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.
Related
The above statement is taken from www.uml-diagrams.org section Assocation ends from the rather beginning.
Is there a real-world example for this?
What exactly means owned by association itself?
What is association class or how is it implement in code?
Here's my attempt. I also was unsure about the nature of an association in the past and until not so long ago where I sort of grasped it. For the full pain you have to go through pp. 197 of UML 2.5 (11.5 Associations)
What is an association?
When you have a number of classes (we may call them A, B, etc. here for brevity. They are designed to be somewhat self-consistent. So you have properties and operations which work with the latter. If our A is some mathematical class you may have a well working black box. Fine.
Now, when any class is in need of another (e.g. the B class needs objects of type A to have that math toolbox handy) you create some reference. In a programming language that will be some variable holding a reference to an instance of the needed class. That's the moment where you have an association.
In UML you can denote such an association as a property typed with the other class. Or (!) you draw a line for an association between both class boxes and place the role name towards the referenced class (telling the name of the variable the programmer shall use). This being quite simple got me some headaches since I saw notations with both property and role notation. I have not fosteres the UML spec as tho what this would mean, but I established a rule I was also teaching that this is not allowed. Tools like EA do not support roles in a correct way which forces you to manually maintain your diagrams. That is, if you have an association role from B to A and the diagram does not contain A then you should show the role as property (see my note about dot notation below). But once you place A on that diagram you should see the assocation line with the role name, but not the property (pretty confusing I admit).
Note: Dot notation
Since UML 2.5 (or was it 2.1?; but see p. 200 of UML 2.5) the OMG guys introduced the so called dot notation. That is, you have a little dot at the association line end along where the role name is. This dot indicates that the role name is actually the property of the referencing class. Pretty much what I was talking about above. And what I thought was the intention. So, I have no idea what a not-owned property would be in any case. May be a good question to be asked.
Associaton class
Assume you have class C which reflects some kind of relation between A and B (and eventually D, E, etc.). Doing that makes it an association class. It's doing that by exactly the same technique as above, just for multiple classes. (If a class has multiple relations to other classes it's not necessarily an association class, but only if it's main purpose is to set these into some relation. YMMV)
So here you have the case where an association owns the role since you have a real object. The association explained above exists only as a reference in the programming world, not a complex storage structure like an association class.
First of all, you should understand that, as opposed to the explanation of #qwerty_so, in data/information modeling, relations between two objects (called "links" in UML) are a first-class citizen. Logically/conceptually, they exist next to objects. In terms of OO implementation, they may exist either within one or both of the two participating objects in the form of object references, or outside of the two participating objects in the form of another object representing the "link".
Likewise, in data/information modeling, an Assocation, which classifies "links", is a first-class citizen that logically/conceptually exists next to the involved Classes. A binary Assocation is implementated either within one or both of the two (Java/C#/JS/etc.) classes involved in the form of a reference property (or possibly two mutually inverse reference properties), or outside of the two classes in the form of another class representing the Assocation.
In the case where a binary association has one ownership dot at one of its ends, it is unidirectional and implemented as a reference property in the corresponding class, while in the case where it has ownership dots at both of its ends, it is bidirectional and implemented as two mutually inverse reference properties, one in each of the involved classes.
What exactly means owned by association itself?
An Assocation End of a binary Assocation can be owned either by the Class on the opposite side or by the Assocation itself. In the former (more common) case, the Assocation End corresponds to a reference property of the (Java/C#/JS/etc.) class on the opposite side. In the former (less common) case, the Assocation End corresponds to a reference property of the (Java/C#/JS/etc.) class that represents the Assocation.
In principle, there could be an OOP language that supports Assocations as first-class citizens, having a keyword "assocation", in addition to "class".
What is association class or how is it implement in code?
The need for an Association Class arises when the "links" of an Association have to be characterized with the help of attributes, or when they have to be processed with the help of special operations. In such a case, an Association Class allows defining these attributes or operations. An Association Class is implemented in the form of a (Java/C#/JS/etc.) class that has corresponding reference properties for each of the Association's ends.
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.
I am designing a class diagram for an assignment. In this design, I use a separate class called Currency, to define currency values and their functionality. there are at least four other classes have to use this Currency class.
How can I show it in the class diagram ? I mean, do I need to draw relationships (connecting lines) from the Currency class to all the others ?
Is there a better way ?
What am I doing wrong here ?
There is nothing wrong and a reusability of a class is valuable. Actually that's a standard situation.
If you use this class in another class as an attribute you have two options to depict that:
draw an association relationship (line) from the class using to the class that is used.
put the attribute in a proper compartment of a class that is using and as a type of an attribute (after a colon) put the name of the used class.
The benefit of the first approach is that you immediately see the dependency between the classes.
If you use a class but not directly as an attribute type you use other relationship types that suit best to the situation you want to describe.
As I imagine one of your concerns is that you'll have a lot of relationships pointing to your class (in your case Currency). Don't worry about that. You don't have to put everything in a single diagram. Put a full specification of your class on one diagram with those relationships where it uses something else and then put only the class box with a name (without any compartment) on diagrams defining those elements that use your class. It will make your model readable. And with a support of some CASE tool you will be able to see all relationship and dependencies of this class anyway. By the way that's how the UML specification is written. Look for example how Namespace is used in the diagrams there (and many others as well).
Of course I'm not suggesting creating one diagram per one element to define it. No. Collect them in logical Packages (hey - that's exactly what Packages are for!) and make a class diagram per Package. If the Package becomes too large - you might need to split it into smaller subpackages.
For Currency your Package would be probably something like Utils. It can also contain other elements like Date, Address etc. Note - these are typical examples, probably every analyst/designer/programmer sooner or later has to cope with those elements. If you build them well, you'll be really able to reuse them in future applications as well.
One last thought. While you build "package based" Class diagram you might also need a diagram that shows just specific parts coming from several Packages to clarify some bit of your system/business/whatsoever. This is also absolutely fine. Again a benefit of CASE tool here is that it keeps consistency in your model.
Suppose I have such requirement:
The objects in the system all derive from a base class named IObject, and it may have objects with color, objects with transformations, and both.
Now there are 2 approach to design the class hierarchy.
The first one is:
just let concrete class derived from
IObject, and also select "capability"
interfaces as its base class to
indicate it support such behavior,
like interface: IHasColor,
IHasTransformation
The second one is:
Organize the base classes, and let
concrete classes derived from one of
them: IObject, IColorObject,
ITransfromationObject ,
IColorAndTransformationObject
I prefer the first one (Does it have a formal name? ) as it is more flexible, and as you can see the second one may have class combination explosion problem when there are many attributes like color, transformation...
I would like to know your ideas and suggestions.
Thanks.
Classes abstract the real concept of types of objects.
Interfaces abstract the real concept of behaviors or abilities for an object.
So the questions becomes, is the "color" a property of the object or is it a capability of the object?
When you design a hierarchy you are constraining the world into a narrower space. If you take the color as a property of the object then you will have two kind of objects, the ones that have colors and the ones that do not. Does that fit your "world"?
If you model it as a capability (Interface) then you'll have objects that are able to provide, lets say cast, colors to the world.
For the transformation the same logic applies. You can either split the world into two kind of objects, the ones who can transform and the ones who can not, or you can view it as a capability, an object may have the ability to transform itself into another thing.
For me, from that point of view, what would make sense would be:
Color is a property of the object. In fact every object should have a color, even if its transparent, even if is reflection, even if its "none" (good luck figuring out what an object with color = none means in the real world, still it might make sense in your program logic).
Transformation is a capability, that is, an interface, something the object is capable of doing, among other things the object may or may not be able of doing.
I'm working on classes hierarchy in my project and basically I have similar situation like you described in your question.
Let's say I have base type Object which is absolute root of all other classes in my toolkit. So naturally everything derives from it directly or through it's subclasses. There is a common functionality that every Object-derived class has to provide but in some leaf classes effects are little different than in others. For example every object have size and position which can be changed with properties or methods like Position = new Point(10, 10), Width = 15, etc. But there are classes that should ignore setting of a property or modify it according to self inner state. Think about control docked to left side of parent window. You can set Height property all you like but it will be generally ignored because this property really depend on Height of parent container control (or it's ClientArea height or sth like that).
So having Object abstract class implementing basic common functionality is ok until you reach a point of where you need "customize" behavior. If Object provides protected virtual SetHeight method that is called in setter of Height property you can override it in you DockedControl class and allow change of height only if docking is None, in other cases you limit it or ignore completely.
So we are happy but now we have requirement for object that react on mouse events like Click or Hover. So we derive MouseAwareObject from abstract Object class and implement events and stuff.
And now client want dockable, mouse aware objects. So we derive from DockableObject and... hmm, what now? If we can do multiple inheritance we can do it but we hit diamond problem with ambiguity of duplicated interface and we need to deal with it. We can have two memeber of Dockable and MouseAware types in new class and proxy external calls to them to provide functionality.
And last thing that comes to mind is to make IDockable and IMouseAware interfaces and let them define functionality and add them freely only to objects that need to deliver concrete behaviors/implementations.
I think I will split my base class into parts and leave my Object with very limited "core" set of properties and methods and rest of functionality that is in fact optional to Objects as a type but needed in concrete cases move to interfaces like IResizable, IDockable, IMakeAWorldABetterPlaceAble, etc. With this solution it is possible to "attach" behaviors to Object-derived classes without need for draggin virtual or pure virtual (abstract) methods from root base class all the way down to leaf class.
There is of course inconvenience of implementing interfaces in all affected classes but you can always implement some "adapters" and just forward calls to them. That way you don't have duplicated implementation (to some extend of course) and have decoupling between realization of task (Resize can mean different things for different classes) and expectation of client code.
Probably this is not ideal answer for your question but maybe it will hint you to your own solution.
I think you jump directly into interfaces, skipping classes. Is it required for you app. to have a "IObject" interface ? Maybe a "CObject" root class for your class hierarchy, may help you.
It think the winner is No. 1 solution, you may have a "MyObject", whether is an implementation of an interface, or direct class. Later you can add additional classes or interfaces in your class hierarchy, as you need.
After seeing several applications (some mine, some others), I think there should be a "My Custom Application Class Hierarchy Root Object" or "My Custom Application Class Hierarchy Root Interface" Design Pattern.
I have recently been studying UML and drawing simple diagrams with ordinary plain arrows between classes, but I know it's not enough. There are plenty of other arrows: generalization, realisation and etc. which have meaning to the diagram reader.
Is there a nice resource which could explain each arrow (ordinary, plain, dotted, diamond-filled, diamond)?
It would be the best if it will have some code examples for them.
Here's some explanations from the Visual Studio 2015 docs:
UML Class Diagrams: Reference: https://learn.microsoft.com/en-us/previous-versions/visualstudio/visual-studio-2015/modeling/uml-class-diagrams-reference
5: Association: A relationship between the members of two classifiers.
5a: Aggregation: An association representing a shared ownership relationship. The
Aggregation property of the owner role is set to Shared.
5b: Composition: An association representing a whole-part relationship. The Aggregation
property of the owner role is set to Composite.
9: Generalization: The specific classifier inherits part of its definition from the general
classifier. The general classifier is at the arrow end of the connector. Attributes, associations, and
operations are inherited by the specific classifier. Use the Inheritance tool to create a
generalization between two classifiers.
13: Import: A relationship between packages, indicating that one
package includes all the definitions of another.
14: Dependency: The definition or implementation of the dependent classifier might change if
the classifier at the arrowhead end is changed.
15: Realization: The class implements the operations and attributes defined by the interface.
Use the Inheritance tool to create a realization between a class and an interface.
16: Realization: An alternative presentation of the same relationship. The label on the
lollipop symbol identifies the interface.
UML Class Diagrams: Guidelines: http://msdn.microsoft.com/library/dd409416%28VS.140%29.aspx
Properties of an Association
Aggregation: This appears as a diamond shape at one end of the connector. You can use it to
indicate that instances at the aggregating role own or contain instances of the other.
Is Navigable: If true for only one role, an arrow appears in the navigable direction. You can use
this to indicate navigability of links and database relations in the software.
Generalization: Generalization means that the specializing or derived type inherits attributes,
operations, and associations of the general or base type. The general type appears at the arrowhead
end of the relationship.
Realization: Realization means that a class implements the attributes and operations specified by
the interface. The interface is at the arrow end of the connector.
Let me know if you have more questions.
I think these pictures are understandable.
A nice cheat sheet (http://loufranco.com/wp-content/uploads/2012/11/cheatsheet.pdf):
It covers:
Class Diagram
Sequence Diagram
Package Diagram
Object Diagram
Use Case Diagram
And provides a few samples.
My favourite UML "cheat sheet" is UML Distilled, by Martin Fowler. It's the only one of his books that I've read that I do recommend.
For quick reference along with clear concise examples, Allen Holub's UML Quick Reference is excellent:
http://www.holub.com/goodies/uml/
(There are quite a few specific examples of arrows and pointers in the first column of a table, with descriptions in the second column.)
The accepted answer being said, It is missing some explanations.
For example, what is the difference between a uni-directional and a bi-directional association? In the provided example, both do exist. ( Both '5's in the arrows)
If looking for a more complete answer and have more time, here is a thorough explanation.
A very easy to understand description is the documentation of yuml, with examples for class diagrams, use cases, and activities.
Aggregations and compositions are a little bit confusing. However, think like compositions are a stronger version of aggregation. What does that mean?
Let's take an example:
(Aggregation)
1. Take a classroom and students:
In this case, we try to analyze the relationship between them. A classroom has a relationship with students. That means classroom comprises of one or many students. Even if we remove the Classroom class, the Students class does not need to destroy, which means we can use Student class independently.
(Composition)
2. Take a look at pages and Book Class.
In this case, pages is a book, which means collections of pages makes the book. If we remove the book class, the whole Page class will be destroyed. That means we cannot use the class of the page independently.
If you are still unclear about this topic, watch out this short wonderful video, which has explained the aggregation more clearly.
https://www.youtube.com/watch?v=d5ecYmyFZW0
If you are more of a MOOC person, one free course that I'd recommend that teaches you all the in and outs of most UML diagrams is this one from Udacity: https://www.udacity.com/course/software-architecture-design--ud821