CS 4310: Software Engineering

CS 4310: Software Engineering Lecture 15 Object Oriented Software Design in Java

Analysis leads to Design • goals of the system • scenarios • properties of interest Analysis • 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 Design • check properties of interest

Design leads to the Design Document Design • identify the main data design (structures) • identify component-level design • identity software interface design • identity user interface design Design Document

Design leads to Source Code Design • identify the main active entities • identify the main (shared) passive entities • structure the classes as a class diagram Java

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.

Frame TextField setLayout Private data and methods Private data and methods setText add . . . pack . . . getText show 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 Frame

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).

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.

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) .

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.

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

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

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 java. • Invoking a method causes the behavior to take place.

Classes • Classes are the definitions (or blueprints) used to create objects. I’d say: descriptions of objects. • To make acar 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.

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.

Object example Audi 6 BMW Z3 Corvette Car Car Car • Notice that all objects are of the same type. All objects are cars!

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.

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.

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.

Defining a class Behavior or Method 1 Behavior Behavior or or Method 4 Method 2 Behavior or Method 3 Properties or Instance Variables

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.

Instance variables Car MaxSpeed Audi 6 BMW Z3 Corvette Car Car Car MaxSpeed = 155 MaxSpeed = 165 MaxSpeed = 145

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.

Class variables Car MaxSpeed MaxSpeed=155 Audi 6 BMW Z3 Corvette Car Car Car MaxSpeed = 155 MaxSpeed = 165 MaxSpeed = 145

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.

Class variables Car MaxSpeed MaxSpeed=155 NumCars = 3 Audi 6 BMW Z3 Corvette Car Car Car MaxSpeed = 155 MaxSpeed = 165 MaxSpeed = 145

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. Messages Audi 6 • turnOnHazard() For Humans • Who the message is for. • What we want the person to do. • What information is needed to do it.

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.

Messages and Methods Car MaxSpeed MaxSpeed=155 NumCars = 3 turnOnHazard() Audi 6 BMW Z3 Corvette Car Car Car MaxSpeed = 155 turnOnHazard() MaxSpeed = 165 turnOnHazard() MaxSpeed = 145 turnOnHazard()

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.

Messages and Methods Car MaxSpeed MaxSpeed=155 NumCars = 3 turnOnHazard() addGass(amount) Audi 6 BMW Z3 Corvette Car Car Car MaxSpeed = 155 turnOnHazard() addGass(amount) MaxSpeed = 165 turnOnHazard() addGass(amount) MaxSpeed = 145 turnOnHazard() addGass(amount)

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.

turnOnHazard() turnOnHazard() addGass(amount) addGass(amount) getGasLevel():GasLevel getGasLevel():GasLevel Messages and Methods Car MaxSpeed GasLevel MaxSpeed=155 NumCars = 3 Audi 6 BMW Z3 Corvette Car Car Car MaxSpeed = 155 GasLevel = 4 turnOnHazard() addGass(amount) getGasLevel():GasLevel MaxSpeed = 165 GasLevel = 10 MaxSpeed = 145 GasLevel = 6 turnOnHazard() addGass(amount) getGasLevel():GasLevel

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( )

MaxSpeed GasLevel MaxSpeed=155 NumCars = 3 Messages and Methods Car carIn() carOut() turnOnHazard() getNumCars():NumCars addGass(amount) getGasLevel():GasLevel Audi 6 BMW Z3 Corvette Car Car Car MaxSpeed = 155 GasLevel = 4 turnOnHazard() addGass(amount) getGasLevel():GasLevel MaxSpeed = 165 GasLevel = 10 turnOnHazard() addGass(amount) getGasLevel():GasLevel MaxSpeed = 145 GasLevel = 6 turnOnHazard() addGass(amount) getGasLevel():GasLevel

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.

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.

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.

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.

Inheritance • Our Car class is very general. • Let's define a new class called BMW that contains the parameters: model, color, engine size.

MaxSpeed GasLevel MaxSpeed=155 turnOnHazard() addGass(amount) getGasLevel():GasLevel Model Color EngineSize MaxSpeed GasLevel turnOnHazard() addGass(amount) getGasLevel():GasLevel Inheritance Car BMW

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

BMW Z3 3 series MaxSpeed GasLevel MaxSpeed=155 turnOnHazard() addGass(amount) getGasLevel():GasLevel Model Color EngineSize MaxSpeed GasLevel turnOnHazard() addGass(amount) getGasLevel():GasLevel Model Color EngineSize Roof MaxSpeed GasLevel turnOnHazard() addGass(amount) getGasLevel():GasLevel Model Color EngineSize NumDoors MaxSpeed GasLevel turnOnHazard() addGass(amount) getGasLevel():GasLevel Inheritance Car

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

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

Project Work • Next project deliverable – Design Specification Document • Last deliverable - Prototype

CS 4310: Software Engineering