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I want to understand a class diagram more fully and I am finding lots of conflicting information.
My first question is, what is the difference between class diagram and ERD? Not necessarily in look, but in classification. e.g. I have read that a class diagram is a type of ERD and I have read that a class diagram and an ERD are two different things.
My second question is around how the class diagram should look, I was given a basic tutorial on how to create a class diagram and I was taught that each class should be connected with a single line, with an arrow that looks like a 'Play' symbol (example 1 in the attached image)
But since doing some research into it, I am finding lots of examples where different connectors are used to denote association, aggregation, composition, inheritance etc. (example 2 in the attached image)
As mine is more simplistic, just showing the relationship and the multiplicities, does that mean that I have just learned a more basic version of class diagram and the extra connectors are an advanced step?
Or are they both something different?
Thanks for your help
Holly
First of all, welcome to Stack Overflow!
A class diagram is a type of static structure diagram that describes the structure of a system by showing the system's classes, their attributes, operations (or methods), and the relationships among objects. Wiki link
An entity relationship diagram (ERD) shows the relationships of entity sets stored in a database. Link
Therefore the answer to your question of "what is the difference between class diagram and ERD"?
The class diagram has nothing to do with fact how the classes are persisted in the data layer. It shows only the logical relationship between classes and the properties of the classes. While the ERD diagram illustrates the logical structure of database; what the database tables, table-column, primary keys, foreign keys, etc. are, and last but not least the relationships between database tables.
As for the question "Is this just a more advanced version of class diagram? Or a more updated version?":
There are cases when the ERD diagram can look similar to the corresponding class diagram, but the persistence data model can be way different from the class (domain) model. Furthermore a class diagram has no any information about how a class is persisted in the database - as I've already mentioned -, therefore an ERD has other kind of information than a class diagram.
As for the notations you linked:
A proper class diagram contains notations like in the second link. An example is the following diagram:
For more info what those arrows mean, click here for the corresponding SO answer.
What you are taught about how to make a class diagram (like at the first link you shown), can also be useful but it is a customized class diagram rather than an proper class diagram following the UML standards and notations because:
I find it strange that the arrow is not on the line itself
There is a shared ownership relationship (aggregation) between Customer and Vehicle. It means that a Customer can have (own) a Vehicle but the Vehicle can still exist as its own, without a Customer. This relationship can be represented with the aggregation notation. (See arrow 5a, or the class diagram below)
I find it also strange that a vehicle can have multiple Customers, as you notated with "0..*". But of course it is possible, since I do not know what kind of domain you try to model with the diagram...I made an UML diagram with proper signs, check this out:
Summing up, it is wise to follow the UML standards and conventions, since it is widely accepted and known so the information can be exchanged as efficient as possible, without misunderstanding.
Is there a term for the top-most parent class in OOP where some specific method (e.g. newMethod as per the diagram below) is introduced?
Every intro, walk-through, wiki page, or example hierarchy diagram I looked through seems to be lacking the term.
UML Generator
Maybe in languages like Java where there can only be one parent this could be easier to define, but what about other languages that allow for multiple parents? Would such a term be a moot effort to try to define in this case?
Composition: A class can have references to objects of other classes as members. This is called composition and is sometimes referred to as a has-a relationship.
By Deitel P.J., Deitel H.M. - Java How to Program 9th Edition.
This viewpoint is discussed in this topic:
Prefer composition over inheritance?
Composition: Composite aggregation (composition) is a "strong" form of aggregation with the following characteristics:
*it is binary association,
*it is a whole/part relationship,
*a part could be included in at most one composite (whole) at a time, and
*if a composite (whole) is deleted, all of its composite parts are "normally" deleted with it.
Found on http://www.uml-diagrams.org/composition.html
(actually, Deitel presents UML examples following this idea, in the same book, but did not bother to explain the difference).
This viewpoint is discussed in this topic:
What is the difference between association, aggregation and composition?
Fine, BOTH ARE CORRECT. And this introduces the problem of homonym concepts.
For instance: don't draw a UML model with composition arrows to exemplify the first definition: In UML, any association is a composition by Deitels' the first definition.
Here are some aspects of my question that may help in the correct answer:
How I can say (and know) which composition are we talking about?
Where we draw the line between the two definitions (in contextual terms)?
Can I say that the first is object oriented programming and the second is software engineering/modeling?
Is the UML composition a model-only concept/jargon?
Is the UML composition an UML exclusive thing? or is also applied in the programming field?
How to avoid miscommunication of "what composition are we talking about" in a team?
Please, answer with references, evidences, it is not a philosophical/opinion problem, it is a "scope" problem that I´m trying to address.
And it is not "what is composition" question.
Edit: I´m thinking if the distinction is verb x adjective: "to compose" a class (first def.) and "a composite relation" (second def.).
I found it hard to explain the difference between UML association and implementation references without explaining at least a little bit what UML associations actually are, and what they can do, so here we go.
Association & Link
Lets start by looking at what a UML Association and a link (Association's instance) are.
[11.5.3.1] An Association specifies a semantic relationship that can occur between typed instances.
[11.8.1.1] A link is a tuple of values that refer to typed objects. An Association classifies a set of links, each of which is an instance of the Association. Each value in the link refers to an instance of the type of the corresponding end of the Association.
So the following is a valid implementation of a limited association.
class Brain { }
class Head { }
a = new Brain;
b = new Head;
link = (new Array).add(a).add(b);
Ownership
[9.5.3] When a Property is owned by a Classifier other than an Association via ownedAttribute, then it represents an attribute of the Classifier.
(Note: Class is a subclass of a Classifier.)
Navigability
[11.5.3.1] An end Property of an Association that is owned by an end Class or that is a navigableOwnedEnd of the Association indicates that the Association is navigable from the opposite ends; otherwise, the Association is not navigable from the opposite ends. Navigability means that instances participating in links at runtime (instances of an Association) can be accessed efficiently from instances at the other ends of the Association. The precise mechanism by which such efficient access is achieved is implementation specific. If an end is not navigable, access from the other ends may or may not be possible, and if it is, it might not be efficient.
Why are those concepts relevant? Imagine the following example.
We see that brain is an attribute of Head class (the black dot signifies ownership by the opposite Class), and that it is navigable (the arrow).
We also see that head is NOT an attribute of Brain (no black dot ⇒ not owned by the Brain class ⇒ not an attribute of Brain), however it is still navigable. This means that in UML the head Property is held by the association itself.
The implementation could, for example, look like this (the association itself is represented by a tuple of two references (see link description earlier)).
class Head {
public Brain brain;
}
class Brain {
}
h = new Head;
b = new Brain;
h.brain = b;
link = (new Array).add(h).add(b);
So as you hopefully start to see, UML association is not such a simple concept as a has-a relationship.
Composition
Lets add another piece, composition.
[11.5.3.1] A binary Association may represent a composite aggregation (i.e., a whole/part relationship). Composition is represented by the isComposite attribute
[9.9.17] The value of isComposite is true only if aggregation is composite.
With the aggregation being
none - Indicates that the Property has no aggregation semantics.
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
Again we see, that a UML association is explicitly specifying concepts that are hard to perceive from implementation (e.g. who is responsible for object management/destruction).
Model Composition vs Object Implementation Composition
So from the description above we can construct a more precise description of what an implementation composition (has-a relationship) would be.
[Deteils] Composition: A class can have references to objects of other classes as members. This is called composition and is sometimes referred to as a has-a relationship.
McConnell [Code Complete 2, 6.3] also refers to has-a relationship as a Containment.
Neither of them however talk about HOW the objects (container-contained, composer-composite) are related to one another, who is responsible for lifecycles, or whether the contained element knows about the container.
So just by saying that objects have a has-a relationship (and call it composition), you could actually mean any of these (and several more)
So if you call something composition in programming, you can mean pretty much any relationship/reference (or rather not an inheritance), so the word by itself is not very useful.
In UML on the other hand you are trying to capture all such information about how the objects are related to one another. Therefore there's a focus on giving terms a more precise meaning. So when you call something composition in UML you have in mind a very specific has-a relationship, where the container is responsible for the lifecycle of the contained items.
Implementation of UML associations
All those extra concepts information mean that there is really no precise way how to even implement associations. This makes sense as the implementation would depend on the target programming language or environment (e.g. executable models, where the UML concepts are used as the final product).
As an example I can recommend a paper describing UML association implementation in Java with enforced concepts such as multiplicity, navigability, and visibility Implementing UML Associations in Java.
More subquestions
How I can say (and know) which composition are we talking about?
By context, or you can just ask (which is always a good thing to do when unsure). Personally I've heard the use of composition as "has-a relationship" only when differentiating from inheritance; and in the rest in terms of UML. But then again I am in academia, so my view is biased.
Where we draw the line between the two definitions (in contextual terms)?
As the "programming" term composition doesn't actually mean anything (only that it is has-a), I'd recommend drawing the line yourself and pushing others to use more precise terminology.
Can I say that the first is object oriented programming and the second is software engineering/modeling?
More or less, with all the nuances mentioned in this answer.
Is the UML composition a model-only concept/jargon?
Is the UML composition an UML exclusive thing? or is also applied in the programming field?
No, you can use it in programming to mean the same thing as it means in UML, but you might need to state it more obviously. E.g. "This class is a composite for those classes, because it manages their lifecycle.".
The point is to teach people to differentiate between regular-old has-a relationships, and relationships that have more precise semantics.
How to avoid miscommunication of "what composition are we talking about" in a team?
This is a very broad question that you could apply to any term to which you want attach special meaning (what even is software engineering?), and there is no best way. Have a team-shared vocabulary (you are probably already having a lots of specific terms in your domain), and guide people to use more precise terminology.
numbered quotes refers to sections in UML 2.5 Specifications.
To cite the UML 2.5 specification on page 110:
Sometimes a Property is used to model circumstances in which one instance is used to group together a set of instances; this is called aggregation. To represent such circumstances, a Property has an aggregation property, of type AggregationKind; the instance representing the whole group is classified by the owner of the Property, and the instances representing the grouped individuals are classified by the type of the Property. AggregationKind is an enumeration with the following literal values:
none: Indicates that the Property has no aggregation semantics.
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).
Personally I see it the way that notion of a composite aggregation is about object lifetime, not about static relation. A composite aggregation kills aggregate members when their parent dies. None leaves this open. And shared aggregation is a bastard that OMG should not have introduced at all since it's semantics is domain dependent.
Is the arrow read like "Arena depends on LeagueStore" ? How is this implemented ? Here is a similar question, but it doesn't include such an arrow.
picture taken from slide 9
The UML relationship "depends on" is deliberately wide in scope. It means that some aspect of the "classifier" (class, interface, package, ...) referenced by the relationship is used by the classifier at the other end of the relationship. This can include calling a method, using a type, including a package and so on.
In this case I think it can clearly be interpreted as "uses", that is, calls one or more of its methods. Today, this relationship has its own UML representation as a stereotype called "uses" on the dependency relationship to make it a little more specific.
The diagram is not a very good example of a bridge. The name comes from the whole idea that there are two hierarchies connected at the top. All this diagram is depicting is the fact that the outer class (Arena) manipulates the LeagueStore through an outer class. That's not even a pattern, that's the Envelope-Letter Idiom from Coplien's Advanced C++.
Bridge would be LeagueStore having a delegate inside (impl, as depicted), but then also having specializations of LeagueStore. For example, if you had a class called Report, it would have ReportImpl inside, that could have subclasses like JasperReport and BirtReport, but then Report could have subclasses like CrosstabReport.
In Programming in Scala, page 239, the first paragraph says:
Composition and inheritance are two ways to define a new class in
terms of another existing class. If what you’re after is primarily
code reuse, you should in general prefer composition to inheritance.
Only inheritance suffers from the fragile base class problem, in which
you can inadvertently break subclasses by changing a superclass.
To me it is unclear. Can anyone show an example of such a situation, preferably with some code?
The fragile base class problem is general to all systems that support inheritance. It means that changes to your supertype (the class you are inheriting from) can get you unexpected results : change makes you break your assumptions on your base class. See this related SO question for explanation & examples.
By contrast Scala exports posterior additions to parent classes to exterior, independent traits, that you can add to a subtype using Mixin Composition. See this example, and consider "RichIterator" as change that you would like to effect on the base AbsIterator, once it has been defined. See how the mixin doesn't change anything of the parent, yet is still easily reusable in a subtype ?