Object-oriented programming is a programming paradigm that uses "Object_" and their interactions to design applications and computer programs.... , a class is a programming language
Programming language
A programming language is a machine-readable artificial language designed to express computations that can be performed by a machine, particularly a computer.... 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.... to create objects. This blueprint includes attribute
Attribute (computing)
In computing, an attribute is a specification that defines a property of an object, element, or file. An attribute of an object usually consists of a name and a value; of an element, a type or class name; of a file, a name and extension.... s and method
Method (computer science)
In object-oriented programming, a method is a subroutine that is exclusively associated either with a class or with an object . Like a procedure in procedural programming languages, a method usually consists of a sequence of statement to perform an action, a set of input parameter to customize those actions, and possibly an output value... s that the created objects all share.
Usually, a class represents a person, place, or thing - it is an abstraction of a concept within a computer program. Fundamentally, it encapsulates the state and behavior of that which it conceptually represents. It encapsulates state through data placeholders called member variables; it encapsulates behavior through reusable code called methods.
In computer programming, cohesion is a measure of how strongly-related and focused the various responsibilities of a software module are. Cohesion is an level of measurement#Ordinal measurement type of measurement and is usually expressed as "high cohesion" or "low cohesion" when being discussed.... package that consists of a particular kind of metadata
Metadata
Metadata is "data about other data", of any sort in any media. An item of metadata may describe an individual datum, or content item, or a collection of data including multiple content items and hierarchical levels, for example a database schema.... .
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Object-oriented programming is a programming paradigm that uses "Object_" and their interactions to design applications and computer programs.... , a class is a programming language
Programming language
A programming language is a machine-readable artificial language designed to express computations that can be performed by a machine, particularly a computer.... 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.... to create objects. This blueprint includes attribute
Attribute (computing)
In computing, an attribute is a specification that defines a property of an object, element, or file. An attribute of an object usually consists of a name and a value; of an element, a type or class name; of a file, a name and extension.... s and method
Method (computer science)
In object-oriented programming, a method is a subroutine that is exclusively associated either with a class or with an object . Like a procedure in procedural programming languages, a method usually consists of a sequence of statement to perform an action, a set of input parameter to customize those actions, and possibly an output value... s that the created objects all share.
Usually, a class represents a person, place, or thing - it is an abstraction of a concept within a computer program. Fundamentally, it encapsulates the state and behavior of that which it conceptually represents. It encapsulates state through data placeholders called member variables; it encapsulates behavior through reusable code called methods.
In computer programming, cohesion is a measure of how strongly-related and focused the various responsibilities of a software module are. Cohesion is an level of measurement#Ordinal measurement type of measurement and is usually expressed as "high cohesion" or "low cohesion" when being discussed.... package that consists of a particular kind of metadata
Metadata
Metadata is "data about other data", of any sort in any media. An item of metadata may describe an individual datum, or content item, or a collection of data including multiple content items and hierarchical levels, for example a database schema.... . It describes the rules by which objects behave; these objects are referred to as instances of that class. A class has both an interface and a structure. The interface describes how the class and its instances can be interacted with via methods, while the structure describes how the data
Data (computing)
In computer science, data is anything in a form suitable for use with a computer. Data is often distinguished from computer programs. A program is a set of instruction that detail a task for the computer to perform.... is partitioned into attributes within an instance. A class may also have a representation (metaobject
Metaobject
In computer science, a metaobject or meta-object is any entity that manipulates, creates, describes, or implements other object s. The object that the metaobject is about is called the base object.... ) at runtime, which provides runtime support for manipulating the class-related metadata. In object-oriented design, a class is the most specific type
Data type
A data type in programming languages is an attribute of a data which tells the computer something about the kind of data it is. This involves setting constraints on the datum, such as what values it can take and what operations may be performed upon it.... of an object in relation to a specific layer
Layer (object-oriented design)
In object-oriented design, a layer is a group of class es that have the same set of link-time Module dependency to other modules. In other words, a layer is a group of reusable software componentrys that are reusability in similar circumstances.... .
Programming languages that support classes all subtly differ in their support for various class-related features. Most support various forms of class inheritance
Inheritance (computer science)
In object-oriented programming, inheritance is a way to form new class es using classes that have already been defined. The inheritance concept was invented in 1967 for Simula.... . Many languages also support features providing encapsulation
Information hiding
Information hiding in computer science is the principle of hiding of design decisions in a computer program that are most likely to change, thus protecting other parts of the program from change if the design decision is changed.... , such as access specifiers.
Reasons for using classes
Classes, when used properly, can accelerate development by reducing redundant code entry, testing
Software testing
Software Testing is an empirical investigation conducted to provide stakeholders with information about the quality of the product or service under test , with respect to the context in which it is intended to operate.... and bug
Software bug
A software bug is an error, flaw, mistake, failure, or fault in a computer program that prevents it from behaving as intended . Most bugs arise from mistakes and errors made by people in either a program's source code or its software architecture, and a few are caused by compilers producing incorrect code.... fixing
Debugging
Debugging is a methodical process of finding and reducing the number of computer bugs, or defects, in a computer program or a piece of electronic hardware thus making it behave as expected.... . If a class has been thoroughly tested and is known to be a solid work, it is typically the case 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 GUI
Graphical user interface
A graphical user interface is a type of user interface which allows people to human-computer interaction such as computers; hand-held devices such as MP3 Players, Portable Media Players or Gaming devices; household appliances and office equipment.... window
Window (computing)
In computing, a window is a visual area, usually rectangular in shape, containing some kind of user interface, displaying the output of and allowing input for one of a number of simultaneously running computer processes.... drawing subroutine on the terminal screen (as would be typical for structured programming
Structured programming
Structured programming can be seen as a subset or subdiscipline of procedural programming, one of the major programming paradigms. It is most famous for removing or reducing reliance on the GOTO Statement .... ), it is more intuitive to represent the window as an object and tell it to draw itself as necessary. 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.
Instantiation
A class is used to create new instances (objects) by instantiating the class.
Instances of a class share the same set of attributes, yet may differ in what those attributes contain. For example, a class "Person" would describe the attributes common to all instances of the Person class. Each person is generally alike, but varies in such attributes as "height" and "weight". The class would list types of such attributes and also define the actions which a person can perform: "run", "jump", "sleep", "walk", etc. One of the benefits of programming with classes is that all instances of a particular class will follow the defined behavior of the class they instantiate.
In most languages, the structures as defined by the class determine how the memory used by its instances will be laid out. This technique is known as the cookie-cutter model. The alternative to the cookie-cutter model is the model of Python
Python (programming language)
Python is a general-purpose high-level programming language. Its design philosophy emphasizes code readability. Python's core syntax and semantics are Minimalism , while the standard library is large and comprehensive.... , wherein objects are structured as associative key-value containers. In such models, objects that are instances of the same class could contain different instance variables, as state can be dynamically added to the object. This may resemble prototype-based languages in some ways, but it is not equivalent.
Interfaces and methods
Note: the term "interface" here isn't referring to a Java interface
Interface (Java)
An interface in the Java is an abstract type that is used to specify an interface that class must implement. Interfaces are declared using the interface Java keywords, and may only contain Method signatures and constant declarations .... , although the two are closely related.
Objects define their interaction with the outside world through the methods that they expose. A method, or instance method, is a subroutine
Subroutine
In computer science, a subroutine or subprogram is a portion of computer code within a larger computer program, which 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). Methods that manipulate the data of the object and perform tasks are sometimes described as behavior
Behavior
Behavior or behaviour refers to the action s or reactions of an object or organism, usually in Relational theory to the environment. Behavior can be conscious or Unconscious mind, overt or covert, and voluntary or involuntary.... .
Methods form the object's interface with the outside world; 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.
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, and in the Dylan language, class definitions do not even define interfaces.
Interfaces may also be defined to include a set of auxiliary functions called static methods or class methods. Static methods, like instance methods, are exclusively associated with the class. They differ from instance methods in that they do not work with instances of the class; that is, static methods neither require an instance of the class nor can they access the data of such an instance. For 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 sets.
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 client-server
Client-server
The client-server software architecture model distinguishes client systems from server systems, which communicate over a computer network. A client-server application is a distributed system comprising both client and server software.... (or layered) design where servers do not depend in any way on the clients. 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.
Structure of a class
Along with having an interface, a class contains a description of structure of data
Data (computing)
In computer science, data is anything in a form suitable for use with a computer. Data is often distinguished from computer programs. A program is a set of instruction that detail a task for the computer to perform.... stored in the instances of the class. The data is partitioned into attributes (or properties, fields, data members). Going back to the television set example, the myriad 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).
In computer science and automata theory, a state is a unique configuration of information in a program or machine. It is a concept that occasionally extends into some forms of systems programming such as Lexical analysiss and parsers.... of an instance's data is stored in some resource, such as memory or a file. The storage is assumed to be located in a specific location, such that it is possible to access the instance through reference
Reference (computer science)
In computer science, a reference is an object containing information about how to locate and access the particular data item, as opposed to containing the data itself.... s to the identity
Identity (object-oriented programming)
An identity in object-oriented programming, object-oriented design and object-oriented analysis describes the property of object s that distinguishes them from other objects.... of the instances. However, the actual storage location associated with an instance may change with time. In such situations, the identity of the object does not change. The state is encapsulated and every access to the state occurs through methods of the class.
In computer programming, a class invariant is an invariant used to constrain object s of a class . Method s of the class should preserve the invariant.... s that are preserved by every method in the class. An invariant is a constraint on the state of an object that should be satisfied by every object of the class. The main purpose of the invariants is to establish what objects belong to the class. An invariant is what distinguishes data type
Data type
A data type in programming languages is an attribute of a data which tells the computer something about the kind of data it is. This involves setting constraints on the datum, such as what values it can take and what operations may be performed upon it.... s and classes from each other; that is, a class does not allow use of all possible values for the state of the object, and instead allows only those values that are well-defined by the semantics of the intended use of the data type. The set of supported (public) methods often implicitly establishes an invariant. Some programming languages support specification of invariants 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.
Some languages allow an implementation of a class to specify constructor
Constructor (computer science)
In object-oriented programming, a constructor in a class is a special block of statements called when an object lifetime#Creating objects, either when it is declared or dynamically constructed on the heap-based memory allocation through the keyword ?new?.... (or initializer) and destructor
Destructor (computer science)
In object-oriented programming, a destructor is a method which is automatically invoked when the object is destroyed. Its main purpose is to clean up and to free the Resource which were acquired by the object along its life cycle and unlink it from other objects or resources invalidating any references in the process.... (or finalizer) methods that specify how instances of the class are created and destroyed, respectively. A constructor that takes arguments can be used to create an instance from passed-in data. The main purpose of a constructor is to establish the invariant of the class, failing if the invariant isn't valid. The main purpose of a destructor is to destroy the identity of the instance, invalidating any references in the process. Constructors and destructors are often used to reserve and release, respectively, resources associated with the object. In some languages, a destructor can return a value which can then be used to obtain a public representation (transfer encoding) of an instance of a class and simultaneously destroy the copy of the instance stored in current thread's memory.
A class may also contain static attributes or class attributes, which contain data that are specific to the class yet are common to all instances of the class. If the class itself is treated as an instance of a hypothetical metaclass
Metaclass
In object-oriented programming Computer programming, a metaclass is a Class whose instances are classes. Just as an ordinary class defines the behavior of certain objects, a metaclass defines the behavior of certain classes and their instances.... , static attributes and static methods would be instance attributes and instance methods of that metaclass.
Run-time representation of classes
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 original type, form, or instance of something serving as a typical example, basis, or standard for other things of the same category.... or factory
Factory method pattern
The factory method pattern is an object-oriented design pattern .Like other creational patterns, it deals with the problem of creating object without specifying the exact class of object that will be created.... metaobject
Metaobject
In computer science, a metaobject or meta-object is any entity that manipulates, creates, describes, or implements other object s. 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 behaviour. The programming paradigm driven by reflection is called reflective programming.... 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 keeps information about an object's data type in memory 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 object s. 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 object s. The object that the metaobject is about is called the base object.... .
Information hiding and encapsulation
Many languages support the concept of information hiding and encapsulation, typically with access specifiers for class members. Access specifiers specify constraints on who can access which class members. Some access specifiers may also control how classes inherit such constraints. Their primary purpose is to separate the interface of a class with 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.
A common usage of access specifiers is to separate the internal data structures of a class from its interface; that is, the internal data structures are private. Public accessor methods can be used to inspect or alter such private data. The various object-oriented programming languages enforce this to various degrees. For example, the 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 .... language does not allow client code to access the private data of a class at all, whereas in languages like Objective-C
Objective-C
Objective-C is a Reflection , Object-oriented programming programming language which adds Smalltalk-style message passing to C .Today it is used primarily on Mac OS X, iPhone OS, and GNUstep, three environments based on the OpenStep standard, and is the primary language used for the NEXTSTEP, OpenStep#OPENSTEP, and Cocoa application framew... or Perl
Perl
In computer programming, Perl is a high-level programming language, List of programming languages by category, Interpreter , dynamic programming language.... client code can do whatever it wants. In C++
C++
C++ is a general-purpose programming language. It is regarded as a middle-level language, as it comprises a combination of both high-level programming language and low-level programming language language features.... 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.
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. Object-oriented design uses the access specifiers in conjunction with careful design of public method implementations to enforce class invariants. Access specifiers are intended to protect against accidental use of members by clients, but are not suitable for run-time protection of object's data.
Ruby is a dynamic programming language, reflection , general purpose object-oriented programming language that combines syntax inspired by Perl with Smalltalk-like features.... , support instance-private and instance-protected access specifiers in lieu of (or in addition to) class-private and class-protected, respectively. They differ in that they restrict access based on the instance itself, rather than the instance's class.
In addition, some languages, such as C++, support a mechanism where a function explicitly declared as friend of the class may access the members designated as private or protected.
Associations between classes
In object-oriented design 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.
Composition
Composition between class A and class B describes 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.... relationship where instances of class B 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.... than the lifetime of the corresponding instances of the enclosing class. Class B is said to be a part of class A. This is often implemented in programming languages by allocating the data storage of instances of class A to contain a representation of instances of class B.
Aggregation is a variation of composition that describes that instances of a class are part of instances of the other class, but the constraint on lifetime of the instances is not required. The implementation of aggregation is often via a pointer or reference to the contained instance. In both cases, method implementations of the enclosing class can invoke methods of the part class. A common example of aggregation is a list class. When a list's lifetime is over, it does not necessarily mean the lifetimes of the objects within the list are also over.
Inheritance
Another type of class association is inheritance, which involves subclasses and superclasses, also known respectively as child classes (or derived classes) and parent classes (or base classes). If [car] was a class, then [station wagon] and [mini-van] might be two subclasses. If [Button] is a subclass of [Control], then all buttons are controls. In other words, inheritance is an is-a
Is-a
In knowledge representation and object-oriented programming and Object-oriented design , is-a is a relationship where one class D is a subclass of another class B .... relationship between two classes. Subclasses usually consist of several kinds of modifications (customizations) to their respective superclasses: addition of new instance variables, addition of new methods and overriding of existing methods to support the new instance variables.
Conceptually, a superclass should be considered as a common part of its subclasses. This factoring of commonality is one mechanism for providing reuse
Reuse
Reuse is to use an item more than once. This includes conventional reuse where the item is used again for the same function, and new-life reuse where it is used for a new function.... . 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.
C++ is a general-purpose programming language. It is regarded as a middle-level language, as it comprises a combination of both high-level programming language and low-level programming language language features.... ) allow multiple inheritance
Multiple inheritance
Multiple inheritance refers to a feature of some object-oriented programming programming languages in which a class can inheritance behaviors and features from more than one superclass .... - they allow a child class to have more than one parent class. This technique has been criticized by some for its unnecessary complexity and being difficult to implement efficiently, though some projects have certainly benefited from its use. 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 .... , for example has no multiple inheritance, as its designers felt that it would add unnecessary complexity. Java instead allows inheriting from multiple pure abstract classes (called interfaces in Java).
Sub- and superclasses are considered to exist within a hierarchy
Hierarchy (object-oriented programming)
In computer science's object-oriented programming, the mapped relationships of class is known as a hierarchy. This can be visualized as an upside-down tree , the top of which is known as the root .... defined by the inheritance relationship. If multiple inheritance is allowed, this hierarchy is a directed acyclic graph
Directed acyclic graph
In computer science and mathematics, a directed acyclic graph, also called a DAG, is a with no ; that is, for any Vertex v, there is no nonempty directed path that starts and ends on v.... (or DAG for short), otherwise it is a tree
Tree (graph theory)
In mathematics, more specifically graph theory, a tree is a graph in which any two Vertex are connected by exactly one path . Alternatively, any connectedness graph with no Cycle is a tree.... . The hierarchy has classes as nodes and inheritance relationships as links. The levels of this hierarchy are called layers
Layer (object-oriented design)
In object-oriented design, a layer is a group of class es that have the same set of link-time Module dependency to other modules. In other words, a layer is a group of reusable software componentrys that are reusability in similar circumstances.... or levels of abstraction. 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 which allows one object instance to cause another to perform an action on its behalf.... than classes in different levels.
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
In simple terms, polymorphism is the ability of one type, A, to appear as and be used like another type, B. In strongly typed languages, this usually means that type A somehow derives from type B, or type A implements an interface that represents type B.... 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.
Languages without 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 programming language event-driven programming and integrated integrated development environment from Microsoft for its Component Object Model 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 from 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 passing to a specific sequence of code at runtime. This is done to support the cases where the appropriate method cannot be determined at compile-time .... and polymorphism).
Categories of classes
There are many categories of classes depending on modifiers. Note that these categories do not necessarily categorize classes into distinct partitions. For example, while it is impossible to have a class that is both abstract and concrete, a sealed class is implicitly a concrete class, and it may be possible to have an abstract partial class.
Concrete classes
A concrete class is a class that can be instantiated. This contrasts with abstract classes as described below.
Abstract classes
An abstract class, or abstract base class (ABC), is a class that cannot be instantiated. Such a class is only meaningful if the language supports inheritance. An abstract class is designed only as a parent class from which child classes may be derived. Abstract classes are often used to represent abstract
Abstraction (computer science)
In computer science, abstraction is a mechanism and practice to reduce and factor out details so that one can focus on a few concepts at a time.... concepts or entities. The incomplete features of the abstract class are then shared by a group of subclasses which add different variations of the missing pieces.
Abstract classes are superclasses which contain abstract methods and are defined such that concrete subclasses are to extend them by implementing the methods. The behaviors defined by such a class are "generic" and much of the class will be undefined and unimplemented. Before a class derived from an abstract class can become concrete, i.e. a class that can be instantiated, it must implement particular methods for all the abstract methods of its parent classes.
When specifying an abstract class, the programmer is referring to a class which has elements that are meant to be implemented by inheritance. The abstraction of the class methods to be implemented by the subclasses is meant to simplify software development
Software development
Software development is the set of activities that results in software products. Software development may include research, new development, modification, reuse, re-engineering, maintenance, or any other activities that result in software products.... . This also enables the programmer to focus on planning and design.
Most object oriented programming languages allow the programmer to specify 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 .... , the keyword abstract is used. In C++
C++
C++ is a general-purpose programming language. It is regarded as a middle-level language, as it comprises a combination of both high-level programming language and low-level programming language language features.... , an abstract class is a class having at least one abstract method (a pure virtual function in C++ parlance).
Some languages, notably Java and C#, additionally support a variant of abstract classes called an interface
Interface (Java)
An interface in the Java is an abstract type that is used to specify an interface that class must implement. Interfaces are declared using the interface Java keywords, and may only contain Method signatures and constant declarations .... . Such a class can only contain abstract publicly-accessible methods. In this way, they are closely related - but not equivalent - to the abstract concept of interfaces described in this article.
Sealed classes
Some languages also support sealed classes. A sealed class cannot be used as a base class. For this reason, it cannot also be an abstract class. Sealed classes are primarily used to prevent derivation. They add another level of strictness during compile-time, improve memory usage, and trigger certain optimizations that improve run-time efficiency.
Local and inner classes
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.... other than the global scope. There are various types of such classes.
In object-oriented programming, an inner class is a class declared entirely within the body of another class or interface.Inner classes became a feature of the Java starting with version 1.1.... or nested class, 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 general-purpose programming language. It is regarded as a middle-level language, as it comprises a combination of both high-level programming language and low-level programming language language features.... is an example of a language that supports both inner classes and inner types (via typedef 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.
Named vs. anonymous classes
In most languages, a class is bound to a name or identifier upon definition. However, some languages allow classes to be defined without names. Such a class is called an anonymous class (analogous to named vs. anonymous function
Anonymous function
In computing, an anonymous function is a function defined, and possibly called, without being bound to a name. In lambda calculus, all functions are anonymous.... s).
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 in software design. A design pattern is not a finished design that can be transformed directly into code .... or describe a shorthand for particular kinds of classes. Metaclasses are often used to describe framework
Framework
A framework is a basic conceptual structure used to solve or address complex issues. This very broad definition has allowed the term to be used as a buzzword, especially in a software context.... s.
Python is a general-purpose high-level programming language. Its design philosophy emphasizes code readability. Python's core syntax and semantics are Minimalism , while the standard library is large and comprehensive.... , Ruby
Ruby (programming language)
Ruby is a dynamic programming language, reflection , general purpose object-oriented programming language that combines syntax inspired by Perl with Smalltalk-like features.... , 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 .... , and Smalltalk
Smalltalk
Smalltalk is an Object-oriented programming, Type system, reflection computer programming 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 learning, at PARC by Al... , 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 Reflection , Object-oriented programming programming language which adds Smalltalk-style message passing to C .Today it is used primarily on Mac OS X, iPhone OS, and GNUstep, three environments based on the OpenStep standard, and is the primary language used for the NEXTSTEP, OpenStep#OPENSTEP, and Cocoa application framew... , each object and class is an instance of NSObject. The Common Lisp
Common Lisp
Common Lisp, commonly abbreviated CL, is a dialect of the Lisp programming language, published in American National Standards Institute standard document Information Technology - Programming Language - Common Lisp, formerly X3.226-1994 .... 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 object s. The object that the metaobject is about is called the base object.... (MOPs) to implement those classes and metaclasses.
Partial classes
Partial classes are classes that can be split over multiple definitions (typically over multiple files), making it easier to deal with large quantities of code. At compile
Compiler
A compiler is a computer program that transforms source code written in a programming language into another computer language . The most common reason for wanting to transform source code is to create an executable program.... time the partial classes are grouped together, thus logically make no difference to the output. An example of the use of partial classes may be the separation of user interface logic and processing logic. A primary benefit of partial classes is allowing different programmers to work on different parts of the same class at the same time. They also make automatically generated code easier to interpret, as it is separated from other code into a partial class.
Smalltalk is an Object-oriented programming, Type system, reflection computer programming 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 learning, at PARC by Al... under the name of Class Extensions for considerable time. With the arrival of the .NET framework 2
.NET Framework
The Microsoft .NET Framework is a software framework that is available with several Microsoft Windows operating systems. It includes a large Library of coded solutions to prevent common programming problems and a virtual machine that manages the execution of programs written specifically for the Software framework.... , Microsoft
Microsoft
Microsoft Corporation is a multinational corporation computer technology corporation that develops, manufactures, licenses, and supports a wide range of computer software products for computing devices.... introduced partial classes, supported in both C# 2.0 and Visual Basic 2005
Visual Basic .NET
Visual Basic , formerly called Visual Basic .NET , is an object-oriented programming computer language that can be viewed as an evolution of Microsoft Visual Basic implemented on the .NET Framework.... .
Non-class-based object-oriented programming
To the surprise of some familiar with the use of classes, classes are not the only way to approach object-oriented programming. Another common approach is prototype-based programming
Prototype-based programming
Prototype-based programming is a style of object-oriented programming in which class es are not present, and behavior reuse is performed via a process of cloning existing object s that serve as prototypes.... . Languages that support non-class-based programming are usually designed with the motive to address the problem of tight-coupling between implementations and interfaces due to the use of classes. For example, the Self
Self programming language
Self is an Object-oriented programming computer programming programming language based on the concept of Prototype-based programming. It was used mainly as an experimental test system for language design in the 1980s and 1990s.... language, a prototype-based language, was designed to show that the role of a class can be substituted by using an extant object which serves as a prototype to a new object, and the resulting language is as expressive as Smalltalk
Smalltalk
Smalltalk is an Object-oriented programming, Type system, reflection computer programming 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 learning, at PARC by Al... with more generality in creating objects.
C++ is a general-purpose programming language. It is regarded as a middle-level language, as it comprises a combination of both high-level programming language and low-level programming language language features.... class named "Hello". It has a private string attribute named "what", and a public method named "say".
Example 2
class MyAbstractClass
;
class MyConcreteClass : public MyAbstractClass
;
An object of class MyAbstractClass cannot be created because the function MyVirtualMethod has not been defined (the =0 is C++ syntax for a pure virtual function, a function that must be part of any derived concrete class but is not defined in the abstract base class. The MyConcreteClass class is a concrete class because its functions (in this case, only one function) have been declared and implemented.
C#
Example 1
using System;
public class Program
This example demonstrates a traditional "Hello world!" example in Microsoft's C# language. The Program class contains a single static method, Main, which calls System.Console.WriteLine to print text onto the console.
In computer science, porting is the process of adapting software so that an executable Computer program can be created for a computing environment that is different from the one for which it was originally designed .... of the above C++ example. A class called Hello is created with a constructor that
takes a string parameter. When the Say method is called, the instance of Hello will print "Hello !"
onto the console. Notice that the Main method (the entry point) is actually contained in a class itself.
ECMAScript
ECMAScript (and JavaScript) doesn't directly support classes. It is a prototype-based language. However, classes are often emulated in practice, as shown in the following example.
Example 1
// Constructor
function Hello(s)
var hello_world = new Hello("world");
hello_world.say;
This example is a port of the first C++ example. The print function used here is a placeholder for a standard text output function; ECMAScript (and JavaScript)'s standard library does not include an I/O
I/O
I/O may refer to:* Input/output, a system of communication for information processing systems* The input-output model, an economic model of flow prediction between sectors... API.
A "Hello World" program is a computer program that prints out "Hello world!" on a display device. It is used in many introductory tutorials for teaching a programming language.... .
Example 2
public class Example2 extends Example1
This example shows a class that has a defined constructor, one member data, an accessor method (getData) and a Modifier method (setData) for that member data. It extends the previous example's class. Note that in Java all classes automatically extend the class Object. This allows you to write generic code to deal with objects of any type.
// An Objective C class with class methods and instance methods.
PHP
Example 1
Example 2
Python
Example 1
class Hello:
def __init__(self, s):
self.what = s
def __repr__(self):
return "Hello(%s)" % self.what
def say(self):
print("Hello %s!" % self.what)
hello_world = Hello("world")
hello_world.say
A Python class Hello, which behaves the same as the C++ class Hello, above. Its constructor takes a single string argument, and it has a say method, which prints "Hello <argument>". Instances of the class also display intuitively, due to the __repr__ method.
Ruby
Example 1
class Hello
def hello
"Hello world!"
end
end
A Ruby class Hello, with one method hello. This method returns "Hello world!".
Visual Basic .NET
Example 1
Class Hello
Private what as String
Sub New( ByVal s as String )
what = s
End Sub
Sub Say
MessageBox.Show("Hello " & what )
End Sub
End Class
Sub Main
Dim h As New Hello( "Foobar" )
h.Say
End Sub
This example is a port of the C++ example above. It demonstrates how to make a class named Hello with a private property named what. It also demonstrates the proper use of a constructor, and has a public method named Say. It also demonstrates how to instantiate the class and call the Say method.
In computer science's object-oriented programming, the mapped relationships of class is known as a hierarchy. This can be visualized as an upside-down tree , the top of which is known as the root ....
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 class es, their attributes, and the Object-oriented programming between the classes.... (UML)
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 ....
