Introduction to Classes and Objects Initializing Objects Making Use of Classes in Algorithms Class Examples

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Title: Introduction to Classes and Objects Initializing Objects Making Use of Classes in Algorithms Class Examples

1
Introduction to Classes and Objects
Initializing Objects Making Use of Classesin
Algorithms Class Examples
Lecture 18
2
Introduction to Classes and Objects
3
The Scenario

We want to provide a black box approach to
reusable components

Dont know howit works, but itworks!
Input
Output
4
The Scenario

We want to create reusable components for other
programmers to use.
They are focused on their work and dont want to
know the details of how our component works.
They are also busy trying to do their work, so
maybe theyll take shortcuts and hack.
We want to provide services to them but protect
the implementation details of our components.

5
Classes

A class is the fundamental unit of
object-oriented programming.
It allows us to implement ADTs, combining both
data and operations into one logical bundle.

6
Objects

Once we have a class defined, we can then declare
any number of objects of that class.
For example
Once we have a queue class defined, we can then
declare any number of queue objects.
Each queue object will then have all the data and
procedural abstractions needed for a queue.

7
Classes and Objects

A class is like a blueprint
An object is an instantiated class (like a
building)

8
Classes vs. Objects

Class refers to the template by which we
implement an ADTs functionality. Thus, it is
analogous to data type it is only a definition
or template.
Object refers to a specific instance of the
class. Thus, it is analogous to variable it is
an instance of a template.
Class Object Type Variable

9
Classes Allow for Reuse!

Algorithms which make use of the queue class (by
declaring instances of the queue, or objects) are
clients who obtain services
algorithm Movie_Theatre
algorithm Restaurant
algorithm Check-out_Line
The authors of these algorithms can instantiate
and use queues without writing them!

10
Components of a Class

The individual data elements of a class are
referred to as attributes of the class.
The entire collection of data maintained within
an object is referred to as the state of the
object.
The operations (procedures and functions) that
are provided by the class (and thus allowed on
the data) are called the methods of the class.

11
Controlling Visibility

Recall we want to protect the implementation of
the class.
Users shouldnt need to know how it works
Users shouldnt be able to violate the behaviors
(may only do what we allow them).
Encapsulation allows us to hide the
implementation details.

12
Public vs. Protected
Protected
Public
13
Visibility Within a Class
The rest of the algorithm
Public Section (Interface)
Protected Section(Implementation)
14
Anatomy of a Class

Classes have two sections
Public - the specification of the "visible part
of the class
Protected - the specification of the "hidden
part of the class

15
Public Section

Contains everything that the client algorithm
needs to know to use the class
Is the visible part of the class
Defines the interface and contract with the user
(algorithm)
Contains the header lines of those methods that
are available for clients of the class to call.

16
Protected Section

Contains the details of the implementation of the
class
Cannot be seen by the client algorithm
Hides attributes and method implementation
Contains all method declarations and
implementations.

17
Interactions
Client Algorithm
Public Module Definitionsand Contract
Supplier Object
Protected Data Implementation of Modules
18
Encapsulation and Hiding

A client algorithm can see only the public
section of the class. A client has no idea what
is within the protected section.
A client algorithm can interact with an object
only by calling the methods listed in the public
section of the class.
A client algorithm cannot directly access any of
the data within an object. It may get at data
only via calls to public methods.

19
Public vs. Protected Methods
Some Class
publicmethod 1
publicmethod 3
class data
publicmethod 2
a protectedmethod

Clients may call any/all of the three public
methods.
Clients dont know the hidden method exists they
cannot call it.

20
Protected Methods

A protected method is one which is defined in the
protected section that does not have its header
line in the public section.
Clients dont even know it exists.
Such a method may only be called by other methods
within the protected section.

21
Structure of a Class Definition

class ltidentifiergt
public
ltmethod definitions and contractsgt
protected
ltconstantsgt
lttype definitionsgt
ltattribute declarationsgt
ltmethod implementationsgt
endclass // ltidentifiergt

22
A Bus Class Example
Bus
Initialize
MAX_PASSENGERS Current
Add Passengers
Implementation
RemovePassengers
Number of Passengers
23

class Bus
public
procedure Initialize
// contract here
procedure AddPassengers (to_add iot in num)
// contract here
function NumPassengers returnsa num
// contract here
procedure RemovePassengers (to_remove iot in
num)
// contract here
protected
MAX_PASSENGERS is 50
current isoftype num
procedure Initialize
current lt- 0
endprocedure // Initialize

procedure AddPassengers (to_add iot in num)
if (current to_add lt MAX_PASSENGERS) then
current lt- current to_add
endif
endprocedure // AddPassengers
function NumPassengers returnsa num
NunPassengers returns current
endfunction // NumPassengers
procedure RemovePassengers (to_remove iot in
num)
if (current gt to_remove) then
current lt- current to_remove
endif
endprocedure // RemovePassengers
endclass // Bus

25
Recipe for Writing a Class

Write shell/template
Decide on what the user needs to know about -
PUBLIC
Specify the protected data
Diagram think of sample algorithm
Write public section with contract information
Write protected section with implementation
details

26
Attribute Visibility in the Protected Section

Variables declared at the global level of the
protected section can be directly accessed
throughout the protected section without passing
them via parameter.
protected
current isoftype num
. . .
procedure AddPassengers (to_add iot in num)
if (current to_add lt MAX_PASSENGERS) then
current lt- current to_add
endif
endprocedure // AddPassengers
function NumPassengers returnsa num
NunPassengers returns current
endfunction // NumPassengers
. . .

27
Attribute Visibility in the Protected Section

Allow direct access, bypassing parameters!
May want clients of the class to be able to call
methods
Don't want clients to know the details of the
protected section's data representation.
If method parameter lists included data that's
protected, then would be telling the client about
that representation.
So, allow direct access within protected
Still have encapsulation (the class itself)

28
Summary

Classes implement ADTs
An object is an instance of a class
Encapsulation allows for the protection of
details within the class
Public section provides interface
Protected section provides implementation
Attributes declared at the global level of the
protected section may be directly accessed within
any method in the protected section.

29
Questions?
30
Initializing Objects
31
The Scenario

Youve defined a Queue class and now want to
create an instance of that class.
There are some attributes which should be
initialized when an object is created.
Wed like to initialize our Head and Tail
pointers to NIL.
But the algorithm shouldnt know about the
pointers!

32
Need to Initialize

Our Queue class will thus need a method that
sets things up when an object is created.
Different languages handle this differently.
Well use a normal method that must be invoked
like any other

33
A Method By Any Other Name

An initialization method is just like any other
method.
For clarity and convenience, we always use the
identifier Initialize for such methods.

34
Initialize Common but Not Required

Most classes have attributes that require some
initial values.
We expect most classes to have initialization
methods.
But its not required.

35
Initialize In the Class Definition

class Initialize_Example
public
. . .
procedure Initialize (ltparametersgt)
// contract information
. . .
protected
. . .
procedure Initialize (ltparametersgt)
ltimplementationgt
endprocedure // Initialize
endclass

36
Invoking the Initialize Method

In the algorithm, we invoke (or call) the
Initialize method just as any other method.
It is assumed that Initialize is only called
once, but there is no rule to enforce this.
The algorithm must explicitly call the initialize
method for each object that has attributes that
need to be initialize.

37
An Example

In the algorithm
MyQueue isoftype Queue
MyQueue.Initialize
In the class definition
procedure Initialize
head lt- NIL
tail lt- NIL
endprocedure // Initialize

38
Another Example

In the algorithm
MyAirplane isoftype Plane
MyAirplane.Initialize(Delta 955)
In the class definition
procedure Initialize(flight_name iot in string)
altitude lt- 0
current_flight lt- flight_name
endprocedure // Initialize

39
Summary

Very often, objects have attributes that need
some initial values.
Thus these classes need some method to set these
attributes.
Different languages handle this differently
Some have special methods that are
automatically executed.
Well treat the Initialize method like any other
method and call it from the algorithm for each
object created.

40
Questions
41
Making Use of Classesin Algorithms
42
The Scenario

You need to create a simulation of an airport.
A coworker has developed an airplane class youd
like to use in your algorithm.
How do we get at the airplane class?
Notice we dont have access to modify the class,
just make use of it.

43
Importing the Class

Algorithms and class definitions often reside in
separate computer files.
Recall that a benefit of Object-Oriented
Programming is re-use via class libraries
We need the ability to import a class into our
algorithm or another class
The uses clause allows this import.

44
The Uses Clause

algorithm ltAlgorithm Namegt
uses ltClass Namegt, ltClass Namegt, . . .
. . .
endalgorithm // ltAlgorithm Namegt
class ltClass Namegt
uses ltOther Class Namegt
. . .
endclass // ltClass Namegt

45
An Example

Having previously defined an Airplane class, the
algorithm can make use of the class and create
objects of the class as follows
algorithm Airport
uses Airplane
Cessna, Prop isoftype Airplane
. . .

46
A More Complex Example

algorithm BigAirport
uses Airplane, Helicopter,
FlightController
Plane1, Plane2 isoftype Airplane
ControlDeck isoftype FlightController
. . .
endalgorithm

47
Classes and Objects

A class is like a blueprint
It is analogous to data type
An object is an instantiated class (like a
building)
It is analogous to variable
Class Object Type Variable

48
Creating Objects

Once the algorithm has imported the class
definition, then it can create objects of the
class.
Objects are created just as any variable
ltObject Identifiergt isoftype ltClass Namegt
For example
MyPlane, YourPlane isoftype Airplane
ControlDeck isoftype FlightController

49
Accessing Methods of Objects

After instantiating a class and creating an
object in the algorithm, we want to make use of
its methods.
There are many methods in the class, so how do we
access the one we want?
Just as fields of a record, we access the methods
in an object using the dot (.) operator.

50
Invoking Methods of an Object