hi every body i'm trying to understand UML but there are some questions about it
In UML what is the significance of tagging a class with the stereotype <<abstract>>?
and how to express this constraint as an invariant,
A stereotype "abstract" does not exist - an abstract class should be depicted using italic font. Abstract means that a class cannot be instantiated. It needs a subclass to do so. So as a pseudo-code constraint this would mean
for all instances i of MyAbstractClass holds: i.actualClass != MyAbstractClass
or in ocl for MyAbstractClass holds
self.allInstances()->forAll(i: MyAbstractClass | i.classifier <> self)
As the word 'abstract' was not displayed in your first question version, I expanded on stereotypes in general:
First of all: When learning UML, stereotypes should not be the first things you look into. They are rather complex.
Stereotypes or keywords (both denoted with <<MyStereotype>>) do not have a general meaning. It is defined by the specific stereotype. Commonly you cannot express a stereotype as an invariant instead.
But some other aspects of UML can be shown the same way: A class from the UML Metalevel is marked with <<metaclass>> even though it does not have a stereotype or even is of different actual type. The Stereotypes themselves are shown with a <<stereotype>> marker (even if they are instances of a special class).
An example for a custom stereotype could be "Service". You could mark classes with it which represent a Service. There could be a constraint which tells you that a "Service" must implement a special Interface. In this case you could express this constraint as a (boring) invariant. But probably it is even just a marker. In the latter case you can use a keyword as replacement.
I realize this thread is a couple of years old, but I came to it when it was referenced by someone else, as supporting the assertion that the «abstract» stereotype isn't supported by the UML spec. That assertion isn't quite accurate, and I'd like to explain why. I'll start by clarifying what abstract classes are.
Abstract classes are definitions of classes that don't include complete implementation. Therefore, abstract classes can't be directly instantiated; they have to be specialized (inherited). Abstract classes are notated by italicizing the class name and the methods that are abstract, and additionally by optionally adding an {abstract} property to the class name and/or to the operations (methods, we usually say, but methods are actually the "method" by which the operation is implemented) that are abstract.
Interfaces are actually a specific type of abstract class: a class with zero implementation. Their notation is different from other types of abstract classes (don't italicize, use the «interface» keyword, and notate all the specialization arrows with dotted lines). So, as Christian says here, there is standard notation for abstract classes--at least, there is in class diagrams.
Now, while it is true, as Christian also says, that the «abstract» stereotype doesn't exist, it is also true that you can create it if you want to, and that doing so is supported by the UML spec. It's unlikely that you'll have a reason to (at least in class diagrams), but you still can.
A stereotype is an "extensibility mechanism" for UML (there are three: stereotypes, tagged values, and constraints). It allows you to more specifically define some sort of element. Stereotypes are applied to classes (metaclasses actually, metaclasses are classes whose instances are also classes). A number of stereotypes are pre-defined "Standard Stereotypes" (in UML 1.4 they were called "Standard Elements"). Examples of these are «metaclass» (again, a class whose instances are also classes) and «file» (a physical file in the context of the system developed).
Stereotypes are a type of keyword. The spec (Superstructure 2.0, Annex B, p. 663) has this to say about keywords:
UML keywords are reserved words that are an integral part of the UML
notation and normally appear as text annotations attached to a UML
graphic element or as part of a text line in a UML diagram. These
words...cannot be used to name user-defined model elements where such naming would result in ambiguous interpretation of
the model. For example, the keyword “trace” is a system-defined
stereotype of Abstraction (see Annex C, “Standard Stereotypes”) and,
therefore, cannot be used to define any user-defined stereotype.
In UML, keywords are used for four different purposes:
To distinguish a particular UML concept (metaclass) from others sharing the same general graphical form...
To distinguish a particular kind of relationship between UML concepts (meta-association) from other relationships sharing the same general graphical form...
To specify the value of some modifier attached to a UML concept (meta-attribute value)...
To indicate a Standard Stereotype (see Annex C, “Standard Stereotypes”)...
Keywords are always enclosed in guillemets («keyword»), which serve as visual cues to more readily distinguish when a keyword is being used...In addition to identifying keywords, guillemets are also used to distinguish the usage of stereotypes defined in user profiles. This means that:
Not all words appearing between guillemets are necessarily keywords (i.e., reserved words), and
words appearing in guillemets do not necessarily represent stereotypes.
In other words, you can create any stereotype that you want, so long as it isn't a keyword. Since "abstract" is not a keyword, it follows that you can create an «abstract» stereotype.
In order to do so, however, you would have to go to some trouble, more trouble in UML 2.0 and above than in UML 1.4. UML 1.4 simply stated that a stereotype was an extension mechanism for the UML spec. One could simply define the stereotype, apply it to whichever part of the UML metamodel one wanted, and document the change. UML 2.0 wanted to formalize the relationship of a stereotype to a UML metaclass (any item on a UML diagram is a metaclass, and part of the UML metamodel). So, they came up with Profiles. This sample diagram shows how profiles work:
Now, that black arrow may look a bit strange, since you don't see it in any context but this one. UML 2.0 introduced the concept of an Extension, which it defines as "used to indicate that the properties of a metaclass are extended through a stereotype." This black arrow indicates an extension.
I'll quote Tom Pender (The UML Bible, Wiley Publishing, 2004) for an explanation of this diagram, since he does a better job than the spec (and I certainly can't improve on it):
It shows that a Component is extended by a Bean stereotype, which is required. The Bean stereotype is an abstract type, with two subtypes - Entity and Session. Each instance of Component, therefore, must be extended by an instance of either the Entity stereotype or the Session stereotype. Remember that a stereotype is a kind of class that can have properties - in this case, a Session stereotype has an attribute named state. This corresponds to a tagged definition whose value specifies the state of the Session. The tagged value is an enumeration, StateKind, which has either a stateless or stateful value.
The Component has a constraint on it, displayed in the note attached to the Component symbol, which states that a Component cannot be generalized or
specialized.
The diagram also shows that an Interface metaclass is extended by the Remote and Home stereotypes. The EJB package has a constraint, displayed in the note that sits in the package, that states a Bean must realize exactly one Home interface.
So, you can indeed use an «abstract» stereotype if you have reason to go to the trouble of creating it. The main reason that anyone might want to is to represent an abstract class in some place other than a class diagram.
Related
I want to evaluate constraints in both UML class diagram and Complete OCL file. I searched but I only found examples where the methods are in the same class. Below is an example of an UML diagram class about which I have the following questions:
Using Papyrus, how to set in diagram the Collection types (Bag, Set, etc.), since they are not available in the input and return argument types selection window ? It's available only for Ecore (EEList, EMap)
How to define Context and Operations (parameters/arguments), since both are in other class, according class diagram above ?
How to navigate between both classes in OCL complete file ?
Does the OCL Complete file allow the use of commands: def, let, if-then-else, etc. ?
Thanks in advance.
UML does not support Collections or Maps. Rather it has multiplicities and qualified associations. For the usual common cases it is sufficient to specify an unlimited upper bound, and unique and ordered polarities to configure the UML multiplicity of a Bag/OrderedSet/Sequence/Set for OCL.
UML supports templates but OMG OCL does not, although OMG OCL is aligned to UML. Rather OMG OCL has magic "T" and "T1" types that looks remarkably like template parameters without being defined as such. The Pivot-based Eclipse OCL that prototypes solutions to many OMG OCL problems implements templates and so allows a UML user to define a DataType property whose type templates the relevant collection/map type from the Eclipse OCL Standard Library.
For flat collections using the library type is just an alternative approach. For nested collections, that UML does not support, using the DataType to define all or all but one levels of nesting is the only option.
(Papyrus uses the Pivot-based Eclipse OCL.)
Complete OCL should work normally; UML multiplicities are converted to/interpreted as OCL collection types. In the Pivot-based Eclipse OCL there is a legacy syntax that uses () to reference e.g. Set(MyClass) but a novel/familiar <> to declare e.g. MyAggregate<E>.
I have a question about my UML diagram. I have made a system consisting of a "User" parent class and the two child classes inherit from the "User". Now my question is how to show the multiplicity notation between them, just like the multiplicity notation is shown between "Booking" and "client", where more clients can only have one booking each and the notation would be many to one.
The question is:
The user in our system can only be a practitioner or a client, not both at the same time -->, therefore, the notation would be 1 to 1
There are more users and the different users could be either a practitioner or a client --> therefore the notation would be 1..n to 1..n
What is the right way to write it? Is it wrong to write multiplicity notation when a class inherits? Please help I have spend like 2 hours on figuring it out.
You don't. There are no multiplicities on a Generalization in UML.
The fact that your User is either a Cient or Practitioner can be expressed by setting your User abstract. The notation for abstract classes is to set the name in italic. This way you cannot instantiate User instance directly, but you have to use one of the concrete subclasses.
The second phrase doesn't really constrain anything, so you don't have to mention it at all.
Remark: the guillemets « and » are reserved in UML to denote stereotypes and keywords. As it is right now your User class is really an unnamed class with stereotype «User»
Suppose an abstract class X and its subclasses Y and Z. How do I represent in UML class diagrams that Y and Z should be singletons. Is it possible to represent that all X subclasses must be singletons?
To specify that all subclasses of X are singletons, you can write a constraint in between braces: { every subclass of X is a singleton }. This constraint should be put in a constraints compartment in the class rectangle.
The UML 2.5 specification, §7.6.4 defines the notation for constraints in general and §9.2.4 specifies how to show the constraints of a classifier:
If a Classifier owns Constraints, a conforming tool may implement a compartment to show the owned Constraints listed
within a separate compartment of the owning Classifier’s rectangle. The name of this optional compartment is
“constraints.”
Alternatively, you could give a singleton indication on each and every subclass of X. From your wording, I assume that that is not what you want. Anyway, the latest version of UML (2.5.1) does not have a standard way to indicate that a class is a singleton. Some people indicate it by writing 1 in the top right corner of the rectangle. However, that is not valid UML. You may use that for parts, but not for classes. Instead, you could invent your own stereotype ≪singleton≫.
There is another StackOverflow question about this topic.
Here's another possibility: you can adorn the class with a <<singleton>> stereotype. I always used it that way and the coder knows how to handle that. It's no UML standard, but see the last sentence.
§11.4.4 of the UML 2.5 spec says:
A usage dependency may relate an InstanceSpecification to a
constructor for a Class, describing the single value returned by the
constructor Operation. The Operation is the client, the created
instance the supplier.
If you create a GeneralizationSet that has the meta-property isComplete=true (to say that all possible subclasses are accounted for), and you connect one InstanceSpecification to each constructor by a usage dependency, the model means that every class is a singleton.
I have class Controller in my java project, which has method like this:
As you can see, in the first line I am getting Singleton instance of ActualModes class and call method getActualModes().
So the first question is, which relatinship I should use in class diagram.
After that I am creating new instane of ModeContext class and then call method executeStrategy. In this case, which relatiship is better ?
It should be like this:
Access to the singleton (note the stereotype which is just convenient and no obligation or general standard) is anonymous and so you just have a dependency. The ModeContext in contrast uses a private (I don't know the scoping rules of the language you used, so I made it pivate) property called context. Note the dot which is saying exactly that.
Disclaimer: UML does not specify a mapping between Java and UML, so every answer to your question is open for debate.
I think both relationships are dependencies, drawn as dashed arrows from Controller to ActualModes and from Controller to ModeContext. The definition of 'dependency' according to the UML 2.5 specification (§7.8.4.1) is:
A Dependency is a Relationship that signifies that a single model Element or a set of model Elements requires other
model Elements for their specification or implementation.
An example of a type of relationship which is in my opinion less suited, is the association, although its definition (§11.5) is quite broad:
An Association classifies a set of tuples representing links between typed instances. (...) An Association specifies a semantic relationship that can occur between typed instances.
One could argue that there are links between Controller and the other two classes, in the form of variables, but these variables are local method variables, which exist only temporarily during the execution of the method. Associations represent more durable links, e.g. class members - as far as I understand UML and as far as I have seen associations used in practice.
Is it correct in accordance with UML specification for UML in version 2.5 to model generalization relationship between UML enumeration and UML class? In page 166 the specification states "As a specification of Classifier, Enumerations can participate in generalization relationships". Is it however correct to put generalization relationship between class and enumeration? If yes, how such a enumeration should than be interpreted?
Yes, you can do so. Both elements are Classifiers as the UML 2.5 metamodel shows.
As such you can use inheritance between both.
Now for your question what that would mean. Well, honestly I don't know. Enumerations are basically meant to be just enumerations. In the old days they were just that list of EnumerationLiterals. More recent languages allow for operations/properties as well. But when you pimp them with a Class, then what is the point? Generalization is a blade with two edges. Defining an amphi-car as inheriting from car and ship feels strange. It's neither of both but something new. So in case of Enumeration it's similar.
I still did not find a formal constraint, but you may use the Generalization definition to find evidence whether a generalization between a Class and an Enumeration makes sense (or find examples which have an interpretation):
A Generalization is a taxonomic relationship between a more general
Classifier and a more specific Classifier. Each instance of the
specific Classifier is also an instance of the general Classifier. (UML 2.5.1, p.138)
With your construction, an instance of the Enumeration would also be an instance of the Class (or vice versa). Now let's look at what that means.
When a Classifier is generalized, certain members of its
generalizations are inherited, that is they behave as though they were
defined in the inheriting Classifier itself. For example, an inherited
member that is an attribute may have a value or collection of values
in any instance of the inheriting Classifier... (UML 2.5.1, p.100)
In fact an Enumeration can have attributes just like any Classifier, thus why shouldn't it inherit those from a Class? Looking further into what it means to be an instance of an Enumeration or an instance of a class, we find this:
DataTypes model Types whose instances are distinguished only by their
value. ... Each value of an Enumeration corresponds to one of its
user-defined EnumerationLiterals. ... EnumerationLiterals may not
change during their existence, so any attributes on an Enumeration
shall be read-only. (UML 2.5.1, p. 209 f.)
Aha, maybe we could enumerate objects with their attribute values (instances of Class) as values of an Enumeration? Would the instances of the Enumeration than also be instances of the Class? Would that be a suitable interpretation?
I am just thinking out loud, not being sure if this makes much sense. I just wouldn't rule out that there is an interpretation just because I never encountered one or cannot imagine one. In fact, look at Enums in Java - they implicitly extend the Class java.lang.Enum and the only reason why they can not extend further user-defined classes or other Enums is that Java does not support multiple inheritance of state (Oracle Java tutorials).