Object-Oriented Software Engineering Lecture 2 PDF

Object-Oriented Software Engineering Chapter 2: Review of Object Orientation 2.1 What is Object Orientation? Procedural Oriented Programming: Each steps is executed in a systematic manner so that the computer can understand what to do (step by step). C c...

Object-Oriented Software Engineering Chapter 2: Review of Object Orientation 2.1 What is Object Orientation? Procedural Oriented Programming: Each steps is executed in a systematic manner so that the computer can understand what to do (step by step). C called procedural orientation language because large program is divided into modules. —Groups together the pieces of data that describe some entity —Helps reduce the system’s complexity. Object oriented Programming: Organizing your code by creating objects, and then you can give those objects properties. © Lethbridge/Laganière 2005 Chapter 2: Review of Object Orientation 2 Object Oriented An approach to the solution of problems in which all computations are performed in the context of objects. The objects are instances of classes, which: —are data abstractions —contain procedural abstractions that operate on the objects A running program can be seen as a collection of objects collaborating to perform a given task © Lethbridge/Laganière 2005 Chapter 2: Review of Object Orientation 3 Assignments 1 What are a differences between procedural and object oriented programming? Parameter : Definition Approach Access modifiers Complexity Inheritance © Lethbridge/Laganière 2005 Chapter 2: Review of Object Orientation 4 A View of the Two paradigms See in Umple © Lethbridge/Laganière 2005 Chapter 2: Review of Object Orientation 5 2.2 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 © Lethbridge/Laganière 2005 Chapter 2: Review of Object Orientation 6 Objects © Lethbridge/Laganière 2005 Chapter 2: Review of Object Orientation 7 Classes A class: A unit of abstraction in an object oriented (OO) program Represents similar objects —Its instances A kind of software module —Describes its instances’ structure (properties) —Contains methods to implement their behaviour © Lethbridge/Laganière 2005 Chapter 2: Review of Object Orientation 8 Naming classes Use capital letters —E.g. BankAccount not bankAccount Use singular nouns Use the right level of generality —E.g. Municipality, not City Make sure the name has only one meaning —E.g. ‘bus’ has several meanings © Lethbridge/Laganière 2005 Chapter 2: Review of Object Orientation 9 2.3 Instance Variables Variables defined inside a class and are used to store values in an object Also called fields or member variables Attributes —Simple data —E.g. name, dateOfBirth Associations —Relationships to other important classes —E.g. supervisor, coursesTaken —More on these in Chapter 5 © Lethbridge/Laganière 2005 Chapter 2: Review of Object Orientation 10 Variables vs. Objects A variable Refers to an object May refer to different objects at different points in time An object can be referred to by several different variables at the same time Type of a variable Determines what classes of objects it may contain © Lethbridge/Laganière 2005 Chapter 2: Review of Object Orientation 11 Class variables A class variable’s value is shared by all instances of a class. Also called a static variable If one instance sets the value of a class variable, then all the other instances see the same changed value. Class variables are useful for: —Default or ‘constant’ values (e.g. PI) —Lookup tables and similar structures Caution: do not over-use class variables © Lethbridge/Laganière 2005 Chapter 2: Review of Object Orientation 12 2.4 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) © Lethbridge/Laganière 2005 Chapter 2: Review of Object Orientation 13 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 © Lethbridge/Laganière 2005 Chapter 2: Review of Object Orientation 14 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 that is in a variable Reduces the need for programmers to code many if- else or switch statements © Lethbridge/Laganière 2005 Chapter 2: Review of Object Orientation 15 2.5 Organizing Classes into Inheritance Hierarchies Superclasses Contain features common to a set of subclasses Inheritance hierarchies Show the relationships among superclasses and subclasses A triangle shows a generalization Inheritance The implicit possession by all subclasses of features defined in its superclasses © Lethbridge/Laganière 2005 Chapter 2: Review of Object Orientation 16 An Example Inheritance Hierarchy See in Umple Inheritance The implicit possession by all subclasses of features defined in its superclasses © Lethbridge/Laganière 2005 Chapter 2: Review of Object Orientation 17 A possible inheritance hierarchy of mathematical objects © Lethbridge/Laganière 2005 Chapter 2: Review of Object Orientation 18 Make Sure all Inherited Features Make Sense in Subclasses © Lethbridge/Laganière 2005 Chapter 2: Review of Object Orientation 19 2.6 Inheritance, Polymorphism and Variables © Lethbridge/Laganière 2005 Chapter 2: Review of Object Orientation 20 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 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 © Lethbridge/Laganière 2005 Chapter 2: Review of Object Orientation 21 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 © Lethbridge/Laganière 2005 Chapter 2: Review of Object Orientation 22 2.7 Concepts that Define Object Orientation The following are necessary for a system or language to be OO Identity —Each object is distinct from each other object, and can be referred to —Two objects are distinct even if they have the same data Classes —The code is organized using classes, each of which describes a set of objects Inheritance —The mechanism where features in a hierarchy inherit from superclasses to subclasses Polymorphism —The mechanism by which several methods can have the same name and implement the same abstract operation. © Lethbridge/Laganière 2005 Chapter 2: Review of Object Orientation 23 Other Key Concepts Abstraction Object -> something in the world Class -> objects Superclass -> subclasses Operation -> methods Attributes and associations -> instance variables Modularity Code can be constructed entirely of classes Encapsulation Details can be hidden in classes This gives rise to information hiding: —Programmers do not need to know all the details of a class © Lethbridge/Laganière 2005 Chapter 2: Review of Object Orientation 24 Access control Applies to methods and variables public —Any class can access protected —Only code in the package, or subclasses can access (blank) —Only code in the package can access private —Only code written in the class can access —Inheritance still occurs! © Lethbridge/Laganière 2005 Chapter 2: Review of Object Orientation 25 Programming Style Guidelines Remember that programs are for people to read Always choose the simpler alternative Reject clever code that is hard to understand Shorter code is not necessarily better Choose good names Make them highly descriptive Do not worry about using long names © Lethbridge/Laganière 2005 Chapter 2: Review of Object Orientation 26 Programming style … Comment extensively Comment whatever is non-obvious Do not comment the obvious Comments should be 25-50% of the code Organize class elements consistently Variables, constructors, public methods then private methods Be consistent regarding layout of code © Lethbridge/Laganière 2005 Chapter 2: Review of Object Orientation 27 Programming style … Avoid duplication of code Do not ‘clone’ if possible —Create a new method and call it —Cloning results in two copies that may both have bugs - When one copy of the bug is fixed, the other may be forgotten © Lethbridge/Laganière 2005 Chapter 2: Review of Object Orientation 28 2.10 Difficulties and Risks in Object- Oriented Programming Language evolution and deprecated features: Java is evolving, so some features are ‘deprecated’ at every release But the same thing is true of most other languages Efficiency can be a concern in some object oriented systems Java can be less efficient than other languages —VM-based —Dynamic binding © Lethbridge/Laganière 2005 Chapter 2: Review of Object Orientation 29