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The basic UML notations used to represent a class:
A Table
class shown in UML notation:
The equivalent code
class Table {
Integer number;
Chair[] chairs = null;
Integer getNumber() {
// ...
}
void setNumber(Integer n) {
// ...
}
}
class Table:
def __init__(self):
self.number = 0
self.chairs = []
def get_number(self):
# ...
def set_number(self, n):
# ...
The 'Operations' compartment and/or the 'Attributes' compartment may be omitted if such details are not important for the task at hand. 'Attributes' always appear above the 'Operations' compartment. All operations should be in one compartment rather than each operation in a separate compartment. Same goes for attributes.
The visibility of attributes and operations is used to indicate the level of access allowed for each attribute or operation. The types of visibility and their exact meanings depend on the programming language used. Here are some common visibilities and how they are indicated in a class diagram:
+
: public-
: private#
: protected~
: package privateHow visibilities map to programming language features
Visibility | Java | Python |
---|---|---|
- private |
private |
at least two leading underscores (and at most one trailing underscores) in the name |
# protected |
protected |
one leading underscore in the name |
+ public |
public |
all other cases |
~ package private |
default visibility | not applicable |
Table
class with visibilities shown:
The equivalent code
class Table {
private Integer number;
private Chair[] chairs = null;
public Integer getNumber() {
//...
}
public void setNumber(Integer n) {
//...
}
}
class Table:
def __init__(self):
self.__number = 0
self.__chairs = []
def get_number(self):
# ...
def set_number(self, n):
# ...
You should use a solid line to show an association between two classes.
This example shows an association between the Admin
class and the Student
class:
You should use arrow heads to indicate the navigability of an association.
Logic
is aware of Minefield
, but Minefield
is not aware of Logic
.
class Logic {
Minefield minefield;
}
class Minefield {
...
}
class Logic:
def __init__(self, minefield):
self.minefield = minefield
# ...
class Minefield:
# ...
Here is an example of a bidirectional navigability; each class is aware of the other.
Navigability can be shown in class diagrams as well as object diagrams.
According to this object diagram, the given Logic
object is associated with and aware of two MineField
objects.
Association Role labels are used to indicate the role played by the classes in the association.
This association represents a marriage between a Man
object and a Woman
object. The respective roles played by objects of these two classes are husband
and wife
.
Note how the variable names match closely with the association roles.
class Man {
Woman wife;
}
class Woman {
Man husband;
}
class Man:
def __init__(self):
self.wife = None # a Woman object
class Woman:
def __init__(self):
self.husband = None # a Man object
The role of Student
objects in this association is charges
(i.e. Admin is in charge of students)
class Admin {
List<Student> charges;
}
class Admin:
def __init__(self):
self.charges = [] # list of Student objects
Association labels describe the meaning of the association. The arrow head indicates the direction in which the label is to be read.
In this example, the same association is described using two different labels.
Admin
class is associated with Student
class because an Admin
object uses a Student
object.Admin
class is associated with Student
class because a Student
object is used by an Admin
object.Commonly used multiplicities:
0..1
: optional, can be linked to 0 or 1 objects.1
: compulsory, must be linked to one object at all times.*
: can be linked to 0 or more objects.n..m
: the number of linked objects must be within n
to m
inclusive. In the diagram below, an Admin
object administers (is in charge of) any number of students but a Student
object must always be under the charge of exactly one Admin
object.
In the diagram below,
An association can be shown as an attribute instead of a line.
Association multiplicities and the default value can be shown as part of the attribute using the following notation. Both are optional.
name: type [multiplicity] = default value
The diagram below depicts a multi-player Square Game being played on a board comprising of 100 squares. Each of the squares may be occupied with any number of pieces, each belonging to a certain player.
A Piece
may or may not be on a Square
. Note how that association can be replaced by an isOn
attribute of the Piece
class. The isOn
attribute can either be null
or hold a reference to a Square
object, matching the 0..1
multiplicity of the association it replaces. The default value is null
.
The association that a Board
has 100 Square
s can be shown in either of these two ways:
Show each association as either an attribute or a line but not both. A line is preferred as it is easier to spot.
UML uses a hollow diamond to indicate an aggregation.
Notation:
Example:
Aggregation vs Composition
The distinction between composition (◆) and aggregation (◇) is rather blurred. Martin Fowler’s famous book UML Distilled advocates omitting the aggregation symbol altogether because using it adds more confusion than clarity.
An interface is shown similar to a class with an additional keyword <<interface>>
. When a class implements an interface, it is shown similar to class inheritance except a dashed line is used instead of a solid line.
The AcademicStaff
and the AdminStaff
classes implement the SalariedStaff
interface.
A UML sequence diagram captures the interactions between multiple objects for a given scenario.
Consider the code below.
class Machine {
Unit producePrototype() {
Unit prototype = new Unit();
for (int i = 0; i < 5; i++) {
prototype.stressTest();
}
return prototype;
}
}
class Unit {
public void stressTest() {
}
}
Here is the sequence diagram to model the interactions for the method call producePrototype()
on a Machine
object.
Notation:
This sequence diagram shows some interactions between a human user and the Text UI of a Command Line InterfaceCLI Minesweeper game.
The player runs the newgame
action on the TextUi
object which results in the TextUi
showing the minefield to the player. Then, the player runs the clear x y
command; in response, the TextUi
object shows the updated minefield.
The :TextUi
in the above example denotes an unnamed instance of the class TextUi. If there were two instances of TextUi
in the diagram, they can be distinguished by naming them e.g. TextUi1:TextUi
and TextUi2:TextUi
.
Arrows representing method calls should be solid arrows while those representing method returns should be dashed arrows.
Note that unlike in object diagrams, the class/object name is not underlined in sequence diagrams.
[Common notation error] Activation bar too long: The activation bar of a method cannot start before the method call arrives and a method cannot remain active after the method has returned. In the two sequence diagrams below, the one on the left commits this error because the activation bar starts before the method Foo#xyz()
is called and remains active after the method returns.
[Common notation error] Broken activation bar: The activation bar should remain unbroken from the point the method is called until the method returns. In the two sequence diagrams below, the one on the left commits this error because the activation bar for the method Foo#abc()
is not contiguous, but appears as two pieces instead.
Notation:
The Logic
object creates a Minefield
object.
UML uses an X
at the end of the lifeline of an object to show its deletion.
Although object deletion is not that important in languages such as Java that support automatic memory management, you can still show object deletion in UML diagrams to indicate the point at which the object ceases to be used.
Notation:
Note how the below diagram shows the deletion of the Minefield
object.
Notation:
The Player
calls the mark x,y
command or clear x y
command repeatedly until the game is won or lost.
UML can show a method of an object calling another of its own methods.
Notation:
The markCellAt(...)
method of a Logic
object is calling its own updateState(...)
method.
In this variation, the Book#write()
method is calling the Chapter#getText()
method which in turn does a call back by calling the getAuthor()
method of the calling object.
UML uses alt
frames to indicate alternative paths.
Notation:
Minefield
calls the Cell#setMine
method if the cell is supposed to be a mined cell, and calls the Cell:setMineCount(...)
method otherwise.
UML uses opt
frames to indicate optional paths.
Notation:
Logic#markCellAt(...)
calls Timer#start()
only if it is the first move of the player.
UML uses par
frames to indicate parallel paths.
Notation:
Logic
is calling methods CloudServer#poll()
and LocalServer#poll()
in parallel.
If you show parallel paths in a sequence diagram, the corresponding Java implementation is likely to be multi-threaded because a normal Java program cannot do multiple things at the same time.
Method calls to static
(i.e., class-level) methods are received by the class itself, not an instance of that class. You can use <<class>>
to show that a participant is the class itself.
In this example, m
calls the static method Person.getMaxAge()
and also the setAge()
method of a Person
object p
.
Here is the Person
class, for reference:
To reduce clutter, activation bars and return arrows may be omitted if they do not result in ambiguities or loss of information. Informal operation descriptions such as those given in the example below can be used, if more precise details are not required for the task at hand.
A minimal sequence diagram
An object diagram shows an object structure at a given point of time.
An example object diagram:
Notation:
Notes:
car1:Car
are underlined.objectName:ClassName
is meant to say 'an instance of ClassName
identified as objectName
'.:Car
which is meant to say 'an unnamed instance of a Car object'.Some example objects:
Compared to the notation for class diagrams, object diagrams differ in the following ways:
:
before the class nameFurthermore, multiple object diagrams can correspond to a single class diagram.
Both object diagrams are derived from the same class diagram shown earlier. In other words, each of these object diagrams shows ‘an instance of’ the same class diagram.