Basic Concepts Introduction to OO Development and Software Engineering Principles SYSC System Analysis and Design

Everything is an Object! You, me, your neighbour, desks, chairs, doors, etc. Some of these things are the same, and some of them are different. Some of them are “active”, and others are “passive”.

Objectives Define objects, attributes and operations. Differentiate between OOA, OOD and OOP (!) Describe a Class and an object. Define information hiding and encapsulation. Define generalization and specialization and the isA rule. Define polymorphism and over-riding. Describe active objects and persistence.

Object-Oriented Development (OOD) Objects are abstractions of real-world or system entities and manage themselves Objects are independent and encapsulate state and representation information (attributes). System functionality is expressed in terms of object services (operations) Shared data areas are eliminated. Objects communicate by message passing Objects may be distributed and may execute sequentially or in parallel

Objects, Attributes, Operations

O-O World View An object-oriented system is regarded as a network of cooperating objects which -interact by sending each other messages -maintain their own state -have an individual identity

Advantages of OOD Easier maintenance. Classes may be understood as stand-alone entities, including their own functionality and data Classes are appropriate reusable components For some systems, there may be an obvious mapping from real world entities to system classes, e.g., accounts in banking system

Object-oriented development Object-oriented analysis (OOA), design (OOD) and programming (OOP) are related but distinct OOA is concerned with developing an object model of the application domain OOD is concerned with developing an object- oriented system model (structure, behaviour) to implement requirements OOP is concerned with realising an OOD using an OO programming language such as Java or C++

Principal Concepts –Objects, classification –encapsulation and information hiding –modularity –generalization and inheritance –typing –polymorphism –dynamic binding –concurrency –persistence

Classes and Objects Object – A chunk of structured data in a running software system – Has properties Represent its state – Has behaviour How it acts and reacts May simulate the behaviour of an object in the real world – Has an identity: each object is unique

High-level Examples Class Person. ‘Hussain Pervez.’ Speak, walk, read. Studying, resting, qualified. Shirt. My favourite button white denim shirt. Shrink, stain, rip. Pressed, dirty, worn. Sale. Sale no #0015, 16/06/02. Earn loyalty points.Invoiced, cancelled. IdentityBehaviourState Bottle of ketchup. This bottle of ketchup. Spill in transit.Unsold, opened, empty.

Objects Margaret: date of birth: 1980/03/03 position: Teller Transaction 487: amount: time: 2001/09/01 14:30 Greg: date of birth: 1970/01/01 address: 75 Object Dr. Mortgage Account 29865: balance: opened: 2000/08/12 property: 75 Object Dr. Instant Teller 876: location: Java Valley Cafe Savings Account 12876: balance: opened: 1997/03/03 Jane: date of birth: 1955/02/02 position: Manager address: 99 UML St. address: 150 C++ Rd.

Classes A class: – Is a unit of abstraction in an object oriented analysis, design, or program – Represent instances of real-world and system entities – Represents similar objects Its instances – Is a kind of software module Describes its instances’ structure (properties) Contains operations/methods to implement their behaviour

Classes and Objects: Example

Encapsulation and Information Hiding Encapsulation: The technique of hiding details which are not needed by the user of an abstraction Applied in object-oriented systems by the separation of object interfaces and bodies A_STACK PUSH POP interface body client’s view

Message-passing and Encapsulation Message from another object requests a service. Operation called only via valid operation signature. Data accessed only by object’s own operations. An object’s data is hidden (encapsulated). ‘Layers of an onion’ model of an object: An outer layer of operation signatures… …gives access to middle layer of operations… …which can access inner core of data

Object-Oriented Computation Model draw-line clear Attributes #3 #4 remove insert Attributes #4 #2 remove insert Attributes #2 #3 update draw Attributes #1 #1 print control flowdata flow system operation

Organizing Classes into Inheritance Hierarchies Superclasses – Contain features common to a set of subclasses Inheritance hierarchies – Show the relationships among superclasses and subclasses Inheritance – The implicit possession by all subclasses of features defined in its superclasses

An Example Inheritance Hierarchy Inheritance – The implicit possession by all subclasses of features defined in its superclasses

Generalization and Specialization Classification is hierarchic in nature Generalization: the technique of factoring out common properties into shared classes – facilitates reuse and change control – the reverse operation is the specialization For example, a person may be an employee, a customer, a supplier of a service An employee may be paid monthly, weekly or hourly An hourly paid employee may be a driver, a cleaner, a sales assistant

Specialization Hierarchy Person EmployeeCustomer Supplier monthly paidweekly paidhourly paid DriverCleaner Sales assistant More general (superclasses) More specialized (subclasses)

The Isa Rule Always check generalizations to ensure they obey the “isa” rule – “A checking account is an account” – “A village is a municipality” Should ‘Province’ be a subclass of ‘Country’? – No, it violates the isa rule “A province is a country” is invalid!

Inheritance hierarchy of mathematical objects

Make Sure all Inherited Features Make Sense in Subclasses

Methods, Operations and Polymorphism Operation – A higher-level procedural abstraction that specifies a type of behaviour – Independent of any code which implements that behaviour E.g., calculating area (in general) – Operations are defined during Analysis and Design – They are implemented with methods

Methods, Operations and Polymorphism Method – A procedural abstraction used to implement the behaviour of a class. – Several different classes can have methods with the same name They implement the same abstract operation in ways suitable to each class E.g, calculating area in a rectangle is done differently from in a circle

Polymorphism A property of object oriented software by which an abstract operation may be performed in different ways in different classes. – Requires that there be multiple methods of the same name – The choice of which one to execute depends on the object (i.e., type) that is in a variable, on which the method is executed – Reduces the need for programmers to code many if-else or switch statements

Overriding A method would be inherited, but a subclass contains a new version instead – For restriction E.g. scale(x,y) would not work in Circle – For extension E.g. SavingsAccount might charge an extra fee following every debit – For optimization E.g. The getPerimeterLength method in Circle is much simpler than the one in Ellipse

How a decision is made about which method to run 1.If there is a concrete method for the operation in the current class, run that method. 2.Otherwise, check in the immediate superclass to see if there is a method there; if so, run it. 3.Repeat step 2, looking in successively higher superclasses until a concrete method is found and run. 4.If no method is found, then there is an error – In Java and C++ the program would not have compiled

Dynamic binding Occurs when decision about which method to run can only be made at run time – Needed when: A variable is declared to have a superclass as its type, and There is more than one possible polymorphic method that could be run among the type of the variable and its subclasses

Abstract Classes and Methods An operation should be declared to exist at the highest class in the hierarchy where it makes sense – The operation may be abstract (lacking implementation) at that level – If so, the class also must be abstract No instances can be created The opposite of an abstract class is a concrete class – If a superclass has an abstract operation then its subclasses at some level must have a concrete method for the operation Leaf classes must have or inherit concrete methods for all operations Leaf classes must be concrete

Concurrency The nature of objects as self-contained entities make them suitable for concurrent implementation. There are two forms of objects in concurrent systems. Passive: – operations execute only in response to external stimuli – call other objects only in response to external stimuli Active – operations may execute asynchronously – has an independent thread of control – call other objects “spontaneously” – change state “spontaneously”

Active transponder object Active objects may have their attributes modified by operations but may also update them autonomously using internal operations Transponder object broadcasts an aircraft’s position. The position may be updated using a satellite positioning system. The object periodically update the position by triangulation from satellites

An active transponder object

Persistence The ability of an object to exist after the termination of the program which created it Allows data storage in terms of objects (e.g. files are objects) Provides a model for interaction with a DBMS or FMS E.g., Java serialization, Object-oriented DBMS OO program Database

Summary An object is some “thing” which has attributes (variables) and operations that they can perform A class is the blueprint or template that describes objects that have similar characteristics. Big idea: hide implementation of objects to make it easy to change without affecting the world. Polymorphism allows different types to implement common operations differently. “Active objects” operate spontaneously and independently of each other (like us!).