Object-Oriented Programming is a programming paradigm that organises software around objects or object-like abstractions that encapsulate state and provide operations through which that state and related behaviour can be accessed. Objects typically have identity, state and behaviour, while classes in class-based languages define structures or behaviour shared by instances. Core object-oriented concepts commonly include encapsulation, abstraction, inheritance and polymorphism, although different languages implement these concepts in substantially different ways. Object-Oriented Programming does not require classes in every case because prototype-based systems can create objects and establish relationships between them without conventional class definitions. The paradigm is used extensively in application software, graphical interfaces, business systems, libraries, games and frameworks.
Encapsulation means that the internal representation or implementation details of an object can be restricted behind an interface. Access modifiers, modules, private fields, properties and controlled methods are mechanisms languages may use to enforce or encourage encapsulation. The purpose is not merely to hide data but to establish a boundary through which an object's invariants and behaviour can be controlled.
For example, an object representing an account might expose operations for valid state changes rather than allowing arbitrary external modification of its internal balance representation. Encapsulation can reduce coupling because users of an abstraction depend on its public interface rather than its internal implementation. However, poor encapsulation can result when objects expose excessive mutable state or when interfaces become tightly coupled to implementation details.
Classes and objects are related but distinct concepts. A class in a class-based language can define fields, methods, inheritance relationships and other metadata used to create instances, while an object is a particular runtime entity with its own state and identity. Some languages distinguish classes as compile-time or runtime structures, whereas others treat classes themselves as objects. Constructors can establish initial object state, and destructors or finalisation mechanisms may manage cleanup depending on the language. Prototype-based languages instead allow objects to inherit or delegate behaviour through prototype relationships. Object-oriented terminology therefore varies across languages, and concepts such as class, instance, object, type and prototype should not be assumed to have identical meanings in every language.
Inheritance allows a type or class to derive behaviour and structure from another type or class. Single inheritance gives a class one direct superclass, while multiple inheritance permits several direct bases in languages that support it. Inheritance can facilitate code reuse and represent substitutability relationships, but it can also create tight coupling and complex hierarchies. Composition provides an alternative in which an object contains or collaborates with other objects rather than deriving implementation from them.
The principle often summarised as preferring composition over inheritance is not an absolute language rule but reflects situations where composition can make relationships more flexible and reduce dependency on rigid class hierarchies. Modern object-oriented design frequently uses interfaces, delegation and composition alongside or instead of inheritance.
Polymorphism allows a common interface or operation to work with values of different concrete types. Subtype polymorphism permits a value of a derived type to be used where a compatible base type or interface is expected. Parametric polymorphism, often implemented through generics or templates, allows algorithms to operate over types without committing to one concrete type. Ad hoc polymorphism can be provided through overloading or type-class-like mechanisms. Dynamic dispatch selects an appropriate method implementation at runtime according to the object's actual type or method-resolution rules, while static dispatch can determine the target earlier. These mechanisms are technically distinct even though they are often grouped under the broad term polymorphism.
Object-oriented systems require mechanisms for object identity, method invocation, memory management and type relationships. A method call may involve direct static resolution, virtual dispatch, interface dispatch or dynamic lookup. Virtual tables and related dispatch structures are common implementation techniques in compiled class-based languages, although the exact mechanism is implementation-dependent. Objects may be allocated on a heap or represented through other storage strategies, and garbage-collected languages can automatically reclaim objects that are no longer reachable. The runtime representation of an object can include fields, type metadata, dispatch information or object headers. These implementation details explain how high-level object-oriented constructs ultimately map to lower-level operations and runtime services.
Object-Oriented Programming is frequently confused with using classes or objects in any program, but the presence of a class syntax does not automatically make a design meaningfully object-oriented. Effective object-oriented design usually concerns responsibilities, interfaces, collaborations, state ownership and abstraction boundaries. A class hierarchy can be technically object-oriented yet poorly designed if it creates unnecessary coupling or violates substitutability assumptions. Conversely, some languages support object-based or prototype-oriented designs without traditional classes. Object-oriented programming also does not inherently imply encapsulation, inheritance or polymorphism all at once; these mechanisms can be used independently or in different combinations. The paradigm is best understood through its treatment of objects, responsibilities and interactions rather than through a checklist of keywords.
Object-Oriented Programming became particularly influential through languages and environments such as Simula, Smalltalk, C++, Objective-C, Java, C# and many others. Simula introduced influential class and object concepts in the context of simulation, while Smalltalk developed a particularly comprehensive object-oriented environment in which objects and message passing were central. C++ combined object-oriented features with systems-programming capabilities, while Java and C# popularised class-based object-oriented development within managed runtime ecosystems. Contemporary languages often integrate object-oriented mechanisms with functional, procedural and generic techniques. The practical significance of Object-Oriented Programming therefore lies less in adherence to a single canonical model than in its extensive use of encapsulation, abstraction, polymorphism and object collaboration to manage software complexity.
