Object composition

In computer science, object composition and object aggregation are closely related ways to combine objects or data types into more complex ones. In conversation, the distinction between composition and aggregation is often ignored. Common kinds of compositions are objects used in object-oriented programming, tagged unions, sets, sequences, and various graph structures. Object compositions relate to, but are not the same as, data structures.

Object composition refers to the logical or conceptual structure of the information, not the implementation or physical data structure used to represent it

Object-oriented programming is based on using objects to encapsulate data and behavior. It uses two main techniques for assembling and composing functionality into more complex ones, sub-typing and object composition. Object composition is about combining objects within compound objects, and at the same time, ensuring the encapsulation of each object by using their well-defined interface without visibility of their internals. In this regard, object composition differs from data structures, which do not enforce encapsulation.

Object composition may also be about a group of multiple related objects, such as a set or a sequence of objects. Delegation may enrich composition by forwarding requests or calls made to the enclosing composite object to one of its internal components.

In class-based and typed programming languages, types can be divided into composite and non-composite types, and composition can be regarded as a relationship between types: an object of a composite type (e.g. car) "has" objects of other types (e.g. wheel). When a composite object contains several sub-objects of the same type, they may be assigned to particular roles, often distinguished by names or numbers. For example, a Point object might contain 3 numbers, each representing distance along a different axis, such as 'x', 'y', and 'z'. The study of part-whole relationships in general, is mereology.

Composition must be distinguished from subtyping, which is the process of adding detail to a general data type to create a more specific data type. For instance, cars may be a specific type of vehicle: car is a vehicle. Subtyping doesn't describe a relationship between different objects, but instead, says that objects of a type are simultaneously objects of another type. The study of such relationships is ontology.

In prototype-based programming languages such as JavaScript, objects can dynamically inherit the behaviors from a prototype object at the moment of their instantiation. Composition must be distinguished from prototyping: the newly instantiated object inherits the composition of its prototype, but it may itself be composed on its own.

Composite objects may be represented in storage by co-locating the composed objects, by co-locating references, or in many other ways. The items within a composite object may be referred to as attributes, fields, members, properties, or other names, and the resulting composition as composite type, storage record, structure, tuple, or a user-defined type (UDT). For details, see the aggregation section below.

Object composition using UML properties to compose objects

In UML modeling, objects can be conceptually composed, independently of the implementation with a programming language. There are four ways of composing objects in UML: property, association, aggregation and composition:

• A property represents an attribute of the class.
• An association represents a semantic relationship between instances of the associated classes. The member-end of an association corresponds to a property of the associated class.
• An aggregation is a kind of association that models a part/whole relationship between an aggregate (whole) and a group of related components (parts).
• A composition, also called a composite aggregation, is a kind of aggregation that models a part/whole relationship between a composite (whole) and a group of exclusively owned parts.

The relationship between the aggregate and its components is a weak "has-a" relationship: The components may be part of several aggregates, may be accessed through other objects without going through the aggregate, and may outlive the aggregate object. The state of the component object still forms part of the aggregate object.