Object-oriented programming is a programming paradigm using "objects" – data structures consisting of data fields and methods together with their interactions – to design applications and computer programs. Programming techniques may include features such as data abstraction,...
, a class is a construct that is used as a blueprint
Blueprint
A blueprint is a type of paper-based reproduction usually of a technical drawing, documenting an architecture or an engineering design. More generally, the term "blueprint" has come to be used to refer to any detailed plan....
In object-oriented programming an instance is an occurrence or a copy of an object, whether currently executing or not. Instances of a class share the same set of attributes, yet will typically differ in what those attributes contain....
s of itself – referred to as class instances, class objects, instance objects or simply objects. A class defines constituent members which enable these class instances to have state and behavior. Data field members (member variables or instance variables) enable a class object to maintain state. Other kinds of members, especially method
Method (computer science)
In object-oriented programming, a method is a subroutine associated with a class. Methods define the behavior to be exhibited by instances of the associated class at program run time...
s, enable a class object's behavior. Class instances are of the type
Data type
In computer programming, a data type is a classification identifying one of various types of data, such as floating-point, integer, or Boolean, that determines the possible values for that type; the operations that can be done on values of that type; the meaning of the data; and the way values of...
of the associated class.
A class usually represents a noun, such as a person, place or (possibly quite abstract) thing, or something nominalized
Nominalization
In linguistics, nominalization or nominalisation is the use of a verb, an adjective, or an adverb as the head of a noun phrase, with or without morphological transformation...
. For example, "Banana" is an instance of the class "Fruit" (a "Fruit" object). It is of the type "Fruit".
Design and implementation
Classes consist of and are composed from structural and also behavioral constituents, though there may not be a requirement to include any structural or behavioral constituents at all. Programming languages that include classes as a programming construct offer support for various class-related features. These features, and the syntaxes with which they are provided, vary greatly. Various control features such as member access specifiers may be provided by a language.
In computer science, data is information in a form suitable for use with a computer. Data is often distinguished from programs. A program is a sequence of instructions that detail a task for the computer to perform...
field descriptions (or properties, fields, data members, attributes), i.e., field types and names, that will be associated with either per-instance or per-class state variables at program run time. In most languages, the structure as defined by the class determines how the memory used by its instances will be laid out. Note: Other implementations are possible such as wherein objects are structured as associative key-value containers (Python
Python (programming language)
Python is a general-purpose, high-level programming language whose design philosophy emphasizes code readability. Python claims to "[combine] remarkable power with very clear syntax", and its standard library is large and comprehensive...
).
Some programming languages support specification of invariant
Class invariant
In computer programming, specifically object-oriented programming, a class invariant is an invariant used to constrain objects of a class. Methods of the class should preserve the invariant. The class invariant constrains the state stored in the object....
s as part of the definition of the class, and enforce them through the type system. Encapsulation of state is necessary for being able to enforce the invariants of the class.
Behavior
The behavior to be exhibited by class instances at program run time is defined in methods
Method (computer science)
In object-oriented programming, a method is a subroutine associated with a class. Methods define the behavior to be exhibited by instances of the associated class at program run time...
In computer science, a subroutine is a portion of code within a larger program that performs a specific task and is relatively independent of the remaining code....
(function) with a special property that it has access to data stored in an object (instance) and is thereby able to control the state of the object. (Note: Some languages allow direct access to instance variables (C++
C++
C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell...
)). The various kinds of methods and language support for them differs. Kinds of methods include regular methods that are created and called by programmer code and special methods (constructors, destructors, conversion operators, static methods, e.g.) that are created and called by compiler-generated code. A language may allow the programmer to define, and even call, special methods.
The concepts of class interface
Every class implements (or realizes) an interface by providing structure (i.e. data and state) and method implementations (i.e. providing code that specifies how methods work). There is a distinction between the definition of an interface and the implementation of that interface. In most languages, this line is usually blurred, because a class declaration both defines and implements an interface. Some languages, however, provide features that help separate interface and implementation. For example, an abstract class can define an interface without providing implementation.
Languages that support class inheritance also allow classes to inherit interfaces from the classes that they are derived from. In languages that support access specifiers, the interface of a class is considered to be the set of public members of the class, including both methods and attributes (via implicit getter and setter methods
Mutator method
In computer science, a mutator method is a method used to control changes to a variable.The mutator method, sometimes called a "setter", is most often used in object-oriented programming, in keeping with the principle of encapsulation...
); any private members or internal data structures are not intended to be depended on by client code and thus are not part of the interface.
The object-oriented programming methodology is designed in such a way that the operations of any interface of a class are usually chosen to be independent of each other. It results in a layered design where clients of an interface use the methods declared in the interface. An interface places no requirements for clients to invoke the operations of one interface in any particular order. This approach has the benefit that client code can assume that the operations of an interface are available for use whenever the client holds a valid reference to the object.
Example
The buttons on the front of your television set, for example, are the interface between you and the electrical wiring on the other side of its plastic casing. You press the "power" button to toggle the television on and off. In this example, the television is the instance, each method is represented by a button, and all the buttons together comprise the interface. In its most common form, an interface is a specification of a group of related methods without any associated implementation of the methods.
attributes and interface example: A television set has a myriad of attributes, such as size and whether it supports color, together comprise its structure. A class represents the full description of a television, including its attributes (structure) and buttons (interface).
static method example: Getting the total number of televisions in existence could be a static method of the television class. This method is clearly associated with the class, yet is outside the domain of each individual instance of the class. Another example is a static method that finds a particular instance out of the set of all television objects.
Member accessibility
Many languages support the concept of information hiding and encapsulation, typically with access specifiers for class members. Access specifiers control access to class members. Some access specifiers may also control how classes inherit such constraints. Their primary purpose is to separate the interface of a class from its implementation.
A common set of access specifiers that many object-oriented languages support is:
private (or class-private) restricts the access to the class itself. Only methods that are part of the same class can access private members.
protected (or class-protected) allows the class itself and all its subclasses to access the member.
public means that any code can access the member by its name.
Note that although many languages support the above access specifiers, the semantics of them may subtly differ in each.
Object-oriented design uses the access specifiers in conjunction with careful design of public method implementations to enforce class invariants. A common usage of access specifiers is to separate the internal data (structure)s of a class from its interface: the internal data structures are made private, while public accessor methods can be used to inspect or alter such private data.
Access specifiers do not necessarily control visibility, in that even private members may be visible to client code. In some languages, an inaccessible, but visible member may be referred to at run-time (e.g. pointer to it can be returned from member functions), but all attempts to use it by referring to the name of the member from client code will be prevented by the type checker.
The various object-oriented programming languages enforce member accessibility and visibility to various degrees, and depending on the language's type system
Type system
A type system associates a type with each computed value. By examining the flow of these values, a type system attempts to ensure or prove that no type errors can occur...
and compilation policies, enforced at either compile-time
Compile time
In computer science, compile time refers to either the operations performed by a compiler , programming language requirements that must be met by source code for it to be successfully compiled , or properties of the program that can be reasoned about at compile time.The operations performed at...
Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...
language does not allow client code to access the private data of a class to compile at all, whereas in languages like Objective-C
Objective-C
Objective-C is a reflective, object-oriented programming language that adds Smalltalk-style messaging to the C programming language.Today, it is used primarily on Apple's Mac OS X and iOS: two environments derived from the OpenStep standard, though not compliant with it...
Perl is a high-level, general-purpose, interpreted, dynamic programming language. Perl was originally developed by Larry Wall in 1987 as a general-purpose Unix scripting language to make report processing easier. Since then, it has undergone many changes and revisions and become widely popular...
C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell...
language, private methods are visible, but not accessible in the interface; however, they are commonly made invisible by explicitly declaring fully abstract classes that represent the interfaces of the class.
Other prominent accessibility schemes (with example languages):
Ruby is a dynamic, reflective, general-purpose object-oriented programming language that combines syntax inspired by Perl with Smalltalk-like features. Ruby originated in Japan during the mid-1990s and was first developed and designed by Yukihiro "Matz" Matsumoto...
supports instance-private and instance-protected access specifiers in lieu of class-private and class-protected, respectively. They differ in that they restrict access based on the instance itself, rather than the instance's class.
Friend: C++ supports a mechanism where a function explicitly declared as a friend function
Friend function
A friend function for a class is used in object-oriented programming to allow access to , , or data in the class from the outside. Normally, a function that is not a member of a class cannot access such information; neither can an external class. Occasionally, such access will be advantageous for...
of the class may access the members designated as private or protected.
Path-based: Java supports restricting access to a member within a Java package which is roughly the path of a file.
Inter-class relationship
In addition to the design of standalone classes that do not have a relationship to other classes, programming languages may support more advanced class design based upon inter-class relationship. The inter-class relationship design capabilities commonly provided are compositional and hierarchical.
Compositional
Classes can be composed of other classes, thereby establishing a compositional relationship between the enclosing class and its embedded classes. Compositional relationship between classes is also commonly known as a has-a
Has-a
In database design and object oriented program architecture, has-a is a relationship where one object "belongs" to another object , and behaves according to the rules of ownership. In simple words, has-a relationship in an object is called a member field of an object...
relationship. For example, a class 'Car' could be composed of (i.e., contain) a class 'Engine'. Therefore, a Car has an Engine. Composition is often implemented in programming languages by locating the data storage of embedded class instances within the storage area of the enclosing class instance. The instances of embedded classes have shorter or same lifetime
Object lifetime
In computer science, the object lifetime of an object in object-oriented programming is the time between an object's creation till the object is no longer used, and is destructed or freed.In object-oriented programming , the meaning of creating objects is far more subtle than simple...
than the lifetime of the instances of the enclosing class.
Hierarchical
Classes can be derived from one or more existing classes (not all languages support multiple inheritance), thereby establishing a hierarchial relationship between the derived-from classes (base classes or parent classes or superclasses) and the derived class (or child class or subclass) . The relationship of the derived class to the derived-from classes is commonly known as an is-a
Is-a
In knowledge representation, object-oriented programming and design, is-a or is_a or is a is a relationship where one class D is a subclass of another class B ....
relationship. For example, a class 'Button' could be derived from a class 'Control'. Therefore, a Button is a Control. Structural and behavioral members of the parent classes are inherited by the child class. Derived classes can define additional structural members (data fields) and/or behavioral members (methods) in addition to those that they inherit and are therefore specializations of their superclasses. Also, derived classes can override inherited methods if the language provides for such.
In mathematics and computer science, a directed acyclic graph , is a directed graph with no directed cycles. That is, it is formed by a collection of vertices and directed edges, each edge connecting one vertex to another, such that there is no way to start at some vertex v and follow a sequence of...
In mathematics, more specifically graph theory, a tree is an undirected graph in which any two vertices are connected by exactly one simple path. In other words, any connected graph without cycles is a tree...
. The hierarchy has classes as nodes and inheritance relationships as links. Classes in the same level are more likely to be associated
Association (object-oriented programming)
In object-oriented programming, association defines a relationship between classes of objects that allows one object instance to cause another to perform an action on its behalf...
than classes in different levels. The levels of this hierarchy are called layers
Layer (object-oriented design)
In object-oriented design, a layer is a group of classes that have the same set of link-time module dependencies to other modules. In other words, a layer is a group of reusable components that are reusable in similar circumstances...
or levels of abstraction.
Definitions of subclass
Conceptually, a superclass should be considered as a common part of its subclasses. This factoring of commonality is one mechanism for providing reuse
Code reuse
Code reuse, also called software reuse, is the use of existing software, or software knowledge, to build new software.-Overview:Ad hoc code reuse has been practiced from the earliest days of programming. Programmers have always reused sections of code, templates, functions, and procedures...
. Thus, extending a superclass by modifying the existing class is also likely to narrow its applicability in various situations. In object-oriented design, careful balance between applicability and functionality of superclasses should be considered. Subclassing is different from subtyping in that subtyping deals with common behaviour whereas subclassing is concerned with common structure.
There are two slightly different points of view as to whether subclasses of the same class are required to be disjoint. Sometimes, subclasses of a particular class are considered to be completely disjoint. That is, every instance of a class has exactly one most-derived class, which is a subclass of every class that the instance has. This view does not allow dynamic change of object's class, as objects are assumed to be created with a fixed most-derived class. The basis for not allowing changes to object's class is that the class is a compile-time type, which does not usually change at runtime, and polymorphism
Polymorphism in object-oriented programming
Subtype polymorphism, almost universally called just polymorphism in the context of object-oriented programming, is the ability to create a variable, a function, or an object that has more than one form. The word derives from the Greek "πολυμορφισμός" meaning "having multiple forms"...
is utilized for any dynamic change to the object's behavior, so this ability is not necessary. And design that does not need to perform changes to object's type will be more robust and easy-to-use from the point of view of the users of the class.
From another point of view, subclasses are not required to be disjoint. Then there is no concept of a most-derived class, and all types in the inheritance hierarchy that are types of the instance are considered to be equally types of the instance. This view is based on a dynamic classification of objects, such that an object may change its class at runtime. Then object's class is considered to be its current structure, but changes to it are allowed. The basis for allowing changes to object's class is a perceived inconvenience caused by replacing an instance with another instance of a different type, since this would require change of all references to the original instance to be changed to refer to the new instance. When changing the object's class, references to the existing instances do not need to be replaced with references to new instances when the class of the object changes. However, this ability is not readily available in all programming languages. This analysis depends on the proposition that dynamic changes to object structure are common. This may or may not be the case in practice.
Orthogonality of the class concept and inheritance
Although class-based languages are commonly assumed to support inheritance, inheritance is not an intrinsic aspect of the concept of classes. There are languages that support classes yet do not support inheritance. Examples are earlier versions of Visual Basic
Visual Basic
Visual Basic is the third-generation event-driven programming language and integrated development environment from Microsoft for its COM programming model...
. These languages, sometimes called "object-based languages", do not provide the structural benefits of statically type-checked interfaces for objects. This is because in object-based languages, it is possible to use and extend data structures and attach methods to them at run-time. This precludes the compiler or interpreter being able to check the type information specified in the source code as the type is built dynamically and not defined statically. Most of these languages allow for instance behaviour and complex operational polymorphism (see dynamic dispatch
Dynamic dispatch
In computer science, dynamic dispatch is the process of mapping a message to a specific sequence of code at runtime. This is done to support the cases where the appropriate method can't be determined at compile-time...
and polymorphism).
Within object-oriented analysis
In object-oriented analysis and in UML, an association between two classes is a type of a link between the corresponding objects. A (two-way) association between classes A and B describes a relationship between each object of class A and some objects of class B, and vice versa. Associations are often named with a verb, such as "subscribes-to".
An association role type describes the role type of an instance of a class when the instance participates in an association. An association role type is related to each end of the association. A role describes an instance of a class from the point of view of a situation in which the instance participates in the association. Role types are collections of role (instance)s grouped by their similar properties. For example, a "subscriber" role type describes the property common to instances of the class "Person" when they participate in a "subscribes-to" relationship with the class "Magazine". Also, a "Magazine" has the "subscribed magazine" role type when the subscribers subscribe-to it.
Association role multiplicity describes how many instances correspond to each instance of the other class(es) of the association. Common multiplicities are "0..1", "1..1", "1..*" and "0..*", where the "*" specifies any number of instances.
There are some special kinds of associations between classes.
Taxonomy of classes
There are many categories of classes depending on modifiers. Note that these categories do not necessarily categorize classes into distinct partitions.
Abstract
In a language that supports inheritance, an abstract class, or abstract base class (ABC), is a class that cannot be instantiated for it is either labelled as abstract or it simply specifies abstract methods (or virtual methods). Abstract classes specify virtual methods via signatures, that are to be implemented by direct or indirect descendents of the abstract class. Before a class derived, directly or indirectly, from an abstract class can be instantiated, all abstract methods of its parent classes must be implemented by some class in the derivation chain.
Most object oriented programming languages allow the programmer to specify, though not necessarily by a keyword, which classes are considered abstract and will not allow these to be instantiated. For example, in Java
Java (programming language)
Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...
C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell...
, an abstract class is a class having at least one abstract method given by the appropriate syntax in that language (a pure virtual function in C++ parlance).
A class consisting of only virtual methods is called a Pure Abstract Base Class (or Pure ABC) in C++ and is also known as an interface by users of the language. Other languages, notably Java and C#, support a variant of abstract classes called an interface
Interface (Java)
An interface in the Java programming language is an abstract type that is used to specify an interface that classes must implement. Interfaces are declared using the interface keyword, and may only contain method signature and constant declarations...
via a keyword in the language. Such a class can only contain abstract publicly accessible methods. Abstract classes defined as interfaces are a much more specific usage of the more general meaning of the term interface, even as used in computer science, and the concept of interfaces has seen much usage and popularity within the realm of languages that support object-orientation.
Concrete
One common (if not modern) definition of concrete class is a class that can be instantiated. (In "Advanced C++ Programming Styles and Idioms", James Coplien defines "concrete class" as a class that has implicitly generated or explicitly defined special member functions that enable a class object to behave as do built-in types. These "concrete classes", then, adhere to the "Orthodox Canonical Form". Many developers who first learned the term as such, way back then, still primarily use it that way). This contrasts with abstract classes as described above.
Local and inner
In some languages, classes can be declared in scopes
Scope (programming)
In computer programming, scope is an enclosing context where values and expressions are associated. Various programming languages have various types of scopes. The type of scope determines what kind of entities it can contain and how it affects them—or semantics...
other than the global scope. There are various types of such classes.
In object-oriented programming , an inner class or nested class is a class declared entirely within the body of another class or interface. It is distinguished from a subclass.-Overview:...
, which is a class defined within another class. Since it involves two classes, this can also be treated as another type of class association. The methods of an inner class can access static methods of the enclosing class(es). An inner class is typically not associated with instances of the enclosing class, i.e. an inner class is not instantiated along with its enclosing class. Depending on language, it may or may not be possible to refer to the class from outside the enclosing class. A related concept is inner types (a.k.a. inner data type, nested type), which is a generalization of the concept of inner classes. C++
C++
C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell...
is an example of a language that supports both inner classes and inner types (via typedef
Typedef
typedef is a keyword in the C and C++ programming languages. The purpose of typedef is to assign alternative names to existing types, most often those whose standard declaration is cumbersome, potentially confusing, or likely to vary from one implementation to another.Under C convention , types...
declarations).
Another type is a local class, which is a class defined within a procedure or function. This limits references to the class name to within the scope where the class is declared. Depending on the semantic rules of the language, there may be additional restrictions on local classes compared non-local ones. One common restriction is to disallow local class methods to access local variables of the enclosing function. For example, in C++, a local class may refer to static variables declared within its enclosing function, but may not access the function's automatic variables.
Metaclasses
Metaclasses are classes whose instances are classes. A metaclass describes a common structure of a collection of classes. A metaclass can implement a design pattern
Design pattern (computer science)
In software engineering, a design pattern is a general reusable solution to a commonly occurring problem within a given context in software design. A design pattern is not a finished design that can be transformed directly into code. It is a description or template for how to solve a problem that...
or describe a shorthand for particular kinds of classes. Metaclasses are often used to describe framework
Software framework
In computer programming, a software framework is an abstraction in which software providing generic functionality can be selectively changed by user code, thus providing application specific software...
Python is a general-purpose, high-level programming language whose design philosophy emphasizes code readability. Python claims to "[combine] remarkable power with very clear syntax", and its standard library is large and comprehensive...
Ruby is a dynamic, reflective, general-purpose object-oriented programming language that combines syntax inspired by Perl with Smalltalk-like features. Ruby originated in Japan during the mid-1990s and was first developed and designed by Yukihiro "Matz" Matsumoto...
Smalltalk is an object-oriented, dynamically typed, reflective programming language. Smalltalk was created as the language to underpin the "new world" of computing exemplified by "human–computer symbiosis." It was designed and created in part for educational use, more so for constructionist...
, a class is also an object; thus each class is an instance of the unique metaclass, which is built in the language. For example, in Objective-C
Objective-C
Objective-C is a reflective, object-oriented programming language that adds Smalltalk-style messaging to the C programming language.Today, it is used primarily on Apple's Mac OS X and iOS: two environments derived from the OpenStep standard, though not compliant with it...
, each object and class is an instance of NSObject. The Common Lisp Object System (CLOS) provides metaobject protocols
Metaobject
In computer science, a metaobject or meta-object is any entity that manipulates, creates, describes, or implements other objects. The object that the metaobject is about is called the base object...
(MOPs) to implement those classes and metaclasses.
Non-subclassable
Non-subclassable classes allow programmers to design classes and hierarchies of classes which at some level in the hierarchy, further derivation is prohibited. (A stand-alone class may be also designated as non-subclassable, preventing the formation of any hierarchy). Contrast this to abstract classes, which imply, encourage, and require derivation in order to be used at all. A non-subclassable class is implicitly concrete.
A non-subclassable class is created by declaring the class as sealed in C# or as final in Java and PHP. For example, Java's class is designated as final. Non-subclassable classes may allow a compiler (in compiled languages) to perform optimizations that are not available for subclassable classes.
Partial
In languages supporting the feature, a partial c is a class whose definition may be split into multiple pieces, within a single source-code
Source code
In computer science, source code is text written using the format and syntax of the programming language that it is being written in. Such a language is specially designed to facilitate the work of computer programmers, who specify the actions to be performed by a computer mostly by writing source...
file or across multiple files. The pieces are merged at compile-time, making compiler output the same as for a non-partial class.
The primary motivation for introduction of partial classes is to facilitate the implementation of code generators, such as visual designers. It is otherwise a challenge or compromise to develop code generators that can manage the generated code when it is interleaved within developer-written code. Using partial classes, a code generator can process a separate file or course-grained partial class within a file, and is thus alleviated from intricately interjecting generated code via extensive parsing, increasing compiler efficiency and eliminating the potential risk of corrupting developer code. In a simple implementation of partial classes, the compiler can perform a phase of precompilation where it "unifies" all the parts of a partial class. Then, compilation can proceed as usual.
Other benefits and effects of the partial class feature include:
Enables separation of a class's interface and implementation code in a unique way.
Eases navigation through large classes within a editor
Source code editor
A source code editor is a text editor program designed specifically for editing source code of computer programs by programmers. It may be a standalone application or it may be built into an integrated development environment ....
In computer science, separation of concerns is the process of separating a computer program into distinct features that overlap in functionality as little as possible. A concern is any piece of interest or focus in a program. Typically, concerns are synonymous with features or behaviors...
In computing, aspect-oriented programming is a programming paradigm which aims to increase modularity by allowing the separation of cross-cutting concerns...
but without using any extra tools.
Enables multiple developers to work on a single class concurrently without the need to merge individual code into one file at a later time.
Smalltalk is an object-oriented, dynamically typed, reflective programming language. Smalltalk was created as the language to underpin the "new world" of computing exemplified by "human–computer symbiosis." It was designed and created in part for educational use, more so for constructionist...
under the name of Class Extensions for considerable time. With the arrival of the .NET framework 2
.NET Framework
The .NET Framework is a software framework that runs primarily on Microsoft Windows. It includes a large library and supports several programming languages which allows language interoperability...
Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...
introduced partial classes, supported in both C# 2.0 and Visual Basic 2005
Visual Basic .NET
Visual Basic .NET , is an object-oriented computer programming language that can be viewed as an evolution of the classic Visual Basic , which is implemented on the .NET Framework...
Visual Basic .NET , is an object-oriented computer programming language that can be viewed as an evolution of the classic Visual Basic , which is implemented on the .NET Framework...
, shows how parts of the same class are defined in two different files.
file1.vb:
Partial Class MyClass
Private _name As String
End Class
file2.vb:
Partial Class MyClass
Public Readonly Property Name As String
Get
Return _name
End Get
End Property
End Class
When compiled, the result is the same as if the two files were written as one, like this:
Class MyClass
Private _name As String
Public Readonly Property Name As String
Get
Return _name
End Get
End Property
End Class
Uninstantiable
Uninstantiable classes allow programmers to group together per-class fields and methods (and perhaps other kinds of members depending on the given programming language) that are accessible at runtime without an instance of the class. Indeed, instantiation is prohibited for this kind of class.
For example, in C#, a class marked "static" can not be instantiated, can only have static members (fields, methods, other), may not have instance constructors, and is sealed.
Unnamed
An unnamed class or anonymous class, in languages that support such a concept, is a class which is not bound to a name or identifier upon definition. (Analogous to named vs. unnamed functions
Anonymous function
In programming language theory, an anonymous function is a function defined, and possibly called, without being bound to an identifier. Anonymous functions are convenient to pass as an argument to a higher-order function and are ubiquitous in languages with first-class functions such as Haskell...
)).
Benefits
Computer programs usually model aspects of some real or abstract world (the Domain
Domain-driven design
Domain-driven design is an approach to developing software for complex needs by deeply connecting the implementation to an evolving model of the core business concepts...
). Because each class models a concept, classes provide a more natural way to create such models. Each class in the model represents a noun in the domain, and the methods of the class represent verbs that may apply to that noun (Verbs can also be modeled as classes, see Command Pattern
Command pattern
In object-oriented programming, the command pattern is a design pattern in which an object is used to represent and encapsulate all the information needed to call a method at a later time...
). For example in a typical business system, various aspects of the business are modeled, using such classes as Customer, Product, Worker, Invoice, Job, etc. An Invoice may have methods like Create, Print or Send, a Job may be Performed or Canceled, etc. Once the system can model aspects of the business accurately, it can provide users of the system with useful information about those aspects. Classes allow a clear correspondence (mapping
Map (mathematics)
In most of mathematics and in some related technical fields, the term mapping, usually shortened to map, is either a synonym for function, or denotes a particular kind of function which is important in that branch, or denotes something conceptually similar to a function.In graph theory, a map is a...
) between the model and the domain, making it easier to design, build, modify and understand these models. Classes provide some control over the often challenging complexity of such models.
Classes can accelerate development by reducing redundant program code, testing
Software testing
Software testing is an investigation conducted to provide stakeholders with information about the quality of the product or service under test. Software testing can also provide an objective, independent view of the software to allow the business to appreciate and understand the risks of software...
A software bug is the common term used to describe an error, flaw, mistake, failure, or fault in a computer program or system that produces an incorrect or unexpected result, or causes it to behave in unintended ways. Most bugs arise from mistakes and errors made by people in either a program's...
Debugging is a methodical process of finding and reducing the number of bugs, or defects, in a computer program or a piece of electronic hardware, thus making it behave as expected. Debugging tends to be harder when various subsystems are tightly coupled, as changes in one may cause bugs to emerge...
. If a class has been thoroughly tested and is known to be a 'solid work', it is usually true that using or extending the well-tested class will reduce the number of bugs - as compared to the use of freshly developed or ad hoc code - in the final output. In addition, efficient class reuse means that many bugs need to be fixed in only one place when problems are discovered.
Another reason for using classes is to simplify the relationships of interrelated data. Rather than writing code to repeatedly call a graphical user interface
Graphical user interface
In computing, a graphical user interface is a type of user interface that allows users to interact with electronic devices with images rather than text commands. GUIs can be used in computers, hand-held devices such as MP3 players, portable media players or gaming devices, household appliances and...
In computing, a window is a visual area containing some kind of user interface. It usually has a rectangular shape that can overlap with the area of other windows...
drawing subroutine on the terminal screen (as would be typical for structured programming
Structured programming
Structured programming is a programming paradigm aimed on improving the clarity, quality, and development time of a computer program by making extensive use of subroutines, block structures and for and while loops - in contrast to using simple tests and jumps such as the goto statement which could...
), it is more intuitive. With classes, GUI items that are similar to windows (such as dialog boxes) can simply inherit most of their functionality and data structures from the window class
Window class
In computer programming a window class is a structure fundamental to the Microsoft Windows operating systems and its Application Programming Interface ....
. The programmer then need only add code to the dialog class that is unique to its operation. Indeed, GUIs are a very common and useful application of classes, and GUI programming is generally much easier with a good class framework.
Run-time representation
As a data type, a class is usually considered as a compile-time construct. A language may also support prototype
Prototype
A prototype is an early sample or model built to test a concept or process or to act as a thing to be replicated or learned from.The word prototype derives from the Greek πρωτότυπον , "primitive form", neutral of πρωτότυπος , "original, primitive", from πρῶτος , "first" and τύπος ,...
The factory method pattern is an object-oriented design pattern to implement the concept of factories. Like other creational patterns, it deals with the problem of creating objects without specifying the exact class of object that will be created.The creation of an object often requires complex...
In computer science, a metaobject or meta-object is any entity that manipulates, creates, describes, or implements other objects. The object that the metaobject is about is called the base object...
s that represent run-time information about classes, or even represent metadata that provides access to reflection
Reflection (computer science)
In computer science, reflection is the process by which a computer program can observe and modify its own structure and behavior at runtime....
facilities and ability to manipulate data structure formats at run-time. Many languages distinguish this kind of run-time type information
Run-time type information
In programming, RTTI refers to a C++ system that makes information about an object's data type available at runtime. Run-time type information can apply to simple data types, such as integers and characters, or to generic objects...
about classes from a class on the basis that the information is not needed at run-time. Some dynamic languages do not make strict distinctions between run-time and compile-time constructs, and therefore may not distinguish between metaobjects and classes.
In computer science, a metaobject or meta-object is any entity that manipulates, creates, describes, or implements other objects. The object that the metaobject is about is called the base object...
representing the class Person, then instances of class Person can be created by using the facilities of the Human metaobject
Metaobject
In computer science, a metaobject or meta-object is any entity that manipulates, creates, describes, or implements other objects. The object that the metaobject is about is called the base object...
In software engineering, a class diagram in the Unified Modeling Language is a type of static structure diagram that describes the structure of a system by showing the system's classes, their attributes, operations , and the relationships among the classes.- Overview :The class diagram is the main...
Class-based programming, or more commonly class-orientation, refers to the style of object-oriented programming in which inheritance is achieved by defining classes of objects, as opposed to the objects themselves .The most popular and developed model of OOP is a class-based model, as opposed to an...
Prototype-based programming is a style of object-oriented programming in which classes are not present, and behavior reuse is performed via a process of cloning existing objects that serve as prototypes. This model can also be known as classless, prototype-oriented or instance-based programming...
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A class contains data