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Concept
Object composition
Summary
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. For example, a sequence differs from a set because (among other things) the order of the composed items matters for the former but not the latter. Data structures such as arrays, linked lists, hash tables, and many others can be used to implement either of them. Perhaps confusingly, some of the same terms are used for both data structures and composites. For example, "binary tree" can refer to either: as a data structure it is a means of accessing a linear sequence of items, and the actual positions of items in the tree are irrelevant (the tree can be internally rearranged however one likes, without changing its meaning). However, as an object composition, the positions are relevant, and changing them would change the meaning (as for example in cladograms).
Object-oriented programming is based on 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.
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