Classes & Objects

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Nov 18, 2013 (3 years and 6 months ago)

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Classes & Objects

Computer Science I


Last updated 9/30/10

Object
-
Oriented Programming


OOP: A programming paradigm that uses "objects"


data structures consisting of data fields and methods
together with their interactions


to design
applications and computer programs


Programming techniques may include …


Data abstraction


Encapsulation


Modularity


Polymorphism


Inheritance

Background


Object
-
oriented programming has roots that can be
traced to the 1960s


As hardware and software became increasingly
complex, manageability often became a concern


Researchers studied ways to maintain software
quality and developed object
-
oriented programming
in part to address common problems by strongly
emphasizing discrete, reusable units of programming
logic

Background


OOP

focuses on data rather than processes


Programs are composed of self
-
sufficient modules
("classes")


Each instance of which ("objects") contains all the
information needed to manipulate its own data
structure ("members")


Modular programming

focuses on the
function

of a
module, rather than specifically the data


MP provides for code reuse, and self
-
sufficient
reusable units of programming logic, enabling
collaboration through the use of linked modules
(subroutines)

Background


An object
-
oriented program may be viewed as a
collection of interacting
objects


Each object is capable of …


receiving messages,


processing data, and


sending messages to other objects

Background


Each object can be viewed as an independent
'machine' with a distinct role or responsibility


Actions (methods) on these objects are closely
associated with the object


For example …


OOP data structures tend to carry their own operators
around with them


Or at least
inherit

them from a similar object or class


In the conventional model, the data and operations
on the data don't have a tight, formal association

What is a class?


A template for an object


A user
-
defined datatype that contains the variables,
properties and methods in it


Defines the abstract characteristics of a thing
(object), including its characteristics and the thing's
behaviors


For example …


The class Dog would consist of traits shared by all dogs,
such as breed and fur color (characteristics), and the ability
to bark and sit (behaviors)

What is a class?


Provides modularity and structure in an object
-
oriented computer program


Should typically be recognizable to a non
-
programmer familiar with the problem domain,
meaning that the characteristics of the class should
make sense in context


The code should be relatively self
-
contained


Collectively, the defined properties and methods are
called
members

What is a class?


One can have an instance of a class; the instance is
the actual object created at run
-
time


For example …


The Lassie object is an
instance

of the Dog class


The set of attribute values for a particular object is
called its state


The object consists of state and the behavior that's
defined in the object's classes

What is an object?


A discrete bundle of functions and procedures, often
relating to a particular real
-
world concept such as a
bank account holder or hockey player


Other pieces of software can access the object only
by calling its functions and procedures that have
been allowed to be called by outsiders


Some agree that isolating objects in this way makes
their software easier to manage and keep track of


Others feel that software becomes more complex to
maintain and document, or even to engineer from
the start

OOP Features


Dynamic


Encapsulation

(or multi
-
methods, in which case the state is kept
separate)


Polymorphism


Inheritance

(or delegation)


Open recursion


What is a method?


"
The process by which an object sends data to
another object or asks the other object to invoke a
method
.“ *



Also known as interfacing


For example…


The object called Breeder may tell the Lassie object to sit by
passing a
sit

message that invokes Lassie's sit() method


The syntax varies between languages

* Armstrong,
The Quarks of Object
-
Oriented Development
. In descending order of popularity, the "quarks" are:
Inheritance, Object, Class, Encapsulation, Method, Message Passing, Polymorphism, Abstraction

Dynamic Dispatch


When a method is invoked on an object, the object
itself determines what code gets executed by looking
up the method at run time in a table associated with
the object


This feature distinguishes an object from an abstract
data type (or module), which has a fixed (static)
implementation of the operations for all instances


It is a programming methodology that gives modular
component development while at the same time
being very efficient

Encapsulation


Conceals the functional details of a class from objects
that send messages to it


For example …


The Dog class has a bark() method variable


The code for the bark() method defines exactly how a bark
happens


Timmy, Lassie's friend, however, does not need to know
exactly how she barks


Encapsulation is achieved by specifying which classes may
use the members of an object


The result is that each object exposes to any class a certain
interface



those members accessible to that class

Encapsulation


Rationale:


Prevents clients of an interface from depending on those
parts of the implementation that are likely to change in the
future


Thereby allowing those changes to be made more easily
without

changes to clients


For example…


An interface can ensure that puppies can only be added to an object of
the class Dog by code in that class


Members are often specified as
public
,
protected

or
private
,
determining whether they are available to all classes, sub
-
classes or only the defining class

Polymorphism


Allows the programmer to treat derived class
members just like their parent class's members


More precisely, the ability of objects belonging to
different data types to respond to calls of methods of
the same name, each one according to an
appropriate type
-
specific behavior


One method, or an operator such as +,
-
, or *, can
be abstractly applied in many different situations


For example …


If a Dog is commanded to speak(), this may elicit a bark()


If a Pig is commanded to speak(), this may elicit an oink()

Inheritance


A process in which a class inherits all the state and
behavior of another class


This is called
child
-
parent

or
is
-
a

relationship


Subclasses are more specialized versions of a class,
which
inherit

attributes and behaviors from their
parent classes, and can introduce their own

Inheritance


For example …


The class Dog might have sub
-
classes called Collie,
Chihuahua, and GoldenRetriever


Lassie would be an instance (object) of the Collie subclass


Assume that the Dog class defines a method called bark()
and a property called furColor


Each of the sub
-
classes will inherit these members; the
programmer only needs to write the code for them once

Inheritance


Each subclass can alter its inherited traits


For example …


The Collie subclass might specify that the default furColor
for a collie is brown
-
and
-
white


The Chihuahua subclass might specify that the bark()
method produces a high pitch by default

Inheritance


Subclasses can also add new members


For example …


The Chihuahua subclass could add a method called
tremble()


An individual Chihuahua instance would then use a high
-
pitched bark() from the Chihuahua subclass, which in turn
inherited the usual bark() from Dog


The Chihuahua object would also have the tremble()
method, but Lassie would not, because she is a Collie, not a
Chihuahua

Inheritance


Inheritance is an "
a


is a
" relationship between
classes, while instantiation is an "
is a
" relationship
between an object and a class


For example …


a

Collie is a Dog ("a… is a"), but


Lassie is a Collie ("is a")


Thus, the object named Lassie has the methods from both
classes Collie and Dog


Open recursion


A special variable (syntactically it may be a keyword),
usually called
this

or
self
, that allows a method body
to invoke another method body of the same object


This variable is
late
-
bound
; it allows a method
defined in one class to invoke another method that is
defined later, in some subclass thereof