Object (computer science)

Object (computer science)
Name	Object
Paradigm	Object-oriented programming
Influenced by	Simula, Smalltalk, C++
Influenced	Java (programming language), C Sharp (programming language), Python (programming language), Ruby (programming language)

Object (computer science). In computer science, an object is a fundamental concept within object-oriented programming (OOP) that bundles data and the procedures that operate on that data into a single unit. It is an instance of a class (computer programming), which serves as a blueprint defining the object's structure and behavior. Objects interact by sending messages to each other, enabling modular and reusable software design, a paradigm pioneered by languages like Simula and Smalltalk.

Definition and characteristics

An object is characterized by its state, behavior, and identity, encapsulating data within fields and code within methods. The state is represented by the values of the object's attributes, which are stored in member variables, while behavior is defined by the functions or procedures, known as methods, that can access and modify that state. Identity ensures that each object is a distinct entity, even if its state is identical to another, a concept crucial in systems like the Java Virtual Machine. Core principles such as encapsulation bind the data and methods together, controlling access through mechanisms like access modifiers and information hiding.

Object-oriented programming

The concept of the object is the cornerstone of object-oriented programming, a paradigm formalized through the development of Simula by Kristen Nygaard and Ole-Johan Dahl at the Norwegian Computing Center. This approach was expanded by Alan Kay and his team at Xerox PARC, who created Smalltalk, establishing key concepts like message passing and dynamic dispatch. Major programming languages that adopted this model include C++, designed by Bjarne Stroustrup, Objective-C developed at Stepstone, and Java from Sun Microsystems. The paradigm promotes software design through modeling real-world entities, facilitating features like inheritance and polymorphism.

Object composition and structure

Internally, an object's structure is defined by its class, which specifies the object type and the layout of its instance variables in memory, often within a heap or stack. Objects can be composed of other objects, a relationship known as object composition, which is a fundamental alternative to inheritance for code reuse. The memory footprint of an object includes its data members and a reference to a virtual method table (vtable) in languages like C++ that support dynamic binding. Associations between objects, such as aggregation and association, are key to building complex systems in frameworks like the .NET Framework and Spring Framework.

Object lifecycle

The lifecycle of an object encompasses its creation, usage, and destruction, managed differently across programming environments. Objects are typically instantiated via a constructor, a special method defined in a class, as seen in languages like Python's `__init__`. During its lifetime, an object's state may be altered through method calls and garbage collection mechanisms, such as those in the Java Virtual Machine or the Common Language Runtime, reclaim memory. Finalization before destruction can be handled by a destructor, as in C++, or a finalizer method, though management varies between systems like Objective-C's reference counting and Automatic Reference Counting.

Implementation in programming languages

Different programming languages implement objects with varying syntax and semantics, influencing their use in software development. In C++, objects are compatible with C (programming language) structures but include features like virtual functions and multiple inheritance, while Java enforces a single-inheritance model using the `extends` keyword and interfaces. Prototype-based languages like JavaScript, developed at Netscape, use prototype-based programming where objects inherit directly from other objects without classes. Languages such as Ruby, created by Yukihiro Matsumoto, treat everything as an object, and Swift from Apple Inc. uses structures and classes with value and reference semantics.

Design patterns and principles

Effective object design is guided by established design patterns and principles that address common software engineering challenges. The Gang of Four's seminal work, *Design Patterns: Elements of Reusable Object-Oriented Software*, catalogues patterns like Singleton pattern, Factory method pattern, and Observer pattern, which standardize object interactions. Principles such as SOLID, coined by Robert C. Martin, and GRASP provide guidelines for assigning responsibilities to objects, promoting maintainable systems. These concepts are applied in major frameworks and architectures, including the Model–view–controller pattern used in Ruby on Rails and the Apache Struts framework.

Category:Object-oriented programming Category:Programming constructs