UML Weekend Crash Course™ - To Parent Directory

Session 1—What Is the UML 7
Table 1-1
The UML Four-layer Metamodel Architecture
Layer Description Example
metametamodel Defines the language for Defines the concepts MetaClass,
specifying metamodels. MetaAttribute, MetaOperation, and
so on.
metamodel Defines the language for Defines the concepts Class,
specifying a model.
Attribute, Operation, Component,
and so on.
model Defines the language to use Defines the concepts Order,
to describe a subject domain. Shipment, Product, Product ID,
Buy(), and so on.
user objects Defines specific subject Defines Order #74653, Shipment
domain information.
#87649, the product “CD-ROM
435”, the price $50.00, and so on.
Starting from the bottom and working up, the user object layer is where you find a diagram,
like a Sequence diagram or Object diagram, populated with the facts from the problem domain
like Order #74653 that contains a line item for “CD-ROM 435” with a price of $50.00. The diagram
is built following the rules defined by the next layer above it, the model layer.
The model layer fully explains the classes that describe the subject domain objects, for
example, classes like Order, Shipment, and Product. It tells you what an Order looks like, the
fields it contains, the operations it can perform, and so on, without ever telling you about
any particular Order. These class definitions conform to the rules specified in the next layer
above, the metamodel (2M) layer.
The metamodel (2M) layer defines what a class is so that the model layer knows how to
describe the Order class. It defines a class as a concept having attributes, operations, and
associations. It defines an attribute as having a name, a data type, a default value, and constraints.
These definitions in turn conform to the specifications of the metametamodel (3M).
The metametamodel (3M) layer is the realm of the philosophers and practitioners of the
black arts who determine what makes up a language. Nearly all the definitions at this layer
are abstract, that is, they are more like templates that can be used to build a wide variety
of concrete concepts.
In case I managed to scare you just now, relax, all you really need to know is the metamodel.
That is what this entire course is about, defining the diagrams and the elements
used to construct them. Once you understand the diagrams defined by the metamodel layer,
you will get enough practice building the model and user object layers to become quite
comfortable.
The organization of the metamodel
The metamodel is a bit complex, so it helps to organize the elements into packages. A package
is basically the UML version of a directory, a place to put things. At the first level you