Transcript The Program Life Cycle.

Object Oriented
Programming 2.
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0. Admin.
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Take in any more program 6’s.
Questions on program 7.
Next Week:
Tuesday: Final Review.
Thursday: no class.
Final: Tuesday, December 11, 10:10am.
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1.
Review: Advantages of OOP.
 1) Coherent bundles of “nouns” and
“verbs.” Analogy: an athlete’s performance
data (nouns) and skill set (verbs).
 2) Methods are grouped with the data they
apply to.
 3) Methods in a class automatically “know”
about the data in the class, so except for Set
and Get, parameter passing is unnecessary.
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Advantages of OOP.
 4) Data is protected from the client (who
must use the member methods, rather than
directly accessing it) and from other classes.
 5) Enhanced code re-use: big class libraries
can be instantiated by any program.
 6) Simplifies applications by using client /
server model.
 7) Classes can be customized.
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Review: What is a class?
 A class is a bundle, like a record except it
contains both data members and member
methods.
 The member methods are usually “public”
(visible to the client program—the interface
/ communication mechanism).
 The data members are usually “private”
(invisible to the client which can change the
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data only via the member methods).
Class
Transform
Set
Get
Client
Program
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Multi-file programming.
 Classes can be stored in separate files from
program drivers (example last time).
 Yet we know that there can only be one
executable target…so how is multi-file
programming possible?
 Biblical Analogy (1 Cor 12: 12): “The body is a
unit, though it is made of many parts; and though
its parts are many, they form one body. So it is
with Christ.”
Do all the parts matter?
 Yes v. 21: “The eye cannot say to the hand,
‘I don’t need you.’”
 The driver needs the class because it is
instantiating the class as an object, and
using its methods.
 Without the class, we would have a link
error (unresolved external reference).
Do all the parts matter?
 On the other hand, the class is just a service,
like a fire extinguisher or a toolkit. It
doesn’t do anything unless there is a driver
specifying how the service is to be used
(how the member methods should be
applied).
The art of Object Oriented
Programming.
 OOP changes the focus from solving every
problem within each program (which leads to
reinventing the wheel) to investing time on useful
reusable services.
 From theology of glory (the master program is all)
to theology of the cross (servants making
themselves useful).
 Programs must decrease that services increase.
The art of Object Oriented
Programming.
 But there is a problem:
 A good class is both useful and reusable.
 It would be very useful to have a class that
does everything my application needs,
but…
 It would be nice to have a class that can be
reused by everyone but….
Looking for the tipping point.
 Engineering focuses on the best compromise of
multiple competing and maybe conflicting
constraints.
 The best car has:
 A) super performance;
 B) excellent safety;
 C) excellent fuel economy;
 D) passenger comfort and excellent cargo
capacity.
 But….?
Designing classes is similar.
 A practical class will not be so tailored to
one application that it cannot be re-used by
any other applications (it only provides data
on underwater Serbo-Croatian basket
weaving), or so generic that it is useless (it
does “stuff”).
Some good examples of classes.
 A date class for handling date information
(see text ch. 11).
 A country class for storing info on GNP,
GDP, exchange rate, interest rate,
demographic information, religion, literacy,
crime rate.
 An employee / student / athlete class etc.
What if there is a hierarchy?
 E.g. some information is generic to all
employees: Name, Address, ID, etc. but
some is specific to the type of Employee
(Hourly, Salaried, Contract, Volunteer,
Indentured-Serf, Student. Etc.).
 More specific classes can be defined by
inheritance from a base class (see CSC 300
and ch. 12).
Some useful methods for any class.
 1) A constructor (a typeless method with the
same name as the class) can be used to initialize
the class data, ideally preventing the client from
misusing the class (GIGO).
 In one methodology (Deitel and Deitel), the
constructor sets safe, generic values independently
of any application, while a Set function allows
each particular application to supply data. If the
Set function is not used, the constructor should
guarantee that no disaster occurs.
Some useful methods for any class.
 2) A set method allows a user to override the
initial values of the constructor, but should employ
self-defense, only setting the class’s private data to
reasonable client values.
 3) An observer: a boolean method that reports
back on the state of the data e.g. is a container full
or empty, is a date valid / invalid.
 4) An accessor or Get method: returns a value to
the client e.g. future value of an investment.
Some useful methods for any class.
 5) An iterator: a method which updates a value
e.g. increments or decrements a date or time, does
a “find next.”
 6) A transformation method: maps data
members to other data members e.g. maps a
statistic to the scaled height of a bar-chart.
 7) Private utility methods: a “hands off” method
that the client cannot invoke, but which the object
calls internally at the right time and in the right
way, e.g. to write data to a back-up file.
Object oriented Design.
 2 programming design paradigms:
 (1) Structured design (SD): focuses on procedural
programming (verb based):
HOW (by what sequence of commands) do we
solve the problem. What is the game plan?
 (2) Object oriented design (OOD): focuses on
agents (noun based):
 WHO are the agents involved? Who are the
major players?
Lab exercise.
 Down-load the 2 separate files for the
integer division problem.
 Change to .cs files.
 In a console project, highlight the reference
folder and select Project/Add Existing Item
to add both files.
 Build and execute.