Object - Oriented software engineering practical software development using uml and java - Chapter 2: Review of object orientation

Object-Oriented Software EngineeringPractical Software Development using UML and JavaChapter 2: Review of Object Orientation © Lethbridge/Laganière 2005Chapter 2: Review of Object Orientation*2.1 What is Object Orientation?Procedural paradigm:Software is organized around the notion of procedures Procedural abstractionWorks as long as the data is simpleAdding data abstractions Groups together the pieces of data that describe some entity Helps reduce the system’s complexity. Such as Records and structuresObject oriented paradigm: Organizing procedural abstractions in the context of data abstractions© Lethbridge/Laganière 2005Chapter 2: Review of Object Orientation*Object Oriented paradigmAn 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 abstractionscontain procedural abstractions that operate on the objectsA running program can be seen as a collection of objects collaborating to perform a given task © Lethbridge/Laganière 2005Chapter 2: Review of Object Orientation*A View of the Two paradigmsSee in Umple© Lethbridge/Laganière 2005Chapter 2: Review of Object Orientation*2.2 Classes and ObjectsObjectA chunk of structured data in a running software system Has propertiesRepresent its stateHas behaviourHow it acts and reactsMay simulate the behaviour of an object in the real world© Lethbridge/Laganière 2005Chapter 2: Review of Object Orientation*Objects© Lethbridge/Laganière 2005Chapter 2: Review of Object Orientation*ClassesA class:A unit of abstraction in an object oriented (OO) program Represents similar objectsIts instancesA kind of software moduleDescribes its instances’ structure (properties)Contains methods to implement their behaviour© Lethbridge/Laganière 2005Chapter 2: Review of Object Orientation*Is Something a Class or an Instance?Something should be a class if it could have instancesSomething should be an instance if it is clearly a single member of the set defined by a class FilmClass; instances are individual films.Reel of Film:Class; instances are physical reelsFilm reel with serial number SW19876Instance of ReelOfFilmScience FictionInstance of the class Genre.Science Fiction FilmClass; instances include ‘Star Wars’Showing of ‘Star Wars’ in the Phoenix Cinema at 7 p.m.:Instance of ShowingOfFilm© Lethbridge/Laganière 2005Chapter 2: Review of Object Orientation*Naming classesUse capital lettersE.g. BankAccount not bankAccountUse singular nounsUse the right level of generalityE.g. Municipality, not CityMake sure the name has only one meaningE.g. ‘bus’ has several meanings© Lethbridge/Laganière 2005Chapter 2: Review of Object Orientation*2.3 Instance VariablesVariables defined inside a class corresponding to data present in each instanceAlso called fields or member variablesAttributesSimple dataE.g. name, dateOfBirthAssociationsRelationships to other important classesE.g. supervisor, coursesTakenMore on these in Chapter 5© Lethbridge/Laganière 2005Chapter 2: Review of Object Orientation*Variables vs. ObjectsA variableRefers to an object May refer to different objects at different points in timeAn object can be referred to by several different variables at the same timeType of a variableDetermines what classes of objects it may contain © Lethbridge/Laganière 2005Chapter 2: Review of Object Orientation*Class variables A class variable’s value is shared by all instances of a class. Also called a static variableIf 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 structuresCaution: do not over-use class variables © Lethbridge/Laganière 2005Chapter 2: Review of Object Orientation*2.4 Methods, Operations and PolymorphismOperationA higher-level procedural abstraction that specifies a type of behaviourIndependent of any code which implements that behaviourE.g. calculating area (in general)© Lethbridge/Laganière 2005Chapter 2: Review of Object Orientation*Methods, Operations and PolymorphismMethodA procedural abstraction used to implement the behaviour of a classSeveral different classes can have methods with the same nameThey 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 2005Chapter 2: Review of Object Orientation*PolymorphismA 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 nameThe choice of which one to execute depends on the object that is in a variableReduces the need for programmers to code many if-else or switch statements© Lethbridge/Laganière 2005Chapter 2: Review of Object Orientation*2.5 Organizing Classes into Inheritance HierarchiesSuperclassesContain features common to a set of subclassesInheritance hierarchiesShow the relationships among superclasses and subclassesA triangle shows a generalizationInheritanceThe implicit possession by all subclasses of features defined in its superclasses© Lethbridge/Laganière 2005Chapter 2: Review of Object Orientation*An Example Inheritance HierarchyInheritanceThe implicit possession by all subclasses of features defined in its superclassesSee in Umple© Lethbridge/Laganière 2005Chapter 2: Review of Object Orientation*The Isa RuleAlways 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 2005Chapter 2: Review of Object Orientation*A possible inheritance hierarchy of mathematical objects © Lethbridge/Laganière 2005Chapter 2: Review of Object Orientation*Make Sure all Inherited Features Make Sense in Subclasses© Lethbridge/Laganière 2005Chapter 2: Review of Object Orientation*2.6 Inheritance, Polymorphism and Variables© Lethbridge/Laganière 2005Chapter 2: Review of Object Orientation*Some Operations in the Shape Example© Lethbridge/Laganière 2005Chapter 2: Review of Object Orientation*Abstract Classes and MethodsAn operation should be declared to exist at the highest class in the hierarchy where it makes senseThe operation may be abstract (lacking implementation) at that levelIf so, the class also must be abstractNo instances can be createdThe opposite of an abstract class is a concrete classIf a superclass has an abstract operation then its subclasses at some level must have a concrete method for the operationLeaf classes must have or inherit concrete methods for all operationsLeaf classes must be concrete© Lethbridge/Laganière 2005Chapter 2: Review of Object Orientation*OverridingA method would be inherited, but a subclass contains a new version insteadFor restrictionE.g. scale(x,y) would not work in CircleFor extensionE.g. SavingsAccount might charge an extra fee following every debitFor optimizationE.g. The getPerimeterLength method in Circle is much simpler than the one in Ellipse© Lethbridge/Laganière 2005Chapter 2: Review of Object Orientation*How a decision is made about which method to run1. 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 errorIn Java and C++ the program would not have compiled© Lethbridge/Laganière 2005Chapter 2: Review of Object Orientation*Dynamic bindingOccurs when decision about which method to run can only be made at run timeNeeded when:A variable is declared to have a superclass as its type, andThere is more than one possible polymorphic method that could be run among the type of the variable and its subclasses© Lethbridge/Laganière 2005Chapter 2: Review of Object Orientation*2.7 Concepts that Define Object Orientation The following are necessary for a system or language to be OOIdentityEach object is distinct from each other object, and can be referred toTwo objects are distinct even if they have the same dataClassesThe code is organized using classes, each of which describes a set of objectsInheritanceThe mechanism where features in a hierarchy inherit from superclasses to subclassesPolymorphismThe mechanism by which several methods can have the same name and implement the same abstract operation.© Lethbridge/Laganière 2005Chapter 2: Review of Object Orientation*Other Key ConceptsAbstractionObject -> something in the worldClass -> objectsSuperclass -> subclassesOperation -> methodsAttributes and associations -> instance variablesModularityCode can be constructed entirely of classesEncapsulationDetails can be hidden in classesThis gives rise to information hiding: Programmers do not need to know all the details of a class © Lethbridge/Laganière 2005Chapter 2: Review of Object Orientation*The Basics of JavaHistoryThe 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 programmersIn 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 2005Chapter 2: Review of Object Orientation*Java documentationLooking up classes and methods is an essential skillLooking up unknown classes and methods will get you a long way towards understanding codeJava documentation can be automatically generated by a program called JavadocDocumentation is generated from the code and its commentsYou should format your comments as shown in some of the book’s examplesThese may include embeded html© Lethbridge/Laganière 2005Chapter 2: Review of Object Orientation*Overview of JavaThe next few slides will remind you of several key Java featuresNot in the bookSee the book’s web site forA more detailed overview of JavaPointers to tutorials, books etc.© Lethbridge/Laganière 2005Chapter 2: Review of Object Orientation*Characters and StringsCharacter is a class representing Unicode charactersMore than a byte eachRepresent any world languagechar is a primitive data type containing a Unicode characterString is a class containing collections of characters+ is the operator used to concatenate strings© Lethbridge/Laganière 2005Chapter 2: Review of Object Orientation*Arrays and CollectionsArrays are of fixed size and lack methods to manipulate themArrayList is the most widely used class to hold a collection of other objectsMore powerful than arrays, but less efficientIterators are used to access members of VectorsEnumerations were formally used, but were more complexa = new ArrayList();Iterator i = a.iterator();while(i.hasNext()){ aMethod(i.next());}© Lethbridge/Laganière 2005Chapter 2: Review of Object Orientation*CastingJava is very strict about typesIf variable v is declared to have type X, you can only invoke operations on v that are defined in X or its superclassesEven though an instance of a subclass of X may be actually stored in the variableIf you know an instance of a subclass is stored, then you can cast the variable to the subclassE.g. if I know a Vector contains instances of String, I can get the next element of its Iterator using: (String)i.next();To avoid casting you could also have used templates:: a = ArrayList; i=a.iterator(); i.next()using© Lethbridge/Laganière 2005Chapter 2: Review of Object Orientation*ExceptionsAnything that can go wrong should result in the raising of an ExceptionException is a class with many subclasses for specific things that can go wrongUse a try - catch block to trap an exceptiontry{ // some code}catch (ArithmeticException e){ // code to handle division by zero}© Lethbridge/Laganière 2005Chapter 2: Review of Object Orientation*InterfacesLike abstract classes, but cannot have executable statementsDefine a set of operations that make sense in several classesAbstract Data TypesA class can implement any number of interfacesIt must have concrete methods for the operationsYou can declare the type of a variable to be an interfaceThis is just like declaring the type to be an abstract classImportant interfaces in Java’s library includeRunnable, Collection, Iterator, Comparable, Cloneable© Lethbridge/Laganière 2005Chapter 2: Review of Object Orientation*Packages and importingA package combines related classes into subsystemsAll the classes in a particular directoryClasses in different packages can have the same nameAlthough not recommendedImporting a package is done as follows:import finance.banking.accounts.*;© Lethbridge/Laganière 2005Chapter 2: Review of Object Orientation*Access controlApplies to methods and variablespublicAny class can accessprotectedOnly code in the package, or subclasses can access(blank)Only code in the package can accessprivateOnly code written in the class can accessInheritance still occurs!© Lethbridge/Laganière 2005Chapter 2: Review of Object Orientation*Threads and concurrencyThread:Sequence of executing statements that can be running concurrently with other threadsTo create a thread in Java:1. Create a class implementing Runnable or extending Thread2. Implement the run method as a loop that does something for a period of time3. Create an instance of this class4. Invoke the start operation, which calls run© Lethbridge/Laganière 2005Chapter 2: Review of Object Orientation*Programming Style GuidelinesRemember that programs are for people to readAlways choose the simpler alternativeReject clever code that is hard to understandShorter code is not necessarily betterChoose good namesMake them highly descriptiveDo not worry about using long names© Lethbridge/Laganière 2005Chapter 2: Review of Object Orientation*Programming style Comment extensivelyComment whatever is non-obviousDo not comment the obviousComments should be 25-50% of the codeOrganize class elements consistentlyVariables, constructors, public methods then private methodsBe consistent regarding layout of code© Lethbridge/Laganière 2005Chapter 2: Review of Object Orientation*Programming style Avoid duplication of codeDo not ‘clone’ if possibleCreate a new method and call itCloning results in two copies that may both have bugsWhen one copy of the bug is fixed, the other may be forgotten© Lethbridge/Laganière 2005Chapter 2: Review of Object Orientation*Programming style ...Adhere to good object oriented principlesE.g. the ‘isa rule’Prefer private as opposed to publicDo not mix user interface code with non-user interface codeInteract with the user in separate classesThis makes non-UI classes more reusable© Lethbridge/Laganière 2005Chapter 2: Review of Object Orientation*2.10 Difficulties and Risks in Object-Oriented ProgrammingLanguage evolution and deprecated features: Java is evolving, so some features are ‘deprecated’ at every releaseBut the same thing is true of most other languagesEfficiency can be a concern in some object oriented systems Java can be less efficient than other languagesVM-basedDynamic binding