I'm trying to make a class diagram for an Online Auction System and I'm having this problem. Bids belong to both the Auction and the Buyer (Correct me if i'm wrong). So can I say that User is composed of bid AND auction is composed of Bid or is this against the rules of UML? I'm confused
In general
The term composition is ambiguous and this explains your confusion:
In OOP, object composition means to use an object in another one.
In UML, composition is a special kind of association that represents a part-whole relationship, with an exclusive ownership of the parts by the whole.
So in the UML sense, it is not possible to have an object that is part of two different compositions, because the ownership would no longer be exclusive. But you could use the object in several aggregations, which are a whole-part relations allowing a shared ownership.
In the OOP sense, there is no problem of having the same object used in (or using) several compositions. The object composition corresponds to a navigable UML association.
In your practical case
The situation is straight forward: A Bid has one Buyer, a Buyer may have several Bids, and an Auction has several Bids. You can model this with simple associations:
You could alternatively use an aggregation here, since one could argue that there is a whole-part relation between an Auction and the corresponding Bids (personally, I wouldn't see it like this):
You should however not see an aggregation on the other side, because there is no real whole-part relation between a Buyer and a Bid: a Buyer is not "made of several Bids".
Additional remarks
You could use also an association class here. But it's not required. And the semantic would be different: it would mean that there is a Bid association between a Buyer and an Auction:
Technically speaking, you'd still have three classes.
But the focus is different: the Bid is accessory to the Buyer and the Auction and cannot exist on its own (e.g. if the buyer disappears)
Have you learned about association classes in UML? They represent an object that is created from the relationship between two other classes, exactly what you are trying to map. There are plenty of contents detailing Association Classes (e.g. Correct use of an association class)
That way you shouldn't get confused reading the diagrams (which you were doing correctly btw), it would be clear to you that exists the entity Bid, which exists only associated to both Buyer and Auction.
Related
I have one base-class named Product. Classes "Drink" and "Pizza" are subclasses that inherit from the "Product" class.
I also have a class named "Ingredient" Which is a part of the Pizza class, so the Pizza class should have an instance (a list) of the Ingredient class.
My question is:
Since the Ingredient has the same properties like all the other "Products", can it also inherit from the "Product" class while working together with the "Pizza" subclass?
Yes, they can!
Yes, classes that inherit from the same super-class can perfectly work together. There is no incompatibility. There is even a design pattern that uses both relations on the same classes (the composite pattern).
Remark: The wording "Working together" is somewhat ambiguous and could mean different things. But your class-diagram is precise enough and shows that you mean a possible association.
But how?
Your diagram shows a navigable association between Pizza and Ingredient (the open arrow). This means that the implementation shall make sure that a pizza can easily find its ingredients.
In your narrative you mention a list. I understand that an Ingredient can exist without any Pizza, but a Pizza can have several Ingredient. It is important to specify this multiplicity on the diagram, by indicating * on the Ingredient side of the association.
The relationship in the opposite direction has still some mysteries:
Navigability is unspecified: we don't know if an Ingredient shall be able to easily find the related pizza or if this is not relevant. You may specify this further with an arrow-head in the opposite direction (navigable) or with an X across the line (non-navigable). You have the right to leave this unspecified and decide later. The navigability has an impact on the way the classes can work together: it means that Pizza can "work with" ingredient (e.g. use it as parameter in an operation, invoke an ingredient operation directly, etc...). But the absence of navigability in the opposite direction would mean that Ingredient could not by itself initiate a collaboration with its pizza.
More important, the multiplicity: we do not know if an ingredient is associated only with one pizza (0..1), or if it can be shared between several (0..*). This is very important to know because the implementation would be very different (in the latter case you'd have a many-to-many association).
Overgeneralization ?
Inheritance is tempting when one discovers OOP. However, inheritance has a lot of implications, constraints and consequences. Therefore, use it wisely.
A useful advice is to start considering that A inherits from B, only if A is a more specialized B, or conversely, that B is a more generalization of A. It is dangerous to use inheritance just because some properties or operations share the same name. Names can by the way be misleading.
In your case, I understand a Product as something that the company sells at a given price: Drink, Pizza, maybe Antipasti or Pasta. The price indicated, is the price requested from the customer. I therefore wonder if an Ingredient is sold to the customer. AN ingredient may have a price, but it is a purchase price. A purchase price is different from a sales price: imagine that a restaurant would subcontract some very special pizza: the sales price would be different from the purchasing price.
Of course, if your Product is something more general, in the business meaning, and if it can have a sales price and a purchase price, and is not necessarily on the menu, then no problem, go-ahead. But be aware that such generalized products are much more complex to manage (e.g. here a well-known ERP example with more than 20 different screens to manage all the aspects of a product).
Other remarks
A very common advice is to prefer composition over inheritance. This rule of thumb aims to remind us to think twice before we use inheritance. Let's be clear, in case of doubt: this does not mean that inheritance and composition are incompatible.
It's pretty strange to see all those answers about composition/aggregation/association.
Who/What is the source of those notions?
geeksforgeeks
wikipedia?!?!
stackexchange
and finally my lovely stackoverflow (at least I glad that answers were not marked as verified)
What is the difference between association, aggregation and composition?
What is the difference between aggregation, composition and dependency?
There is a great book "Design Patterns"
GoF
It describes two most common techniques for reusing functionality in object-oriented systems:
1) class inheritance (is-a)
2) object composition (has-a)
"Object composition is an alternative to class inheritance. Here, new functionality is obtained by assembling or composing objects to get more complex functionality."
"Composition" is a very descriptive term to express relationship between objects unlike "Association".
Why all those sources above use term "Composition" in the wrong way?!
Let's go further.
Objects could be composed in two ways:
1) Aggregation
2) Acquaintance
"Consider the distinction between object aggregation and acquaintance and how differently they manifest themselves at compile- and run-times. Aggregation implies that one object owns or is responsible for another object. Generally we speak of an object having or being part of another object. Aggregation implies that an aggregate object and its owner have identical lifetimes."
aggregate object and its owner have identical lifetimes!!!
"Acquaintance implies that an object merely knows of another object. Sometimes acquaintance is called "association" or the "using" relationship. Acquainted objects may request operations of each other, but they aren't responsible for each other. Acquaintance is a weaker relationship than aggregation and suggests much looser coupling between objects."
"It's easy to confuse aggregation and acquaintance, because they are often implemented in the same way. In Smalltalk, all variables are references to other objects. There's no distinction in the programming language between aggregation and acquaintance. In C++, aggregation can be implemented by defining member variables that are real instances, but it's more common to define them as pointers or references to instances. Acquaintance is implemented with pointers and references as well."
Guys, please, help me to figure out what's going on here...
Yes, there is lot of confusion around for these two terms Composition and Aggregation [There is More to add Shared and non-shared Aggregation]. After going through a lot of confusion and getting biased towards UML resources, I have formed my view as follows [need not be taken as final or accurate].
A simplest and loosely coupled relationship I take is Association [it can be unidirectional or bidirectional] where an Object has a reference of other object but both live independently. Association can be Qualified Association if its connected by specific Identity [In OO, identity is an important part of every entity object] e.g. accountNumber for Account of Customer.
Aggregation is collection (of other object types and can be assembled for restricted purpose) maintained by an Object. Still both the objects live independently. e.g. Athletics team. Same student can be part of many such teams like Cycling team [aggregates] and so on. Deleting Athletics team makes no harm to each student entry in college records [they still exist]. Such a relationship can be maintained as Collection of students on Team side or reference of Athletics team for each student being part of team. It depends on more frequently required navigability for application.
Composition is more tight relationship where container object completely holds the contained object and contained object does not have any meaning outside relationship with container object. I can see an example as relationship between Person and Address where for each person we keep separate/ fresh entry of address. For family members Address may have same logical equality, but never physical equality. Change of address for one member does not affect other members [simple test is - DB columns for Person record has extended columns for address as a part of person table.] another example is Single entry (row) of item purchase and Complete bill of items. where deleting bill makes each entry context-less.
If an object instantiates and contains another object completely [never allows outside world to obtain its ref by any means] I would take that as Composition. Techniques like Cloning objects at interaction points instead of passing same ref can be helpful here. In case of association or aggregation, we exchange same reference.
Containment being black-box reuse, is preferred over white-box kind of reuse (implemented using inheritance). Most of the GOF patterns suggest best combinations of Containment for reuse and inheritance for polymorphism. e.g. In case of Adapter pattern, Object Adapter is preferred over Class Adapter.
Implementing all these flavors in Java (Implementations will be language specific) has its own challenge and NOT very straight forward, especially composition. There is a point on learning curve, where one feels (at least I felt) Composition is same as inner class, inner class can help us implement it, but simply having inner class does not give any guarantee of composition.
The image shows the logistics of the Warehouse. Very very simplistic. What is its concept: There are documents: ReceivingWayBill, DispatchingWaybill, ReplacementOrder.
They interact with the main classes: Warehouse, Counterparty, Item.
And the Register class: ItemRemainsInWarehouse. It turns out, the document is confirmation of the operation, reception, sending, and so on. The Register simply stores information about the number of remaining goods.
If you miss a lot of problems of this scheme, such as: the lack of generalization, getters and setters and a heap of everything else.
Who can tell: the relationship between classes, and there is concrete aggregation everywhere, are placed correctly, or can we somehow consider the association in more detail?
It is so hard (maybe impossible) to correct your whole model with provided explanation. I give some improvements.
You should put Multiplicity of you relationships. They are so important. In some relationship, you have 1 (ReplacementOrder , Warehouse) and some of your relatioships are maybe * (Item , ReceivingWayBill)
You put Aggregation between your classes and we know that Aggregation is type of Association. You can put Associations too. You can find a lot of similar questions and answers that explain differences between Association and Aggregation (and Composition). see Question 1, Question 2 and Question 3. But I recommend this answer.
I think, there is NOT a very significant difference between Aggregation and Association. See my example in this question.
Robert C. Martin says (see here):
Association represents the ability of one instance to send a message to another instance.
Aggregation is the typical whole/part relationship. This is exactly the same as an association with the exception that instances
cannot have cyclic aggregation relationships (i.e. a part cannot
contain its whole).
Therefor: some of your relationships are exactly an Aggregation. (relationship between Item and other classes). Your Counterparty has not good API definition. Your other relationships is about using Warehouse class. I think (just guess) the other classes only use Warehouse class services (public methods). In this case, they can be Associations. Otherwise, if they need an instance of Warehouse as a part, they are Aggregations.
Aggregation is evil!
Read the UML specs about the two variants they introduced (p. 110):
none: Indicates that the Property has no aggregation semantics. [hear, hear!]
shared: Indicates that the Property has shared aggregation semantics. Precise semantics of shared aggregation varies by application area and modeler.
composite: Indicates that the Property is aggregated compositely, i.e., the composite object has responsibility for the existence and storage of the composed objects (see the definition of parts in 11.2.3).
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.
Now, that last sentence clearly indicates where you should use composite (!) aggregation: in security related appications. When you delete a person record in a database you need to also delete all related entities. That often used example with a car being composed of motor, tires, etc. does not really fit. The tires do not vanish when you "delete" the car. Simply because you can not delete it. Even worse is the use of a shared composite since it has no definition per definition (sic!).
So what should you do? Use multiplicities! That is what people usually want to show. There are 0..n, 1, etc. elements related to to the class at the other side. Eventually you name these by using roles to make it explicit.
If you consider DispatchingWaybill and ReceivingWaybill it looks like those are association classes. With the right multiplicities (1-* / *-1) you can leave it this way. (Edit: note the little dots at the association's ends which tell that the class at the opposite has an attribute named after the role.)
Alternatively attach either with a dashed line to an association between the classes where they are currently connected to.
Aggregation is defined as a special case of association. However, any association that is not implemented as a field (like having a relationship through method parameters) are being described as "use" relationship.
So, is it possible to have an association that is not aggregation or composition? If yes, I need a code example for such a case, please.
In fact, I'd say that most cases of associations in models are neither aggregations nor compositions (both are forms of part-whole relationship types). For instance, the association between the classes Publisherand Book for assigning the books published by a publisher to this publisher is neither an aggregation nor a composition because the books published by a publisher are not parts or components of this publisher.
For implementing this bidirectional association, we use the two mutually inverse reference properties Publisher::publishedBooks and Book::publisher, as shown in the following class rectangles:
Notice that the multi-valued reference property Publisher::publishedBooks is normally implemented by a list-valued property in Java.
I have explained how to use associations and reference properties in design models in my tutorial Managing Unidirectional Associations in a JavaScript Frontend Web App.
Shared aggregation (empty diamond) is not defined strictly in the UML standard. So, it is not easy to find a situation, where you can not use it.
But it is explicitely forbidden to use it for both sides of association. So, if you have relation many to many, you have to show it on class diagram as many to many association with none aggregation. Of course, you can show it as TWO shared association, but it is not our target, is it? :-)
As for code, if people visit many courses and courses are visited by many people, make a list "courses" for Person class and a list "visitors" for Course class.
Of course, on the other side, you always can use none instead of shared for any shared association, it is at your wish and up to rules of your place of work. But I don't know these rules, sorry :-). But surely, nobody will make you to use aggregation for 1 to 1 association.
I am reviewing my knowledge in object-oriented programming. Under the relationship between classes topic, I have encountered some relationships which are a bit ambiguous to me.
I know dependency "uses-a" and inheritance "is-a" but I'm a bit unfamiliar with Aggregation, Composition, Association and Direct Association; also, which of them is "has-a" relationship. Some use Aggregation interchangeably with Association.
What is Direct Association? Also, what is Composition? In UML diagrams, the arrows that represents them are different. I would be really thankful if you could clear these things out for me.
Please note that there are different interpretations of the "association" definitions. My views below are heavily based on what you would read in Oracle Certification books and study guides.
Temporary association
A usage inside a method, its signature or as a return value. It's not really a reference to a specific object.
Example: I park my Car in a Garage.
Composition association
A so-called "STRONG relationship": The instantiation of the linked object is often hard
coded inside the constructor of the object. It cannot be set from
outside the object. (Composition cannot be a many-to-many
relationship.)
Example: A House is composed of Stones.
Direct association
This is a "WEAK relationships". The objects can live independent and there are usually setters or other ways to inject the dependent objects.
Example: A Car can have Passengers.
Aggregation association
Very similar to a Direct association. It's also a "WEAK relationship" with independent objects. However here the associated objects are a crucial part of the containing object.
Example: A Car should have Tires.
Note: Both Direct associations and Aggregation associations are often generalized as "Associations". The difference is rather subtle.
The whole point of OOP is that your code replicates real world objects, making your code readable and maintainable.
1. Association
Association is: Class A uses Class B.
Example:
Employee uses Bus/train Services for transportation.
Computer uses keyboard as input device
And in In UML diagram Association is denoted by a normal arrow head.
2. Aggregation
Class A contains Class B, or Class A has an instance of Class B.
An aggregation is used when life of object is independent of container object. But still container object owns the aggregated object.
So if we delete class A that doesn't mean that class B will also be deleted. E.g. none, or many, teachers can belong to one or many departments.
The relationship between Teachers and Departments is aggregation.
3. Composition
Class A owns Class B.
E.g. Body consists of Arm, Head, Legs. BankAccount consists of Balance and TransactionHistory.
So if class A gets deleted then also class B will get deleted.
Direct association has nothing in common with the other three. It does not belong to UML at all, it is the IBM requirements modelling term.
As for others,
Association A->B is a child of Dependency. Association means, that A (or its instance) has some easy way to get to instance of B. For example, a.x.y.b. Or by function, or by some local variable. Or by a direct reference or pointer, or something else (there are many languages in the world). As you see, there is no strict border between dependency and association.
One of attributes of Association is Aggregation, it can have values: None, shared (often incorrectly called aggregation), and composition.
If A (or instance) has some (or one) instances of B so, that destroying of association means the destroying of B instances, it is the composition.
If you or a tool author had decided, that some has-a relationship, that is weaker that composition, needs to be specially shown, you can use shared aggregation. Usually it is some collections of references to B in A.
There are some more interesting attributes of associations. Look here if you are interested.
An association between object types classifies relationships between objects of those types. For instance, the association Person-isEmployedBy-Enterprise may classify the relationships PeterMiller-isEmployedBy-IBM, SusanSmith-isEmployedBy-IBM and SarahAnderson-isEmployedBy-Google between the objects PeterMiller, SusanSmith and SarahAnderson of type Person as well as Google and IBM of type Enterprise. In other words, associations are relationship types with two or more object types participating in them. An association between two object types is called binary. While binary associations are more common, we may also have to deal with n-ary associations, where n is a natural number greater than 2. For instance, Person-isTreatedIn-Hospital-for-Disease is a 3-ary ("ternary") association between the object types Person, Hospital and Disease.
I guess that with "direct association" you mean a directional (or directed) association, which is an association (with a domain class and a range class) that represents a reference property in its domain class. Such a directional association has an "ownership dot" at its target end.
Please see this book chapter for more about associations.
And see my answer to this SO question for an explanation of aggregations and compositions.