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2.2 Alphabets, Words, and Sentences
Because requirements and implementation technologies are complex and
constantly changing, a language must not only facilitate
communication about a subject, but also enable us to better manage
change and complexity when we communicate about the subject. A
language is based on
a
paradigm, a way of viewing a subject, that
defines the types of concepts that may be used in the language and
the principles of why they are useful. A language's
syntax specifies the notation used for
communication and is determined by the language's
alphabet. A
language's
semantics specify the meaning that is
communicated and is determined by the language's
words and sentences. The syntax of the UML involves diagrams, and its
semantics are based on the object-oriented paradigm.To communicate using a language, we must understand its alphabet, how
its alphabet is used to form words, and how its words are used to
form sentences. We must also understand the concepts and principles
of its underlying paradigm.
2.2.1 Alphabet
An alphabet defines the simplest
parts of a language: letters, characters,
signs, and marks. For example, the English language has 26 letters.
The UML's alphabet consists of symbol fragments
(rectangles, lines, and other graphical elements) and strings of
characters. These don't have meaning by themselves;
the smallest units of meaning in a language are its
"words."
2.2.2 Words
A word is a grouping of elements
from a
language's alphabet that defines a unit of meaning.
For example, the English language has various words, including
"project,"
"manager,"
"team,"
"lead,"
"execute," and so forth. In the
UML, words belong to two broad categories or types:
Concepts
Concepts are shown as
solid-outline rectangles or symbols
labeled with a name.
Relationships between concepts
Relationships between
concepts
are shown as line paths connecting symbols
labeled with a name.
In addition to their names, concepts and relationships may have other
strings of characters attached to them specifying other information.Figure 2-1 shows various concepts identified from
the project management system requirements by focusing on nouns,
including Project, Manager,
Team, Work Product,
Requirement, and System.
Figure 2-1. Concepts
Likewise, Figure 2-2 shows various relationships
identified from the project management system requirements by
focusing on verbs, including Manage,
Lead, Execute,
Input, and Output.
Figure 2-2. Relationships
You would not normally show all these relationships in isolation as
in Figure 2-2, but would combine them with the
concepts shown in Figure 2-1. When you combine
relationships with concepts, you are really combining UML words to
form UML sentences.
2.2.3 Sentences
A sentence is a grouping of words
from a language's
vocabulary that defines a grammatical unit of meaning containing a
subject and an expression about the subject. A
language's grammar specifies
the
rules for combining words to form
sentences. For example, the English language has rules for combining
words to form sentences, in which the sentence "a
manager leads a team" follows the grammar rules, but
the sentence "leads manager team"
does not. UML sentences are diagram fragments or very simple
diagrams.Figure 2-3 shows a UML sentence communicating that
a team will execute a project as indicated in the project management
system requirements. Team and
Project are concepts (nouns), and
Execute is the relationship (verb) between the two
concepts.
Figure 2-3. A team will execute a project
Figure 2-4 shows a somewhat more elaborate UML
sentence in which a manager manages a project and leads a team.
Figure 2-4. A manager manages a project and leads a team (Version 1)
Notice that Figure 2-4 communicates about a
manager's relationship with a team and project, but
it does not communicate that the team will execute the project, as
Figure 2-3 did. Just like the English language, we
can communicate whatever we want using the UML. Perhaps one way to
look at this is that the UML sentence in Figure 2-4
has a different subject (manager) than the UML sentence in Figure 2-3 (team).The visual location of concepts and relationships does not have any
special meaning so long as symbols are not nested inside one another.
A relationship is usually read from left to right and top to bottom;
otherwise, its name may have a small black solid triangle arrow next
to it where the point of the triangle indicates the direction in
which to read the name; the arrow is purely descriptive, and the name
of the relationship should be understood by the meaning of the
concepts it relates. For example, Figure 2-5
communicates the same information as Figure 2-4.
Figure 2-5. A manager manages a project and leads a team (Version 2)
In Figure 2-5, if I did not show the small black
solid triangle arrows next to the relationship, we would still know
that managers manage projects and lead teams, because that is what it
means to be a manager. This is why it is important to know the
semantics of what we depict in the UML rather than simply the syntax.
Saying that teams lead managers and projects manage managers would be
meaningless!
