A class defines the abstract characteristics of a thing (object), including the thing’s characteristics (its attributes, fields or properties) and the things it can do (its behaviors or methods or features). For example, the class Dog would consist of traits shared by all dogs, for example breed, fur color, and the ability to bark. Classes provide modularity and structure in an object-oriented computer program. A class should typically be recognizable to a non-programmer familiar with the problem domain, meaning that the characteristics of the class should make sense in context. Also, the code for a class should be relatively self-contained. Collectively, the properties and methods defined by a class are called members.
A particular instance of a class. The class of Dog defines all possible dogs by listing the characteristics that they can have; the object Lassie is one particular dog, with particular versions of the characteristics. A Dog has fur; Lassie has brown-and-white fur. In programmer jargon, the object Lassie is an instance of the Dog class. The set of values of the attributes of a particular object is called its state.
An object’s abilities. Lassie, being a Dog, has the ability to bark. So bark() is one of Lassie’s methods. She may have other methods as well, for example sit() or eat(). Within the program, using a method should only affect one particular object; all Dogs can bark, but you need one particular dog to do the barking.
“The process by which an object sends data to another object or asks the other object to invoke a method.”Also known to some programming languages as interfacing
In some cases, a class will have “subclasses,” more specialized versions of a class. For example, the class Dog might have sub-classes called Collie, Chihuahua, and GoldenRetriever. In this case, Lassie would be an instance of the Collie subclass. Subclasses inherit attributes and behaviors from their parent classes, and can introduce their own. Suppose the Dog class defines a method called bark() and a property called furColor. Each of its sub-classes (Collie, Chihuahua, and GoldenRetriever) will inherit these members, meaning that the programmer only needs to write the code for them once. Each subclass can alter its inherited traits. So, for example, the Collie class might specify that the default furColor for a collie is brown-and-white. The Chihuahua subclass might specify that the bark() method is high-pitched by default. Subclasses can also add new members. The Chihuahua subclass could add a method called tremble(). So an individual chihuahua instance would use a high-pitched bark() from the Chihuahua subclass, which in turn inherited the usual bark() from Dog. The chihuahua object would also have the tremble() method, but Lassie would not, because she is a Collie, not a Chihuahua. In fact, inheritance is an “is-a” relationship: Lassie is a Collie. A Collie is a Dog. Thus, Lassie inherits the members of both Collies and Dogs. When an object or class inherits its traits from more than one ancestor class, and neither of these ancestors is an ancestor of the other, then it’s called multiple inheritance. For example, independent classes could define Dogs and Cats, and a Chimera object could be created from these two which inherits all the (multiple) behaviour of cats and dogs. This is not always supported, as it can be hard both to implement and to use well.
Conceals the exact details of how a particular class works from objects that use its code or send messages to it. So, for example, the Dog class has a bark() method. The code for the bark() method defines exactly how a bark happens (e.g., by inhale() and then exhale(), at a particular pitch and volume). Timmy, Lassie’s friend, however, does not need to know exactly how she barks. Encapsulation is achieved by specifying which classes may use the members of an object. The result is that each object exposes to any class a certain interface — those members accessible to that class. The reason for encapsulation is to prevent clients of an interface from depending on those parts of the implementation that are likely to change in future, thereby allowing those changes to be made more easily, that is, without changes to clients. For example, an interface can ensure that puppies can only be added to an object of the class Dog by code in that class. Members are often specified as public, protected or private, determining whether they are available to all classes, sub-classes or only the defining class. Some languages go further: Java uses the protected keyword to restrict access also to classes in the same package, C# and VB.NET reserve some members to classes in the same assembly using keywords internal (C#) or Friend (VB.NET), and Eiffel and C++ allows one to specify which classes may access any member.
Simplifying complex reality by modeling classes appropriate to the problem, and working at the most appropriate level of inheritance for a given aspect of the problem. For example, Lassie the Dog may be treated as a Dog much of the time, a Collie when necessary to access Collie-specific attributes or behaviors, and as an Animal (perhaps the parent class of Dog) when counting Timmy’s pets.
Polymorphism is the ability of behavior to vary based on the conditions in which the behavior is invoked, that is, two or more methods, as well as operators (such as +, -, *, among others) can fit to many different conditions. For example, if a Dog is commanded to speak() this may elicit a Bark; if a Pig is commanded to speak() this may elicit an Oink. This is expected because Pig has a particular implementation inside the speak() method. The same happens to class Dog. Considering both of them inherit speak() from Animal, this is an example of Overriding Polymorphism. Another good example is about Overloading Polymorphism, a very common one considering operators, like “+”. Once defined an operator used to add numbers, given a class Number and also given two other classes that inherits from Number, such as Integer and Double. Any programmer expects to add two instances of Double or two instances of Integer in just the same way, and more than this: Any programmer expects the same behavior to any Number. In this case, the programmer must overload the concatenation operator, “+”, by making it able to operate with both Double and Integer instances. The way it is done varies a little bit from one language to another and must be studied in more details according to the programmer interest. Most of the OOP languages support small differences in method signatures as polymorphism. it’s very useful, once it improves code readability, to enable implicit conversions to the correct handling method when apply add() method to integers, like in add(1,2), or to strings like in add(“foo”,”bar”) since the definitions of these signatures are available. In many OOP languages, such method signatures would be, respectively, very similar to add(int a, int b) and add(String a, String b). This is an example of Parametric Polymorphism. The returned type and used modifiers, of course, depend on the programmer interests and intentions.
Archive for the ‘OOP & UML’ Category
Posted by keremkosaner on 3 May 2007
Posted by keremkosaner on 2 May 2007
In the field of software engineering, the Unified Modeling Language (UML) is a standardized specification language for object modeling. UML is a general-purpose modeling language that includes a graphical notation used to create an abstract model of a system, referred to as a UML model.
UML is officially defined at the Object Management Group (OMG) by the UML metamodel, a Meta-Object Facility metamodel (MOF). Like other MOF-based specifications, the UML metamodel and UML models may be serialized in XMI. UML was designed to specify, visualize, construct, and document software-intensive systems.
UML is not restricted to modeling software. UML is also used for business process modeling, systems engineering modeling, and representing organizational structures. The Systems Modeling Language (SysML) is a Domain-Specific Modeling language for systems engineering that is defined as a UML 2.0 profile.
UML has been a catalyst for the evolution of model-driven technologies, which include Model Driven Development (MDD), Model Driven Engineering (MDE), and Model Driven Architecture (MDA). By establishing an industry consensus on a graphic notation to represent common concepts like classes, components, generalization, aggregation, and behaviors, UML has allowed software developers to concentrate more on design and architecture.
UML models may be automatically transformed to other representations (e.g. Java) by means of QVT-like transformation languages, supported by the OMG. UML is extensible, offering the following mechanisms for customization: profiles and stereotype.
Posted by keremkosaner on 1 May 2007
Object-oriented programming (OOP) is a programming paradigm that uses “objects” to design applications and computer programs. It utilizes several techniques from previously established paradigms, including inheritance, modularity, polymorphism, and encapsulation. Even though it originated in the 1960s, OOP was not commonly used in mainstream software application development until the 1990s. Today, many popular programming languages support OOP.
Object-oriented programming’s roots reach all the way back to the 1960s, when the nascent field of software engineering had begun to discuss the idea of a software crisis. As hardware and software became increasingly complex, how could software quality be maintained? Object-oriented programming addresses this problem by strongly emphasizing modularity in software.
The Simula programming language was the first to introduce the concepts underlying object-oriented programming (objects, classes, subclasses, virtual methods, coroutines, garbage collection and discrete event simulation) as a superset of Algol. Smalltalk was the first programming language to be called “object-oriented”.
Object-oriented programming may be seen as a collection of cooperating objects, as opposed to a traditional view in which a program may be seen as a list of instructions to the computer. In OOP, each object is capable of receiving messages, processing data, and sending messages to other objects. Each object can be viewed as an independent little machine with a distinct role or responsibility.
Object-oriented programming allegedly came into existence because human consciousness, understanding and logic are highly object-oriented. By way of “objectifying” software modules, it is intended to promote greater flexibility and maintainability in programming, and is widely popular in large-scale software engineering. By virtue of its strong emphasis on modularity, object oriented code is intended to be simpler to develop and easier to understand later on, lending itself to more direct analysis, coding, and understanding of complex situations and procedures than less modular programming methods.
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