1 software engineering lecture 15 object oriented software design in java

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Software Engineering

Lecture 15

Object Oriented Software Designin Java

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Analysis leads to Design

goals of the system scenarios properties of interest

Analysis

Design

identify the main events, actions and interactions

identify and define the main processes

identify and define the properties of interest

structure the processes into a design architecture

check properties of interest

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Design leads to the Design Document

Design

Design Document

identify the main data design (structures)

identify component-level design

identity software interface design

identity user interface design

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Design leads to Source Code

Design

Java

identify the main active entities

identify the main (shared) passive entities

structure the classes as a class diagram

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Software Design Strategies

FUNCTIONAL OBJECT-ORIENTED DECOMPOSITION DESIGN

The solution is expressed

in terms of objects

(self-contained entities

composed of data and

operations on that data) that

interact by sending messages

to one another.

The problem is divided into

more easily handled

subproblems, the solutions

of which together create a

solution to the overall

problem.

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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.

TextField

setText. ..

getText

Private dataand

methods

setLayout

add

pack. ..

show

Frame

Private dataand

methods

TextField Frame

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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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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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• 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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• 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 java.

• 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 2003 Porsche 911, Porsche will use an assembly line to build as many instances of the 2003 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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Defining a class

Behavior or Method 1

Behavior Behavior or or

Method 4 Method 2

Behavior or Method 3

Properties

or

Instance Variables

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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 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


Car Car Car

Car


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


Car Car Car

Car


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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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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Car Car Car

Car

MaxSpeed = 155

turnOnHazard()addGass(amount)

MaxSpeed = 165


MaxSpeed = 145


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 GasLevel variable for our car.

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Car Car Car

Car

MaxSpeed = 155GasLevel = 4

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




MaxSpeedGasLevelMaxSpeed=155NumCars = 3

addGass(amount) getGasLevel():GasLevel

turnOnHazard()


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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Car Car Car

Car







MaxSpeedGasLevelMaxSpeed=155NumCars = 3


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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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

• 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


BMWModelColorEngineSize

MaxSpeedGasLevel


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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


BMWModelColorEngineSize

MaxSpeedGasLevel


Z3ModelColorEngineSizeRoof

MaxSpeedGasLevel


3 seriesModelColorEngineSizeNumDoors

MaxSpeedGasLevel


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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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Project Work

• Next project deliverable – Design Specification Document

• Last deliverable - Prototype

1 software engineering lecture 15 object oriented software design in java

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