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Object-Oriented Software Engineering Practical Software Development using UML and Java Chapter 2: Review of Object Orientation 2.1 What is Object Orientation? Procedural paradigm: • Software is organized around the notion of procedures • Procedural abstraction —Works as long as the data is simple • Adding data abstractions —Groups together the pieces of data that describe some entity —Helps reduce the system’s complexity. - Such as Records and structures Object oriented paradigm: • Organizing procedural abstractions in the context of data abstractions © Lethbridge/Laganière 2001 Chapter 2: Review of Object Orientation 2 Object Oriented paradigm 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 operation on the objects • A running program can be seen as a collection of objects collaborating to perform a given task © Lethbridge/Laganière 2001 Chapter 2: Review of Object Orientation 3 A View of the Two paradigms main Account credit perform transaction debit credit debit compute interest if checking then xxx if savings then xxx etc. © Lethbridge/Laganière 2001 compute fees if checking CheckingAccount SavingsAccount then xxx compute interest compute interest compute fees compute fees if savings then xxx etc. Chapter 2: Review of Object Orientation 4 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 2001 Chapter 2: Review of Object Orientation 5 Objects Jane: date of birth: 1955/02/02 address: 99 UML St. position: Manager Greg: Savings Account 12876: balance: 1976.32 opened: 1997/03/03 date of birth: 1970/01/01 address: 75 Object Dr. Margaret: Mortgage Account 29865: balance: 198760.00 opened: 2000/08/12 property: 75 Object Dr. date of birth: 1980/03/03 address: 150 C++ Rd. position: Teller Transaction 487: amount: 200.00 time: 2001/09/01 14:30 Instant Teller 876: location: Java Valley Cafe © Lethbridge/Laganière 2001 Chapter 2: Review of Object Orientation 6 Classes A class: • Is a unit of abstraction in an object oriented (OO) program • Represents similar objects —Its instances • Is a kind of software module —Describes its instances’ structure (properties) —Contains methods to implement their behaviour © Lethbridge/Laganière 2001 Chapter 2: Review of Object Orientation 7 Is Something a Class or an Instance? • Something should be a class if it could have instances • Something should be an instance if it is clearly a single member of the set defined by a class Film • Class; instances are individual films. Reel of Film: • Class; instances are physical reels Film reel with serial number SW19876 • Instance of ReelOfFilm Science Fiction • Instance of the class Genre. Science Fiction Film • Class; instances include ‘Star Wars’ Showing of ‘Star Wars’ in the Phoenix Cinema at 7 p.m.: • Instance of ShowingOfFilm © Lethbridge/Laganière 2001 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 2001 Chapter 2: Review of Object Orientation 9 2.3 Instance Variables Variables defined inside a class corresponding to data present in each instance • 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 2001 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 2001 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 2001 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 2001 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 2001 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 ifelse or switch statements © Lethbridge/Laganière 2001 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 2001 Chapter 2: Review of Object Orientation 16 An Example Inheritance Hierarchy Account SavingsAccount ChequingAccount MortgageAccount Inheritance • The implicit possession by all subclasses of features defined in its superclasses © Lethbridge/Laganière 2001 Chapter 2: Review of Object Orientation 17 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! © Lethbridge/Laganière 2001 Chapter 2: Review of Object Orientation 18 A possible inheritance hierarchy of mathematical objects MathematicalObject Shape Point Shape2D Matrix Shape3D Ellipse Polygon Line Circle Quadrilateral Plane Rectangle © Lethbridge/Laganière 2001 Chapter 2: Review of Object Orientation 19 Make Sure all Inherited Features Make Sense in Subclasses Account balance opened creditOrOverdraftLimit credit debit calculateInterest SavingsAccount ChequingAccount highestChequeNumber MortgageAccount collateralProperty collateralValue withdrawUsingCheque calculateServiceCharge © Lethbridge/Laganière 2001 setCollateralValue Chapter 2: Review of Object Orientation 20 Shape2D 2.6 Inheritance, Polymorphism and Variables center translate getCenter rotate changeScale getArea getPerimeterLength getBoundingRect EllipticalShape Polygon semiMajorAxis getBoundingRect getVertices Ellipse Circle SimplePolygon rotate changeScale ArbitraryPolygon semiMinorAxis orientation points rotate addPoint getOrientation removePoint orientation getArea rotate getPerimeterLength changeScale rotate getBoundingRect getArea changeScale getRadius getPerimeterLength getArea getBoundingRect Rectangle RegularPolygon getPerimeterLength getOrientation getVertices getSemiMajorAxis height numPoints getSemiMinorAxis width radius changeScale changeNumPoints setHeight changeScale setWidth getArea getArea getPerimeterLength getPerimeterLength getVertices getFocus1 getFocus2 getVertices getBoundingRect © Lethbridge/Laganière 2001 Chapter 2: Review of Object Orientation 21 Origin (show Rotate (show Trans (show Scaled (50%) Scaled (150% Some Operations in the Shape Example © Lethbridge/Laganière 2001 Chapter 2: Review of Object Orientation 22 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 © Lethbridge/Laganière 2001 Chapter 2: Review of Object Orientation 23 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 2001 Chapter 2: Review of Object Orientation 24 Immutable objects • Instance variables may only be set when an object is first created. • None of the operations allow any changes to the instance variables —E.g. a scale method could only create a new object, not modify an existing one © Lethbridge/Laganière 2001 Chapter 2: Review of Object Orientation 25 How a decision is made about which method to run 1. 2. 3. 4. If there is a concrete method for the operation in the current class, run that method. Otherwise, check in the immediate superclass to see if there is a method there; if so, run it. Repeat step 2, looking in successively higher superclasses until a concrete method is found and run. If no method is found, then there is an error • In Java and C++ the program would not have compiled © Lethbridge/Laganière 2001 Chapter 2: Review of Object Orientation 26 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 © Lethbridge/Laganière 2001 Chapter 2: Review of Object Orientation 27 2.7 Concepts that Define Object Orientation 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 2001 Chapter 2: Review of Object Orientation 28 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 2001 Chapter 2: Review of Object Orientation 29 The Basics of Java History • The first object oriented programming language was Simula-67 —designed to allow programmers to write simulation programs • In the early 1980’s, Smalltalk was developed at Xerox PARC —New syntax, large open-source library of reusable code, bytecode, platform independence, garbage collection. • late 1980’s, C++ was developed by B. Stroustrup, —Recognized the advantages of OO but also recognized that there were tremendous numbers of C programmers • In 1991, engineers at Sun Microsystems started a project to design a language that could be used in consumer ‘smart devices’: Oak —When the Internet gained popularity, Sun saw an opportunity to exploit the technology. —The new language, renamed Java, was formally presented in 1995 at the SunWorld ’95 conference. © Lethbridge/Laganière 2001 Chapter 2: Review of Object Orientation 30 Java documentation Looking up classes and methods is an essential skill • Looking up unknown classes and methods will get you a long way towards understanding code Java documentation can be automatically generated by a program called Javadoc • Documentation is generated from the code and its comments • You should format your comments as shown in some of the book’s examples —These may include embeded html © Lethbridge/Laganière 2001 Chapter 2: Review of Object Orientation 31 Overview of Java The next few slides will remind you of several key Java features • Not in the book • See the book’s web site for —A more detailed overview of Java —Pointers to tutorials, books etc. © Lethbridge/Laganière 2001 Chapter 2: Review of Object Orientation 32 Characters and Strings Character is a class representing Unicode characters • More than a byte each • Represent any world language char is a primitive data type containing a Unicode character String is a class containing collections of characters • + is the operator used to concatenate strings © Lethbridge/Laganière 2001 Chapter 2: Review of Object Orientation 33 Arrays and Collections Arrays are of fixed size and lack methods to manipulate them Vector is the most widely used class to hold a collection of other objects • More powerful than arrays, but less efficient Iterators are used to access members of Vectors • Enumerations were formally used, but were more complex v = new Vector(); Iterator i = v.iterator(); while(i.hasNext()) { aMethod(v.next()); } © Lethbridge/Laganière 2001 Chapter 2: Review of Object Orientation 34 Casting Java is very strict about types • If a variable is declared to have the type X, you can only invoke operations on it that are defined in class X or its superclasses —Even though an instance of a subclass of X may be actually stored in the variable • If you know an instance of a subclass is stored, then you can cast the variable to the subclass —E.g. if I know a Vector contains instances of String, I can get the next element of its Iterator using: (String)iterator.next(); © Lethbridge/Laganière 2001 Chapter 2: Review of Object Orientation 35 Exceptions Anything that can go wrong should result in the raising of an Exception • Exception is a class with many subclasses for specific things that can go wrong Use a try - catch block to trap an exception try { // some code } catch (ArithmeticException e) { // code to handle division by zero } © Lethbridge/Laganière 2001 Chapter 2: Review of Object Orientation 36 Interfaces Like abstract classes, but cannot have executable statements • Define a set of operations that make sense in several classes • Abstract Data Types A class can implement any number of interfaces • It must have concrete methods for the operations You can declare the type of a variable to be an interface • This is just like declaring the type to be an abstract class Important interfaces in Java’s library include • Runnable, Collection, Iterator, Comparable, Cloneable © Lethbridge/Laganière 2001 Chapter 2: Review of Object Orientation 37 Packages and importing A package combines related classes into subsystems • All the classes in a particular directory Classes in different packages can have the same name • Although not recommended Importing a package is done as follows: import finance.banking.accounts.*; © Lethbridge/Laganière 2001 Chapter 2: Review of Object Orientation 38 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 2001 Chapter 2: Review of Object Orientation 39 Threads and concurrency Thread: • Sequence of executing statements that can be running concurrently with other threads To create a thread in Java: • 1. Create a class implementing Runnable or extending Thread • 2. Implement the run method as a loop that does something for a period of time • 3. Create an instance of this class • 4. Invoke the start operation, which calls run © Lethbridge/Laganière 2001 Chapter 2: Review of Object Orientation 40 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 2001 Chapter 2: Review of Object Orientation 41 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 2001 Chapter 2: Review of Object Orientation 42 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 2001 Chapter 2: Review of Object Orientation 43 Programming style ... Adhere to good object oriented principles • E.g. the ‘isa rule’ Prefer private as opposed to public Do not mix user interface code with non-user interface code • Interact with the user in separate classes —This makes non-UI classes more reusable © Lethbridge/Laganière 2001 Chapter 2: Review of Object Orientation 44 2.10 Difficulties and Risks in ObjectOriented Programming Language evolution and deprecated features: • Java can be less efficient than other languages —VM-based —Dynamic binding Efficiency can be a concern in some object oriented systems • Java is evolving, so some features are ‘deprecated’ at every release • But the same thing is true of most other languages © Lethbridge/Laganière 2001 Chapter 2: Review of Object Orientation 45