The basic UML notations used to represent a class:
A Table
class shown in UML notation:
The 'Operations' compartment and/or the 'Attributes' compartment may be omitted if such details are not important for the task at hand. Similarly, some attributes/operations can be omitted if not relevant to the purpose of the diagram. '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 private Table
class with visibilities shown:
There is no default visibility in UML. If a class diagram does not show the visibility of a , it simply means the visibility is unspecified (for reasons such as the visibility not being decided yet or it being not important to the purpose of the diagram).
Generic classes can be shown as given below. The notation format is shown on the left, followed by two examples.
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:
Use arrowheads to indicate the navigability of an association.
In this example, the navigability is unidirectional, and is from the Logic
class to the Minefield
class. That means if a Logic
object L
is associated with a Minefield
object M
, L
has a reference to M
but M
doesn't have a reference to L
.
class Logic {
Minefield minefield;
// ...
}
class Minefield {
//...
}
class Logic:
def __init__(self):
self.minefield = None
# ...
class Minefield:
# ...
Here is an example of a bidirectional navigability; i.e., if a Dog
object d
is associated with a Man
object m
, d
has a reference to m
and m
has a reference to d
.
class Dog {
Man man;
// ...
}
class Man {
Dog dog;
// ...
}
class Dog:
def __init__(self):
self.man = None
# ...
class Man:
def __init__(self):
self.dog = None
# ...
The arrowhead (not the entire arrow) denotes the navigability. The line denotes the association, as before. So, the navigability (i.e., the arrowhead) is an extra annotation added to an association line.
For example, both diagrams below show an association between Foo
and Bar
. But the one on the right shows the navigability as well.
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 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 roles are optional to show. They are particularly useful for differentiating among multiple associations between the same two classes.
In each the three associations between the Flight
class and the Airport
class given below, the Airport
class plays a different role.
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 e.g., 2..5
, 1..*
(one or more), *..5
(up to five) 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,
There is no default multiplicity in UML. If a class diagram does not show the multiplicity of an association, it simply means the multiplicity is unspecified.
UML uses a dashed arrow to show dependencies.
Two examples of dependencies:
Dependencies vs associations:
Foo
accessing a constant in Bar
but there is no association/inheritance from Foo
to Bar
.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.
Diagram (a) given below shows the 'author' association between the Book
class and the Person
class as a line while (b) shows the same association as an attribute in the Book
class. Both are correct and the two are equivalent. But (c) is not correct as it uses both a line and an attribute to show the same association.
(a)
(b)
(c)
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.
You can use a triangle and a solid line (not to be confused with an arrow) to indicate class inheritance.
Notation:
Examples: The Car
class inherits from the Vehicle
class. The Cat
and Dog
classes inherit from the Pet
class.
It does not matter whether the triangle is filled or empty.
Here's an example that combines inheritance with generics:
class Foo<T> {
}
class Bar<T> extends Foo<T> {
}
class Goo extends Foo<String> {
}
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 entities 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 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.
The arrowhead style depends on the type of method call. Arrows showing synchronous (i.e., the caller method is blocked from doing anything else until the called method returns) should use filled arrowheads (e.g., ). Most method calls (e.g., normal Java method calls) are synchronous. Asynchronous method calls (i.e., the caller method does not have to wait till the called method returns) are shown using lined arrowheads (e.g., ). As the latter is out of scope for this textbook, all sequence diagram arrow heads you encournter in this textbook will be of the first type.
[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 while the method is being executed, 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 broken in the middle.
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.
Notation:
Note how the below diagram shows the deletion of the Minefield
object.
Although languages such as Java do not support a delete
operation (because they use automatic memory management), you can use the object deletion notation to indicate the point at which the object becomes ready to be garbage-collected (i.e., the point at which it ceases to be referenced).
Note how d
lifeline ends with an X
to show that it is 'deleted' (i.e., ready to be garbage collected) after the cook()
method returns.
class Chef {
void cook() {
Dish d = new Dish();
}
}
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.
'Unroll' chained/compound method calls before drawing sequence diagram. Consider the Java statement new Book().add(new Chapter());
. How do we show it as a sequence diagram? First, 'unroll' it into a simpler series of statements, which can then be drawn as a sequence diagram easily. For example, that statement is equivalent to the following:
Book b = new Book();
Chapter c = new Chapter();
b.add(c);
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.
No more than one alternative partitions be executed in an alt
frame. That is, it is acceptable for none of the alternative partitions to be executed but it is not acceptable for multiple partitions to be executed.
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 LocalData#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.
UML uses ref frame to allow a segment of the interaction to be omitted and shown as a separate sequence diagram. Reference frames help you to break complicated sequence diagrams into multiple parts or simply to omit details you are not interested in showing.
Notation:
The details of the get minefield appearance
interactions have been omitted from the diagram.
Those details are shown in a separate sequence diagram given below.
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, optional elements (e.g, activation bars, return arrows) may be omitted if the omission does not result in ambiguities or loss of . 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
If method parameters don't matter to the purpose of the sequence diagram, you can omit them using ...
e.g., use foo(...)
instead of foo(int size, double weight)
.
An object diagram shows an object structure at a given point of time.
An example object diagram:
Notation:
Notes:
car1:Car
.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:
An activity diagram (AD) captures an activity through the actions and control flows that make up the activity.
Note the slight difference between the start node and the end node which represent the start and the end of the activity, respectively.
This activity diagram shows the action sequence of the activity a passenger rides the bus:
A branch node shows the start of alternate paths. Each control flow exiting a branch node has a guard condition: a boolean condition that should be true for execution to take that path. Exactly one of the guard conditions should be true at any given branch node.
A merge node shows the end of alternate paths.
Both branch nodes and merge nodes are diamond shapes. Guard conditions must be in square brackets.
The AD below shows alternate paths involved in the workflow of the activity shop for product:
Some acceptable simplifications (by convention):
[Else]
condition.The AD below illustrates the simplifications mentioned above:
Fork nodes indicate the start of flows of control.
Join nodes indicate the end of parallel paths.
Both have the same notation: a bar.
In a , execution along all parallel paths should be complete before the execution can start on the outgoing control flow of the join.
In this activity diagram (from an online shop website) the actions User browses products and System records browsing data happen in parallel. Both of them need to finish before the log out action can take place.
The rake notation is used to indicate that a part of the activity is given as a separate diagram.
Here is the AD for a game of ‘Snakes and Ladders’.
The rake symbol (in the Move piece
action above) is used to show that the action is described in another subsidiary activity diagram elsewhere. That diagram is given below.
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.
When the class diagram has an inheritance relationship, the object diagram should show either an object of the parent class or the child class, but not both.
Suppose Employee
is a child class of the Person
class. The class diagram will be as follows:
Now, how do you show an Employee
object named jake
?
This is not correct, as there should be only one object.
This is OK.
This is OK, as jake
is a Person
too.
That is, we can show the parent class instead of the child class if the child class doesn't matter to the purpose of the diagram (i.e., the reader of this diagram will not need to know that jake
is in fact an Employee
).
Association labels/roles can be omitted unless they add value (e.g., showing them is useful if there are multiple associations between the two classes in concern -- otherwise you wouldn't know which association the object diagram is showing)
Consider this class diagram and the object diagram:
We can clearly see that both Adam and Eve lives in hall h1 (i.e., OK to omit the association label lives in
) but we can't see if History is Adam's major or his minor (i.e., the diagram should have included either an association label or a role there). In contrast, we can see Eve is an English major.