In database design , object-oriented programming and design , has-a ( has_a or has a ) is a composition relationship where one object (often called the constituted object, or part/constituent/member object) "belongs to" (is part or member of ) another object (called the composite type), and behaves according to the rules of ownership. In simple words, has-a relationship in an object is called a member field of an object. Multiple has-a relationships will combine to form a possessive hierarchy.
30-605: [REDACTED] Look up has-a or hasa in Wiktionary, the free dictionary. Hasa may refer to: Hasa (Korean military) , a rank in the Korean military Hasa, Al Madinah , Saudi Arabia Hearing and Speech Agency of Baltimore Hasa of Eshtemoa (3rd–4th century CE), Jewish scholar Hasa oasis , a historical region now in Saudi Arabia Has-a ,
60-398: 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
90-406: 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
120-429: A university owns various departments (e.g., chemistry ), and each department has a number of professors. If the university closes, the departments will no longer exist, but the professors in those departments will continue to exist. Therefore, a university can be seen as a composition of departments, whereas departments have an aggregation of professors. In addition, a professor could work in more than one department, but
150-557: A user-defined type (UDT) . For details, see the aggregation section below. 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: 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
180-609: 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 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
210-462: A department could not be part of more than one university. Composition is usually implemented such that an object contains another object. For example, in C++ : In aggregation, the object may only contain a reference or pointer to the object (and not have lifetime responsibility for it). Sometimes aggregation is referred to as composition when the distinction between ordinary composition and aggregation
240-510: A record a struct or structure; object-oriented languages such as Java , Smalltalk , and C++ often keep their records hidden inside objects ( class instances); languages in the ML family simply call them records. COBOL was the first widespread programming language to support records directly; ALGOL 68 got it from COBOL and Pascal got it, more or less indirectly, from ALGOL 68. Common Lisp provides structures and classes (the latter via
270-592: A relationship between objects in object-oriented programming Hasa, Jordan , a village in Jordan Wadi al-Hasa , also called Wadi Hasa, a wadi in Jordan Highly accelerated stress audit , a test method to find electronic product defects See also [ edit ] All pages with titles containing Hasa Al-Hasa (disambiguation) Topics referred to by the same term [REDACTED] This disambiguation page lists articles associated with
300-445: 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:
330-489: 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
SECTION 10
#1732765569198360-489: Is also known as composition. As you can see from the Class Diagram on the right a car "has-a" carburetor , or a car is "composed of" a carburetor. When the diamond is coloured black it signifies composition , i.e. the object on the side closest to the diamond is made up of or contains the other object. While the white diamond signifies aggregation , which means that the object closest to the diamond can have or possess
390-516: Is an object-oriented design based on composite types, that combines recursive composition and containment to implement complex part-whole hierarchies. This is an example of composition in C . In this example, the primitive (noncomposite) types int , enum {job_seeking, professional, non_professional, retired, student } and the composite array type char[] are combined to form the composite structure Person . Each Person structure then "has an" age, name, and an employment type. C calls
420-477: Is deleted, all of its part instances that are objects are deleted with it ". Thus in UML, composition has a more narrow meaning than the usual object composition. The graphical notation represents: Aggregation differs from ordinary composition in that it does not imply ownership. In composition, when the owning object is destroyed, so are the contained objects. In aggregation, this is not necessarily true. For example,
450-404: Is to be contrasted with an is-a ( is_a or is a ) relationship which constitutes a taxonomic hierarchy ( subtyping ). The decision whether the most logical relationship for an object and its subordinate is not always clearly has-a or is-a . Confusion over such decisions have necessitated the creation of these metalinguistic terms. A good example of the has-a relationship is containers in
480-425: Is unimportant. The above code would transform into the following UML Class diagram: [REDACTED] In Microsoft's Component Object Model , aggregation means that an object exports, as if it were their owner, one or several interfaces of another object it owns. Formally, this is more similar to composition or encapsulation than aggregation. However, instead of implementing the exported interfaces by calling
510-500: The C++ STL . To summarize the relations, we have In databases has-a relationships are usually represented in an Entity–relationship model . As you can see by the diagram on the right an account can have multiple characters. This shows that account has a "has-a" relationship with character. In object-oriented programming this relationship can be represented with a Unified Modeling Language Class diagram . This has-a relationship
540-695: The Common Lisp Object System ). Arrays were the only composite data type in Algol 60 . For example, a linked list might be declared as: For ALGOL 68 only the type name appears to the left of the equality, and most notably the construction is made – and can be read – from left to right without regard to priorities. Fortran 77 has arrays, but lacked any formal record/structure definitions. Typically compound structures were built up using EQUIVALENCE or COMMON statements: Ada 95 brought OOP concepts through tagged types (the equivalent of
570-409: The aggregate, and may outlive the aggregate object. The state of the component object still forms part of the aggregate object. The relationship between the composite and its parts is a strong “has-a” relationship: The composite object has sole " responsibility for the existence and storage of the composed objects ", the composed object can be part of at most one composite, and " If a composite object
600-453: The composed class. Objects can be composed recursively, and their type is then called recursive type . Examples includes various kinds of trees , DAGs , and graphs . Each node in a tree may be a branch or leaf; in other words, each node is a tree at the same time when it belongs to another tree. In UML, recursive composition is depicted with an association, aggregation or composition of a class with itself. The composite design pattern
630-472: 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,
SECTION 20
#1732765569198660-506: 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,
690-446: The interfaces of the owned object, the interfaces of the owned object themselves are exported. The owned object is responsible for assuring that methods of those interfaces inherited from IUnknown actually invoke the corresponding methods of the owner. This is to guarantee that the reference count of the owner is correct and all interfaces of the owner are accessible through the exported interface, while no other (private) interfaces of
720-477: 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
750-501: The other hand, if the Car object contains a set of Tire objects, these Tire objects may wear out and get replaced several times. Or if the Car becomes unusable, some Tires may be salvaged and assigned to another Car. At any rate, the Tire objects have different lifetimes than the Car object; therefore the relationship is one of aggregation . If one were to make a C++ software Class to implement
780-405: The other object. Another way to distinguish between composition and aggregation in modeling the real world, is to consider the relative lifetime of the contained object. For example, if a Car object contains a Chassis object, a Chassis will most likely not be replaced during the lifetime of the Car. It will have the same lifetime as the car itself; so the relationship is one of composition . On
810-406: The owned object are accessible. Composition that is used to store several instances of the composited data type is referred to as containment. Examples of such containers are arrays , associative arrays , binary trees , and linked lists . In UML , containment is depicted with a multiplicity of 0..* or 1..*, indicating that the composite object is composed of an unknown number of instances of
840-471: The relationships described above, the Car object would contain a complete Chassis object in a data member. This Chassis object would be instantiated in the constructor of the Car class (or defined as the data type of the data member and its properties assigned in the constructor.) And since it would be a wholly contained data member of the Car class, the Chassis object would no longer exist if a Car class object
870-479: The title Hasa . If an internal link led you here, you may wish to change the link to point directly to the intended article. Retrieved from " https://en.wikipedia.org/w/index.php?title=Hasa&oldid=1226493447 " Categories : Disambiguation pages Place name disambiguation pages Hidden categories: Short description is different from Wikidata All article disambiguation pages All disambiguation pages has-a "Has-a"
900-547: Was to be deleted. On the other hand, the Car class data members that point to Tire objects would most likely be C++ pointers. Tire objects could be instantiated and deleted externally, or even assigned to data members of a different Car object. Tire objects would have an independent lifetime separate from when the Car object was deleted. 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,
#197802