Transcript Object-Oriented Design

Overview of
Object-Oriented Software
Design and Java Programming
Putting the Pieces Together!
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Object-Oriented Design
A technique for developing a
program in which the solution
is expressed in terms of
objects -- self- contained
entities composed of data and
operations on that data.
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Object Oriented Programming

Programmer thinks about and defines
the attributes and behavior of
objects.

Often the objects are modeled after
real-world entities.

Very different approach than
function-based programming (like C).
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Reasons for OOP
Abstraction
Polymorphism
Inheritance
Encapsulation
Software Engineering Issues
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Objects to Classes
• A class defines the pattern used when
instantiating an object of that type.
• A class generally contains private data and
public operations (called methods).
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Class: Object Types

A Java class is an object type.

When you create the definition of a class
you are defining the attributes and
behavior of a new type.
 Attributes
are data members.
 Behavior is defined by methods.
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Creating an object

Defining a class does not result in
creation of an object.

Declaring a variable of a class type
creates an object. You can have many
variables of the same type (class).
Instantiation
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Superclass and Subclass
•
Inheritance enables us to define a new class
(called a subclass) that inherits the properties of
an already existing class.
•
The newly derived class is then specialized by
adding properties specific to it.
•
The class being inherited from is the superclass.
The class that inherits properties is the subclass.
•
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Defining Objects
•
An object-oriented program consists of
many objects.
•
An object is composed of identity, state
(attributes, data, and their current values)
and behavior (operations) .
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Identity, State, Behavior
•
Identity is the property of an object that
distinguishes it from all other objects.
•
The failure to recognize the difference
between the name of the object and the
object itself is the source of many errors
in object-oriented (OO) programming.
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Identity, State, Behavior
•
The state of an object encompasses all of the
(static) properties of the object plus the current
(dynamic) values of each of these properties
•
A property is an inherent or distinctive
characteristic, trait, quality, or feature that
contribute to making an object uniquely that
object
•
We will use the word attribute, or data
member, to refer to the state of an object
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Examples of State
•
Properties
•
Elevators travel up or down
• Vending machines accept coins
• Clocks indicate the current time
•
Values
•
Current floor
• Number of coins deposited
• The number of minutes since the last
hour
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Identity, State, Behavior
•
•
•
•
Behavior is how an object acts and reacts,
in terms of state changes and interactions
with other objects.
An operation is some action that one
object performs upon another in order to
elicit a reaction.
We will use the word method to describe
object behavior in C++.
Invoking a method causes the behavior to
take place.
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Classes
•
Classes are the definitions (or
blueprints) used to create objects. I’d
say: descriptions of objects.
•
To make a car the manufacturer must
first have a design from which to build
the first car. Then, once all the problems
are worked out, the design is used to
build all the cars of that model.
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Objects
•
An object is an instance of a class.
•
If we have a class definition called Car,
then we can think of Audi, BMW, and
Corvette as each being an instance
(object) of the class Car, i.e., they are
each a type of car.
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Object example
Audi 6
BMW Z3
Corvette
Car
Car
Car
• Notice that all objects are of the same type. All objects
are cars!
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Classes and Objects
•
An object is an instance of exactly one class
•
•
Corvette can not be an instance of a car class
and an instance of a plane class at the same
time.
An instance of a class, an object, belongs to
that particular class.
•
A Corvette is a car  Corvette belongs to the
class Car.
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Classes
•
Once a class is defined you can create as
many instances of the class (objects from
the class) as you would like.
•
Once a blue print is completed for the 2004
Porsche 911, Porsche will use an assembly
line to build as many instances of the 2004
Porsche 911 as they wish.
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Defining a class
•
Properties are variables which describe the
essential characteristics of an object.
•
•
Properties of a car: color, model, make, how many
doors, transmission type, direction of movement, etc.
Behaviors are methods that describe how the
object behaves and how the properties may be
modified.
•
Behavior of a car: braking, changing gears, opening
doors, moving forwards or backwards, etc.
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Instance variables
•
•
•
•
•
The class definition will include parameter definitions
(properties) that represent data about a particular object,
instance variables.
Example, Joe's car may have 4 gallons of gas in it while
John's car has 10 gallons.
The amount of gas in each car may change without
affecting the amount of gas in the any other cars.
All instances (objects) of a class will have a set of instance
variables that are specific to that individual object.
The combination of the values of these instance variables
is known as the object’s state.
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Instance variables
Car
MaxSpeed
Audi 6
Car
MaxSpeed = 155
BMW Z3
Car
MaxSpeed = 165
Corvette
Car
MaxSpeed = 145
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Class variables
•
The class definitions may also include
parameter definitions that represent data that
is shared by all class instances (objects),
called class variables.
•
In the case of the car class, we will define a
maximum allowed speed, by the law
(variable MaxSpeed). This will be the same
for each individual car.
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Class variables
Car
MaxSpeed
MaxSpeed=155
Audi 6
Car
MaxSpeed = 155
BMW Z3
Car
MaxSpeed = 165
Corvette
Car
MaxSpeed = 145
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Class variables
•
Class variables may also be used to
keep track of things such as how many
instances of a class exist.
•
Example: let’s create a counter the
records how many cars are in the
garage.
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Class variables
Car
MaxSpeed
MaxSpeed=155
NumCars = 3
Audi 6
Car
MaxSpeed = 155
BMW Z3
Car
MaxSpeed = 165
Corvette
Car
MaxSpeed = 145
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Messages
Audi 6 • turnOnHazard()
•
•
•
•
For Objects
The object to whom
the message is being
sent.
The name of the
method that object is
to execute.
Any parameters
(variables) needed by
that method.
•
•
•
For Humans
Who the message
is for.
What we want the
person to do.
What information
is needed to do it.
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Messages and Methods
•
In order to process a message, an object
needs to have a method defined for the
requested task.
•
A method is a small, well-defined piece of
code that completes a specific task.
•
For our previous example, we need to
define a method to turn on the car's
hazard lights.
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Messages and Methods
Car
MaxSpeed
MaxSpeed=155
NumCars = 3
turnOnHazard()
Audi 6
Car
BMW Z3
Car
Corvette
Car
MaxSpeed = 155
MaxSpeed = 165
MaxSpeed = 145
turnOnHazard()
turnOnHazard()
turnOnHazard()
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Instance methods
•
Each class can have methods that are specific to
each object, called instance methods.
•
These can only affect that object's parameters,
i.e., it’s instance variables.
•
Example: If BMW has 4 gallons of gas and
someone puts 6 more gallons of gas in his/her
car, the car now has 10 gallons. The amount of
gas in Audi and Corvette is unchanged.
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Messages and Methods
Car
MaxSpeed
MaxSpeed=155
NumCars = 3
turnOnHazard()
addGass(amount)
Audi 6
Car
BMW Z3
Car
Corvette
Car
MaxSpeed = 155
MaxSpeed = 165
MaxSpeed = 145
turnOnHazard()
addGass(amount)
turnOnHazard()
addGass(amount)
turnOnHazard()
addGass(amount)
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Methods
•
It is also possible that you want information
from an object; in this case, you would define
a method that sends (returns) a message back
to the requester containing that information.
•
We need to know how much gas is in our cars,
so we will create a new method that returns
the value of Gas-Level variable for our car.
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Messages and Methods
Car
MaxSpeed
GasLevel
MaxSpeed=155
NumCars = 3
addGass(amount)
turnOnHazard()
getGasLevel():GasLevel
Audi 6
Car
BMW Z3
Car
Corvette
Car
MaxSpeed = 155
GasLevel = 4
MaxSpeed = 165
GasLevel = 10
MaxSpeed = 145
GasLevel = 6
turnOnHazard()
addGass(amount)
getGasLevel():GasLevel
addGass(amount)
turnOnHazard()
getGasLevel():GasLevel
turnOnHazard()
addGass(amount)
getGasLevel():GasLevel
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Class methods
•
•
Class methods are used to get or
manipulate information about all objects
created from the class.
Typically, class methods are changing
class variables. For example:
•
Each time we move the car in or out of the
garage, we need to add/subtract one to the
number of cars: carIn( ) & carOut( )
• Also, we may want to know how many cars
are actually in the garage: getNumCars( )
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Messages and Methods
Car
MaxSpeed
GasLevel
MaxSpeed=155
NumCars = 3
carIn()
carOut()
turnOnHazard()
getNumCars():NumCars
addGass(amount)
getGasLevel():GasLevel
Audi 6
Car
BMW Z3
Car
Corvette
Car
MaxSpeed = 155
GasLevel = 4
MaxSpeed = 165
GasLevel = 10
MaxSpeed = 145
GasLevel = 6
turnOnHazard()
addGass(amount)
getGasLevel():GasLevel
turnOnHazard()
addGass(amount)
getGasLevel():GasLevel
turnOnHazard()
addGass(amount)
getGasLevel():GasLevel
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Object Oriented
Programming
•
When writing object-oriented
programs, first one must define the
classes (like Car).
•
Then, while the program is running,
the instances of the classes
(objects) (such as Audi, BMW,
Corvette in our example) are
created.
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Object Oriented
Programming - Benefits
•
An object can be written and maintained
separately from the rest of the program,
modularity.
•
An object has a “public face” that it uses to
communicate with other objects, but other
objects can not directly access its instance
variables, information hiding.
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Information Hiding

The interface to a class is the list of public
data members and methods.

The interface defines the behavior of the
class to the outside world (to other classes
and functions that may access variables of
your class type).

The implementation of your class doesn't
matter outside the class – only the
interface.
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Information Hiding (cont.)

You can change the implementation
and nobody cares! (as long as the
interface is the same).

You can use other peoples classes
without fear!
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Polymorphism
The ability of different objects to respond to
the same message in different ways.
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Inheritance
You can create a new class that inherits
from an existing class.
 You can add new members and methods.
 You can replace methods.
 The new class is a specialization of the
existing class.

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Inheritance
•
•
•
All classes in Java are organized into a
class hierarchy.
The highest level classes are very general
and the lower level classes are more
specific.
The lower level classes are based upon the
higher level classes and inherit instance
variables and methods from those higher
level class. They also may contain their
own (new) instance variables and methods
beyond the higher level class definition. 41
Inheritance
•
A higher level class is called a superclass;
a lower level class is called a subclass.
•
A subclass may also be a superclass
•
Inheritance allows you to define certain
behaviors once and then to reuse those
behaviors over and over again in the
subclasses. This is called reusability.
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Inheritance Example

Base class is shape, represents the
abstract notion of a shape.

Derived classes:
 rectangle
 circle
 triangle.

An object that is a circle is also a shape!
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Inheritance Example
•
Our Car class is very general.
•
Let's define a new class called BMW
that contains the parameters: model,
color, engine size.
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Inheritance
Car
BMW
MaxSpeed
GasLevel
MaxSpeed=155
Model
Color
EngineSize
turnOnHazard()
addGass(amount)
getGasLevel():GasLevel
MaxSpeed
GasLevel
turnOnHazard()
addGass(amount)
getGasLevel():GasLevel
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Inheritance
•
•
•
•
•
Now let's define two new classes. One for
the Z3 and another for the 3 Series
Sedan.
What might be some of the differences
between the two classes?
Number of doors (3, 5)
Roof (soft or hardtop)
Therefore, we add variables NumDoors
and Roof
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Inheritance
Car
BMW
MaxSpeed
GasLevel
MaxSpeed=155
Model
Color
EngineSize
turnOnHazard()
addGass(amount)
getGasLevel():GasLevel
MaxSpeed
GasLevel
turnOnHazard()
addGass(amount)
getGasLevel():GasLevel
Z3
3 series
Model
Color
EngineSize
Roof
Model
Color
EngineSize
NumDoors
MaxSpeed
GasLevel
MaxSpeed
GasLevel
turnOnHazard()
addGass(amount)
getGasLevel():GasLevel
turnOnHazard()
addGass(amount)
getGasLevel():GasLevel
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Views of the class
•
•
•
•
A class can be viewed as a sort of contract that
specifies what instances of the class can, and
cannot do
It is possible to distinguish between the outside
and inside view of a class
The interface of a class provides its outside
view and emphasizes the abstraction
The implementation of a class is its inside view
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Access
•
Most classes provide three levels of access
to their members (state and behavior):
•
Public
• The part of the class that is visible to all clients of the
class
•
Protected
• The part of the class that is only visible to
subclasses of the class
•
Private
• A part of the class that is not visible to any other
classes
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Private vs. Public

Public data members and methods can
be accessed outside the class directly.

The public stuff is the interface.

Private members and methods are for
internal use only.
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Protected Class
members/methods

We've already seen private and public.

Protected means derived classes have
access to data members and methods,
but otherwise the members/methods are
private.
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Special Member Functions

Constructors: called when a new object
is created (instantiated).
 can
be many constructors, each can take
different arguments.

(C++) Destructor: called when an object
is eliminated (not in Java)
 only
one, has no arguments.
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Accessing Data Members

Data members are available within each
method (as if they were local variables).

Public data members can be accessed
by other functions using the member
access operator "." (just like C++ struct).
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Accessing class methods

Within other class methods, a method
can be called just like a function.

Outside the class, public methods can
be called only when referencing an
object of the class.
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static methods

A static method is a class method that
can be called without having an object.

Method can't access non-static data
members! (they don't exist unless we
have an object).
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Static Data Members

It is possible to have a single variable that is
shared by all instances of a class (all the
objects).
 declare the variable as static.

Data members that are static must be
declared and initialize outside of the class.
 at global or file scope.
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Static data member example
class foo {
static int cnt;
foo() {
foocount++;
}
}
...
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Two Programming Paradigms
Structural (Procedural)
PROGRAM
Object-Oriented
PROGRAM
OBJECT
FUNCTION
Operations
FUNCTION
Data
OBJECT
OBJECT
FUNCTION
Operations
Operations
Data
Data
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Object-Oriented Programming
Language Features
1. Data abstraction
2. Inheritance of properties
3. Dynamic binding of operations to objects
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OOP Terms
Java Equivalents
Object
Class object or class instance
Instance variable
Private data member
Method
Public member function
Message passing
Function call ( to a public
member function )
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What is an object?
OBJECT
Operations
Data
set of methods
(public member functions)
internal state
(values of private data members)
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Inheritance Hierarchy Among
Vehicles
vehicle
wheeled vehicle
car
two-door
boat
bicycle
four-door
Every car is a wheeled vehicle.
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Inheritance

is a mechanism by which one class
acquires (inherits) the properties (both data
and operations) of another class

the class being inherited from is the Base
Class (Superclass)

the class that inherits is the Derived Class
(Subclass)

the derived class is then specialized by
adding properties specific to it
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Composition (or Containment)

is a mechanism by which the internal
data (the state) of one class includes
an object of another class
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TimeCard Class
TimeCard has a Time object
Private data:
Punch
id
timeStamp
Print
.
.
.
TimeCard
TimeCard
Set
Increment
Write
.
.
.
Private data:
hrs
mins
secs
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Constructor Rules for Derived Classes

at run time, the base class constructor is
implicitly called first, before the body of the
derived class’s constructor executes

if the base class constructor requires
parameters, they must be passed by the
derived class’s constructor
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Order in Which Constructors are
Executed
Given a class X,

if X is a derived class its base class
constructor is executed first

next, constructors for member objects (if any)
are executed (using their own default
constructors if none is specified)

finally, the body of X’s constructor is executed
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End of Lecture
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