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   • UML aggregation vs association

     Top Answer

     Answered May 23, 2017 · 32 votes

     Rumbaugh's statement is the most telling and Uncle Bob's good advice. As I've said elsewhere, Aggregation is semantically so weak as to offer nothing practically beneficial. It only has one valid corner case (acyclicity of recursive relationships) however few people know and understand that. So you end up having to point out in comments anyway.

     I just don't use it. And have never felt any loss. Stick with simple binary associations and focus on what really matters - getting the cardinality and naming right. You'll get far more from that than trying to decide the undecidable association vs. aggregation.

     hth.

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     Answered Jan 25, 2022 · 31 votes

     Maybe this can help you, but i don't think you will find the perfect explanation:

     The difference is one of implication. Aggregation denotes whole/part relationships whereas associations do not. However, there is not likely to be much difference in the way that the two relationships are implemented. That is, it would be very difficult to look at the code and determine whether a particular relationship ought to be aggregation or association. For this reason, it is pretty safe to ignore the aggregation relationship altogether. [Robert C. Martin | UML]

     And an example for each situation:

     a) Association is a relationship where all object have their own lifecycle and there is no owner. Let’s take an example of Teacher and Student. Multiple students can associate with a single teacher and single student can associate with multiple teachers, but there is no ownership between the objects and both have their own lifecycle. Both can create and delete independently.

     b) Aggregation is a specialized form of Association where all object have their own lifecycle but there is ownership and child object can not belong to another parent object. Let’s take an example of Department and teacher. A single teacher can not belong to multiple departments, but if we delete the department, the teacher object will not be destroyed. We can think about “has-a” relationship.[Maesh | GeeksWithBlogs]

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     Answered May 31, 2013 · 3 votes

     I tend to use Aggregation to show a relation that is the same as a Composition with one big distinction: the containing class is NOT responsible for the life-cycle of the contained object. Typically, a (non-null) pointer or reference to the object-to-be-contained is passed to the containing class's constructor. The containing object, for the duration of its life-cycle, depends upon the contained object existing. The containing object cannot do its job (fully) without the contained object. This is my interpretation of the "Part/Whole" relationship implied by Aggregation.

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   • UML Diagram Help (Aggregation/Composition)

     Top Answer

     Answered Apr 03, 2016 · 1 votes

     What is composition and aggregation ?

     The composition and aggregation represents a whole/part relationship (UML 2.5, section 11.5.3.1):

     A binary Association may represent a composite aggregation (i.e., a whole/part relationship).

     So if you use a diamond, you should first ask yourself if it's really a whole/part relationship, before thinking how objects are created or deleted.

     Then composition have additional constraints over a shared aggregation. In a composition relationship (UML 2.5, section 9.5.3):

     (...) the composite object has responsibility for the existence and storage of the composed objects. Composite aggregation is a strong form of aggregation that requires a part object be included in at most one composite object at a time. If a composite object is deleted, all of its part instances that are objects are deleted with it.

     Analysis of your specific diagramm

     According to your diagram:

     • The players exists only within a game (i.e. temporary identification not accounts existing across several games). The composition could make sense, as players can be seen as parts of the game.
     • The hand exist only in relation to a player. That makes sense. But is it really a composition relationship ? Is the hand a part of a player ? Is the player composed of hands ? Wouldn't a player have several hands sequentially but not in the same time ? I really have my doubt about a composition here; I'd represent this with a normal 1 player to many hands association.
     • The game aggregates several decks. I don't know your game but I'd expect one deck. If several decks are used, and the decks only exists within a game (similarly to the players), I'd rather see a composition instead of an aggregation. Alternatively you could mean not the deck, but the deck together with its state. In this case, I'd opt for a one to many association and not a composition (the deck+state would not be a component of your game, but define the state of the game).
     • A deck is the aggregation of cards that exist independently of the deck. This troubles me a lot, as my world experience has always shown that a card is part of a deck. If I find an isolated card somewhere I always look for it's deck. I'd therefore rather expect a composition between cards and deck.
     • Finally a hand is the aggregation of several cards, which seems to make sense. Note that this is not incompatible with a composition between the deck and the card.

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   • UML definition - Generalization, aggregation and abstract classes

     Top Answer

     Answered Oct 26, 2018 · 1 votes

     You understand Generalization.

     Associations may be Aggregations or Compositions (or neither). This is a good example that Tom Pender used to use. Suppose you have a Car class. That Car class "has" a steering wheel, seats, two axles, four wheels, and so on. If you're creating that Car in a manufacturer context, the relationship between all of those would be Composition: the lifetime of all the car's parts (at your manufacturer) is tied to the lifetime of the car. From the standpoint of the manufacturer, the lifetime of the car and all its parts ends when you deliver it to a dealer.

     Now, let's suppose you're the owner of a junkyard. In this case, a car still "has" all those parts, but they have a lifetime of their own: you can sell parts off of the car, and even make them part of some other car if you need to. The relationship between the car and those parts is Aggregation, because the lifetime of the parts isn't tied to the lifetime of the car itself.

     So, you can see that the same car could actually be modeled in two different ways depending on context.

     As for abstract classes: an abstract class is simply a class that defines methods and/or properties while requiring that they can only be implemented in subclasses. It's easiest to explain why with interfaces. Interfaces are abstract classes, with the added proviso that NONE of the methods or properties can be implemented directly in the class (you might say it's a definition of how to define a subclass). Here's where they come in useful.

     Suppose I want to define an Animal class. What do animals do? Well, they move. They bite. (They do other things too, of course, but let's stick to Move and Bite.) If I create an Animal interface, I'm saying "here's what an animal does. If you want to be an animal, you have to also do these things. However, I'm not going to tell you how to do them." So, I create an iAnimal interface, with two methods, Move and Bite. As an interface, I don't provide any sort of implementation, just empty procedures.

     Now, suppose I have two animals, a Flea and a TRex. Clearly, Fleas and TRexes don't have anything in common in the way that they move or the way that they bite, but they both do both. I'll have Flea and TRex inherit or "implement" the interface, providing implementations of Move and Bite appropriate for the type of animal.

     The advantage of this is that clients of my flea and trex don't have to know which one they are dealing with. They can create an iAnimal, inject an instance of Flea or TRex as appropriate, and safely call iAnimal's Move and/or Bite methods without having to worry about whether they're supported or not. (This is what we mean by "polymorphism").

     So, interface implementation is a form of Generalization (more correctly Specialization, going the other way), as you have probably already surmised.

     From here, an "abstract class" is simply any class with any method or attribute defined that has to be inherited to be implemented. Therefore, as I have said, interfaces are abstract classes. However, in general usage, an abstract class is taken to mean one that is not also an interface, i. e. one that has some concrete methods or attributes. Most programming languages adhere to this definition.

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   • UML - association or aggregation (simple code snippets)

     Top Answer

     Answered May 23, 2017 · 5 votes

     This question has been, and will be, asked many times in many different variants, because many people, including many high-profile developers, are confused about the meaning of these terms, which have been defined in the UML. Since the question has been asked many times, it has also been answered many times. See, e.g. this answer. I'll try to summarize the UML definitions.

     An association between two classes is not established via a method parameter, but rather via reference properties (class attributes), the range/type of which are the associated classes. If the type of a method parameter is a class, this does not establish an association, but a dependency relationship.

     It's essential to understand the logical concept of associations first, before looking at how they are coded. An association between object types classifies relationships between objects of those types. For instance, the association Committee-has-ClubMember-as-chair, which is visualized as a connection line in the class diagram shown below, may classify the relationships FinanceCommittee-has-PeterMiller-as-chair, RecruitmentCommittee-has-SusanSmith-as-chair and AdvisoryCommittee-has-SarahAnderson-as-chair, where the objects PeterMiller, SusanSmith and SarahAnderson are of type ClubMember, and the objects FinanceCommittee, RecruitmentCommittee and AdvisoryCommittee are of type Committee.

     An association is always encoded by means of reference properties, the range/type of which is the associated class. For instance, like so

     class Committee { ClubMember chair; String name;}-

     In the UML, aggregation and composition are defined as special forms of associations with the intended meaning of classifying part-whole-relationships. In the case of aggregation, as opposed to composition, the parts of a whole can be shared with other wholes. This is illustrated in the following example of an aggregation, where a course can belong to many degree programs.

     The defining characteristic of a composition is to have exclusive (or non-shareable) parts. A composition may come with a life-cycle dependency between the whole and its parts implying that when a whole is destroyed, all of its parts are destroyed with it. However, this only applies to some cases of composition, and not to others, and it is therefore not a defining characteristic. An example of a composition where the parts (components) can be detached from the whole (composite) and therefore survive its destruction, is the following:

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   • UML class diagram Association vs ( Aggregation | Composition )-Diamonds

     Top Answer

     Answered Jun 20, 2020 · 18 votes

     I would use the Association for interfaces, because I can't instantiate them. Like they do it here for example. Or for static classes, same reason.

     And the diamonds I use only for objects I can instantiate. Like normal classes.

     That's not really how they work. The three forms (Association, Aggregation and Composition) define different properties about a relationship. All three are normally used between classes although can relate Interfaces too. Association and Composition are the two easiest:

     • Association (no diamond) is the most general form, allowing cardinality and navigability to be defined at both ends.
     • Composition (filled diamond) is a whole-part relationship where the 'whole' (end with the black diamond) 'contains' the part. It imposes two key restrictions: There can only be 1 container (i.e. cardinality at whole end is exactly 1);It imposes a lifecycle responsibility for parts on the whole. So the container is responsible for creating and deleting parts. A part cannot continue to exist if its container is deleted.

     Aggregation (unfilled diamond) sits somewhere in the middle. It's a bit like composition - except it doesn't mandate the properties described above. I don't personally use it. The semantics are too unclear for it to be worthwhile.

     And there is a third manner, the dashed lined <> arrow, but I don't have a glue when to use this one.

     I think you mean the dependency relationship. It's a weaker form of association. As an example, take the following class definition

     class Foo {- def bar(Baz: aParam) { ... }}-

     In this case type Foo has a dependency on type Baz from its use in the bar() method signature. However there's no association between them (can't sensibly discuss e.g. cardinality of relationship between an instance of Foo and an instance of Baz).

     From a practical perspective I'd say:

     • you can use straight Associations for 80%+ of the relationships you're likely to want to model
     • Composition probably accounts for most of the remaining scenarios
     • Dependency can be useful in some circumstances
     • You can get by quite happily without ever using Aggregation.

     hth.

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2. People also ask

   What is aggregation in UML?

   • Aggregation is a special case of association when an object ‘has-a’ another object, which you can have an aggregation between them. The direction between them helps you to specify which object contains the other object. Aggregation is also known as a “Has-a” relationship. It is a subtype of an association relationship in UML. What is Composition?

   Difference Between Aggregation and Composition in UML - Guru99

   www.guru99.com/uml-aggregation-vs-composition.html

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   Is composite aggregation a standard UML relationship?

   • It is not a standard UML Relationship, but it is still used in various applications. Composite aggregation is a subtype of aggregation relation with characteristics as: It is a two-way association between the objects. It is a whole/part relationship. If a composite is deleted, all other parts associated with it are deleted.

   UML Association Vs. Aggregation Vs. Composition [EXAMPLE] - Guru99

   www.guru99.com/association-aggregation-composition-difference.html

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   What does aggregation mean?

   • Aggregation is a specialized form of association between two or more objects in which each object has its own life cycle but there exists an ownership as well. Aggregation is a typical whole/part or parent/child relationship but it may or may not denote physical containment.

   Association, aggregation, and composition in OOP explained

   www.infoworld.com/article/3029325/exploring-association-aggregation-and-composition-in-oop.html

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   What is output aggregation?

   • Output Aggregation (If you remove "whole", “Part” can exist – “ No Ownership”) An aggregation is a specific type of composition where no ownership between the complex object and the subobjects is implied. When an aggregate is destroyed, the subobjects are not destroyed.

   oop - What is the difference between association, aggregation and

   stackoverflow.com/questions/885937/what-is-the-difference-between-association-aggregation-and-composition

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3. www.visual-paradigm.com › guide › uml-unifiedUML Association vs Aggregation vs Composition - Visual Paradigm

   www.visual-paradigm.com › guide › uml-unified

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   Aggregation and Composition are subsets of association meaning they are specific cases of association. In both aggregation and composition object of one class "owns" object of another class. But there is a subtle difference: Aggregation implies a relationship where the child can exist independently of the parent.

4. stackoverflow.com › questions › 885937What is the difference between association, aggregation and ...

   stackoverflow.com › questions › 885937

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   May 20, 2009 · Aggregation. Aggregation is a kind of association.But with specific features.Aggregation is the relationship in one larger "whole" class contains one or more smaller "parts" classes.Conversely, a smaller "part" class is a part of "whole" larger class. for example: club has members

5. www.guru99.com › uml-aggregation-vs-compositionDifference Between Aggregation and Composition in UML - Guru99

   www.guru99.com › uml-aggregation-vs-composition

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   • Key Difference Between Aggregation and Composition
   • What Is Aggregation?
   • What Is Composition?
   • Example of Composition
   • Example of Aggregation

   Aggregation is one type of association between two objects describing the “have a” relationship, while Composition is a specific type of Aggregation which implies ownership.

   Aggregation is indicated using a straight line with an empty arrowhead at one end. On the other hand, the composition is indicated using a straight line with a filled arrowhead at any one of the ends.

   In an aggregation relationship, objects that are associated with each other can remain in the scope of a system without each other. But in a composition relationship, objects that are associated wi...

   In Aggregation, linked objects are not dependent upon the other object, whereas in composition, objects are highly dependent upon each other.

   See full list on guru99.com

   Aggregation is a special case of association when an object ‘has-a’ another object, which you can have an aggregation between them. The direction between them helps you to specify which object contains the other object. Aggregation is also known as a “Has-a” relationship. It is a subtype of an association relationship in UML.

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   The composition is a special case of Aggregation that helps you to specify a whole-part relationship between the composition class and a subordinate (part) class.

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   The folder could contain many files, while each File has exactly one Folder parent. If a folder is deleted, all contained files are removed as well. In a composite aggregation, an object may be a part of only one composite at a time.

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   For example, your car consists of wheels, engine, gearbox, steering, and the main body, etc. It is an assembly, and the other parts are its constituents. Here, car to the wheel is one Aggregation, car to the engine is another aggregation, car to gearbox another, and so on. This type of pairing helps you to define the multiplicity of the constituent...

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7. gleek.io › blog › aggregation-vs-compositionUML Essentials: Aggregation vs Composition Explained

   gleek.io › blog › aggregation-vs-composition

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   May 16, 2024 · Understanding Aggregation in UML. Aggregation is a specialized form of association in the UML that represents a "has-a" or "whole/part" relationship between the aggregate (the whole) and its parts, signifying a weak ownership between them.

8. www.uml-diagrams.org › aggregationUML shared aggregation is relationship between a property and ...

   www.uml-diagrams.org › aggregation

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   Shared aggregation ( aggregation) is a binary association between a property and one or more composite objects which group together a set of instances. It is a "weak" form of aggregation when part instance is independent of the composite. Shared aggregation has the following characteristics: it is binary association,

9. www.guru99.com › association-aggregationUML Association Vs. Aggregation Vs. Composition [EXAMPLE]

   www.guru99.com › association-aggregation

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   Mar 16, 2024 · UML Aggregation. Association Vs. Aggregation Vs. Composition. Summary. UML Association. It is a structural relationship that represents objects can be connected or associated with another object inside the system. Following constraints can be applied to the UML Association relationship.

10. www.infoworld.com › article › 3029325Association, aggregation, and composition in OOP explained

    www.infoworld.com › article › 3029325

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    Nov 19, 2018 · Aggregation is a specialized form of association between two or more objects in which each object has its own life cycle but there exists an ownership as well....