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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

• Notice that all objects are of the same type. All objects are cars!

Car Car Car

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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

Audi 6 BMW Z3 Corvette

Car Car Car

Car

MaxSpeed = 155 MaxSpeed = 165 MaxSpeed = 145

MaxSpeed

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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

Audi 6 BMW Z3 Corvette

Car Car Car

Car

MaxSpeed = 155 MaxSpeed = 165 MaxSpeed = 145

MaxSpeed

MaxSpeed=155

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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

Audi 6 BMW Z3 Corvette

Car Car Car

Car

MaxSpeed = 155 MaxSpeed = 165 MaxSpeed = 145

MaxSpeed

MaxSpeed=155

NumCars = 3

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Messages

• 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.

Audi 6 • turnOnHazard()

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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

Audi 6 BMW Z3 Corvette

Car Car Car

Car

MaxSpeed = 155

turnOnHazard()

MaxSpeed = 165

turnOnHazard()

MaxSpeed = 145

turnOnHazard()

MaxSpeed

MaxSpeed=155

NumCars = 3

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

Audi 6 BMW Z3 Corvette

Car Car Car

Car

MaxSpeed = 155

turnOnHazard()addGass(amount)

MaxSpeed = 165

turnOnHazard()addGass(amount)

MaxSpeed = 145

turnOnHazard()addGass(amount)

MaxSpeedMaxSpeed=155NumCars = 3turnOnHazard()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

Audi 6 BMW Z3 Corvette

Car Car Car

Car

MaxSpeed = 155GasLevel = 4

turnOnHazard()addGass(amount)getGasLevel():GasLevel

MaxSpeed = 165GasLevel = 10

MaxSpeed = 145GasLevel = 6

turnOnHazard()addGass(amount)getGasLevel():GasLevel

MaxSpeedGasLevelMaxSpeed=155NumCars = 3

addGass(amount) getGasLevel():GasLevel

turnOnHazard()

addGass(amount) getGasLevel():GasLevel

turnOnHazard()

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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

Audi 6 BMW Z3 Corvette

Car Car Car

Car

MaxSpeed = 155GasLevel = 4

turnOnHazard()addGass(amount)getGasLevel():GasLevel

MaxSpeed = 165GasLevel = 10

turnOnHazard()addGass(amount)getGasLevel():GasLevel

MaxSpeed = 145GasLevel = 6

turnOnHazard()addGass(amount)getGasLevel():GasLevel

MaxSpeedGasLevelMaxSpeed=155NumCars = 3

addGass(amount) getGasLevel():GasLevel

turnOnHazard()

carIn()

carOut()

getNumCars():NumCars

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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.

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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

CarMaxSpeedGasLevelMaxSpeed=155

turnOnHazard()addGass(amount)getGasLevel():GasLevel

BMWModelColorEngineSize

MaxSpeedGasLevel

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

CarMaxSpeedGasLevelMaxSpeed=155

turnOnHazard()addGass(amount)getGasLevel():GasLevel

BMWModelColorEngineSize

MaxSpeedGasLevel

turnOnHazard()addGass(amount)getGasLevel():GasLevel

Z3ModelColorEngineSizeRoof

MaxSpeedGasLevel

turnOnHazard()addGass(amount)getGasLevel():GasLevel

3 seriesModelColorEngineSizeNumDoors

MaxSpeedGasLevel

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) Object-Oriented PROGRAM PROGRAM

FUNCTION

FUNCTION

FUNCTION

OBJECT

Operations

Data

OBJECT

Operations

Data

OBJECT

Operations

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 boat

bicyclecar

four-door two-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 ClassTimeCard has a Time object

Private data:

hrs

mins

secs

Punch

Private data:

id

timeStamp

Increment

SetPrint . . .

TimeCard

TimeCard

Write . . .

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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