programming-for-dummies

236
Encapsulation Isolates Data and Subprograms
The upper phase shows the main program sending the current temperature
(32 degrees Fahrenheit) to the forecasting subprogram, which then
returns its prediction (as 30 degrees Fahrenheit).
The lower phase shows the same program except now the forecasting
subprogram has been modified to return Celsius temperatures. So now
when the main program sends the current temperature (in Fahrenheit)
to the forecasting subprogram, this subprogram returns its forecast in
Celsius. The main program now uses this faulty value.
Figure 7-4:
Changing a
subprogram
can wreck a
perfectly
working
program.
Main program
Forecast temperature (32)
Display temperature
Tomorrow’s forecast =
30 Degrees
This program measures
temperatures in Fahrenheit.
Subprogram Forecast
Return tomorrow’s temperature
This subprogram also measures
temperatures in Fahrenheit.
Main program
Forecast temperature (32)
Display temperature
Tomorrow’s forecast =
–1.11 Degrees
This program expects to receive
a temperature in Fahrenheit, so
it now displays the wrong forecast.
Subprogram Forecast
Return tomorrow’s temperature
This subprogram now converts
Fahrenheit temperatures into
Celsius and returns a
temperature measured in Celsius.
This problem occurs because the forecasting subprogram has no idea how
its data is being used by another part of the program. Object-oriented programming
can partially solve this problem by organizing data, and all the
subprograms that manipulate that data, into a single location, or an object.
By grouping data and all the subprograms that manipulate that data in one
place, it’s much easier to understand how that data is being used.
The whole idea behind an object is to isolate and “hide” data and subprograms
by using encapsulation. Encapsulation acts like a wall, as shown in
Figure 7-5, that wraps around data and subprograms to
✦ Keep other parts of a program from manipulating data inside an
object.
✦ Keep subprograms inside that object from manipulating data outside
that object.
✦ Keep programmers from modifying code stored in another object.
Shielding data inside an object
Think of data as a wallet full of cash. The more people who handle your
wallet before giving it back to you, the greater the chance that someone
takes money out of that wallet (manipulating the data). Ideally, you want as
few people to handle your wallet as possible and if people absolutely must
handle your wallet, you want them close enough so you can keep an eye on
them.