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.
Defines the abstract characteristics of a thing (object), including the thing's characteristics (its attributes, fields or properties) and the thing's behaviors (the things it can do, or methods, operations or features). One might say that a class is a blueprint or factory that describes the nature of something. For example, the class
Dog would consist of traits shared by all dogs, such as breed and fur color (characteristics), and the ability to bark and sit (behaviors). 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 (generally using encapsulation). 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 and behaviors 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
Lassie object is an instance of the
Dog class. The set of values of the attributes of a particular object is called its state. The object consists of state and the behaviour that's defined in the object's class.
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
walk(). Within the program, using a method usually affects only one particular object; all
Dogs can bark, but you need only 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. E.g. the object called
Breeder may tell the
Lassie object to sit by passing a 'sit' message which invokes Lassie's 'sit' method. The syntax varies between languages, for example:
[Lassie sit] in Objective-C. In Java code-level message passing corresponds to "method calling".
‘Subclasses’ are more specialized versions of a class, which inherit attributes and behaviors from their parent classes, and can introduce their own.
For example, the class
Dog might have sub-classes called
GoldenRetriever. In this case,
Lassie would be an instance of the
Collie subclass. Suppose the
Dog class defines a method called
bark() and a property called
furColor. Each of its sub-classes (
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. 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 produces a high pitch 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
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 is a
Lassie inherits the methods of both
Multiple inheritance is inheritance from more than one ancestor class, neither of these ancestors being an ancestor of the other. For example, independent classes could define
Cats, and a
Chimera object could be created from these two which inherits all the (multiple) behavior of cats and dogs. This is not always supported, as it can be hard both to implement and to use well.
Encapsulation conceals the functional details of a class from objects that send messages to it.
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. Java uses the default access modifier to restrict access also to classes in the same package.
Abstraction is simplifying complex reality by modelling classes appropriate to the problem, and working at the most appropriate level of inheritance for a given aspect of the problem.
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.
Abstraction is also achieved through Composition. For example, a class
Car would be made up of an Engine, Gearbox, Steering objects, and many more components. To build the
Car class, one does not need to know how the different components work internally, but only how to interface with them, i.e., send messages to them, receive messages from them, and perhaps make the different objects composing the class interact with each other.
Polymorphism allows you to treat derived class members just like their parent class' members. More precisely, Polymorphism in object-oriented programming is the ability of objects belonging to different data types to respond to method calls of methods of the same name, each one according to an appropriate type-specific behavior. One method, or an operator such as +, -, or *, can be abstractly applied in many different situations. If a
Dog is commanded to
speak(), this may elicit a
bark(). However, if a
Pig is commanded to
speak(), this may elicit an
oink(). They both inherit
Animal, but their derived class methods override the methods of the parent class; this is Overriding Polymorphism. Overloading Polymorphism is the use of one method signature, or one operator such as ‘+’, to perform several different functions depending on the implementation. The ‘+’ operator, for example, may be used to perform integer addition, float addition, list concatenation, or string concatenation. Any two subclasses of
Number, such as
Double, are expected to add together properly in an OOP language. The language must therefore overload the concatenation operator, ‘+’, to work this way. This helps improve code readability. How this is implemented varies from language to language, but most OOP languages support at least some level of overloading polymorphism. Many OOP languages also support Parametric Polymorphism, where code is written without mention of any specific type and thus can be used transparently with any number of new types. Pointers are an example of a simple polymorphic routine that can be used with many different types of objects.