Software Diagrams
martinpham
SE311SequenceDiagram.pptxUML Sequence Diagram
UML sequence diagrams
Sequence diagram: an "interaction diagram" that models scenarios executing in the system
perhaps 2nd most used UML diagram (behind class diagram)
Relation of UML diagrams to other exercises:
CRC cards -> class diagram
Use cases -> sequence diagrams
One sequence diagram can show multiple scenarios (user stories)
2
Key parts of a sequence diag.
participant: an object or entity that acts in the sequence diagram
sequence diagram starts with an unattached "found message" arrow
message: communication between participant objects
the axes in a sequence diagram:
horizontal: which object/participant is acting
vertical: time (down -> forward in time)
3
Sequence diag. from use case
4
Representing objects
squares with object type, optionally preceded by object name and colon
write object's name if it clarifies the diagram
object's "life line" represented by dashed vert. line
5
Messages between objects
message (method call) indicated by horizontal arrow to other object
write message name and arguments above arrow
dashed arrow back indicates return
different arrowheads for normal / concurrent (asynchronous) methods
6
Lifetime of objects
creation: arrow with 'new' written above it
notice that an object created after the start of the scenario appears lower than the others
deletion: an X at bottom of object's lifeline
Java doesn't explicitly delete objects; they fall out of scope and are garbage-collected
7
Indicating method calls
activation: thick box over object's life line; drawn when object's method is on the stack
either that object is running its code, or it is on the stack waiting for another object's method to finish
nest to indicate recursion
8
Activation
Nesting
Indicating selection and loops
frame: box around part of a sequence diagram to indicate selection or loop
if -> (opt) [condition]
if/else -> (alt) [condition], separated by horiz. dashed line
loop -> (loop) [condition or items to loop over]
9
[
balance
<>
0
]
opt
[
balance
<
100
.
00
]
[
balance
>
=
100
.
00
]
alt
[
balance
<
0
]
loop
linking sequence diagrams
if one sequence diagram is too large or refers to another diagram, indicate it with either:
an unfinished arrow and comment
a "ref" frame that names the other diagram
when would this occur in our system?
10
Example sequence diagram
11
Scenario (User Story)
A player rolls the dice and gets a 6. The player moves 6 cells. The player lands on a cell that is an un-owned property. The player’s turn is over.
You begin to create a sequence diagram by writing a scenario and highlighting all the nouns in the scenario. The nouns generally become the objects that appear in the columns. For example, consider the following scenario. The unique nouns are circled. Each of these nouns is likely to become an object (though not all do, as you will see with the noun “turn”).
[click] This figure shows a possible sequence diagram for this scenario.
Conditional Logic
If the player lands on a cell that is an un-owned property, the player’s turn is over.
If the player lands on a cell that is owned, the player must pay rent to the owner of the property.
Then, the player’s turn is over.
Conditional logic may also be expressed in a sequence diagram as a guard. In this way, a sequence diagram can express a set of scenarios in a more general way.
Consider this scenario and the sequence diagram in the figure that can be used for this scenario:
A player rolls the dice and moves the number of cells indicated on the dice. [click] If the player lands on a cell that is an un-owned property, the player’s turn is over. [click] If the player lands on a cell that is owned, the player must pay rent to the owner of the property. Then, the player’s turn is over.
You can use the “else” part of the sequence diagram to express what could be thought of as a second scenario in which the player lands on an owned property and has to pay rent.
13
(De)centralized system control
What can you say about the control flow of each of the following systems?
centralized?
distributed?
14
Why not just code it?
Sequence diagrams can be somewhat close to the code level. So why not just code up that algorithm rather than drawing it as a sequence diagram?
15
a good sequence diagram is still a bit above the level of the real code (not EVERY line of code is drawn on diagram)
sequence diagrams are language-agnostic (can be implemented in many different languages
non-coders can do sequence diagrams
easier to do sequence diagrams as a team
can see many objects/classes at a time on same page (visual bandwidth)
Verify customer credit
refCustomer Info
Approved?
[guard]
[guard]
[guard]
[guard]
[guard]
[guard]
[guard]
[guard]
Verify customer credit
ref
Customer Info
Approved?
sd Example
loop
StoreFrontCartInventory
AddItem
ReserveItem
PlaceItemInOrder
Checkout
ProcessOrder
ConfirmOrder
[guard]
[guard]
[guard]
[guard]
[guard]
[guard]
[guard]
[guard]
sd Example
StoreFront
Cart
Inventory
loop
AddItem
ReserveItem
PlaceItemInOrder
Checkout
ProcessOrder
ConfirmOrder
Player
DiceCellProperty
rollDice
DiceValue(6)
MoveCell(6)
isOwnedProperty
isOwnedProperty(False)
rollDice
DiceValue(6)
MoveCell(6)
isOwnedProperty
isOwnedProperty(False)
Player
Dice
Cell
Property
Player
Dice
CellProperty
rollDice
DiceValue(n)
MoveCells(n)
[isOwnedProperty]
isOwnedProperty(False)
isOwnedProperty(True,owner)
Owner
PayOwner
[else]
Player
Dice
Cell
Property
rollDice
DiceValue(n)
MoveCells(n)
[isOwnedProperty]
isOwnedProperty(False)
isOwnedProperty(True,owner)
Owner
PayOwner
[else]
