactly how the Monster object calculates its movement,
this type of information would be private and hidden from the rest of the
program.
Ultimately, OOP is another way to make programming easier. Just as highlevel
languages (like BASIC) simplify programming by using real-life commands
(such as PRINT), so does OOP simplify organizing programs by
modeling real-life items. The three advantages that objects have over ordinary
subprograms include
✦ Encapsulation
✦ Inheritance
✦ Polymorphism
Book II
Chapter 7
Breaking a
Large Program
into Objects
Object-oriented programming provides tools for making programming easier,
but it’s still possible to write horrible software with OOP. Think of OOP like
lines painted on the highway. If you follow the lines, you probably arrive
safely at your destination, but if you ignore the lines and do whatever you
want, you probably crash your car. Like traffic lines painted on the road,
OOP guides you into writing software that can be created and modified
easily, but you can still mess up things if you’re not careful.
Encapsulation Isolates Data and Subprograms
Subprograms have two problems. First, subprograms can work with data
from any part of a program. That’s what makes subprograms useful, but
that’s also what makes subprograms harder to modify and fix. If you don’t
know what data a subprogram might manipulate and when, any changes you
make to that subprogram could affect a program in unpredictable ways.
For example, suppose someone writes a weather forecasting program that
has a subprogram for predicting tomorrow’s temperature measured in
Fahrenheit. What happens if another programmer modifies this subprogram
to forecast temperatures in Celsius?
Figure 7-4 shows two phases of a program: