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Sequential Diagrams

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Use Case
This is a collection of interactions between the external actors and the system.

In UML, a use case is

the specification of a sequence of actions
including the variant,
that the system can perform
interacting with the actors of the system

Typically a use case contains

  • One Primary scenario (Main Course)
  • Zero or more secondary scenarios (Alternative Courses)

In Rational Unified Process, user requirements are captured as the Use Cases and they are refined in to the Scenarios.

A Scenario is one path / flow through a USE CASE.
A Scenario describes a sequence of events that occurs during one particular execution of the system.

UML Sequence Diagrams

Describes :
Flow of the Messages, Events and Actions between objects.

Shows :
Concurrent Process and Actions
Time Sequences (Not easily depicted by other diagrams)

Typically used during the AnD during to understand and document the logical flow of a system.

Key Parts.

  • Participants  : Objects / Entity that acts in the diagram
  • Message – communication between participant objects. Here, the diagram starts with and unattached “found message” arrow
  • Axes –
    • Horizontal : which object / participant is acting.
    • Vertical : time

Eg :

Sequential Diagram for making a phone call


Representing Objects

Square – Object type preceded by “name:”
Vertical dashed line – Object’s Life Line.

Object Representation in a Sequencial Diagram







Representing Messages between objects
Messages (method calls) are indicated by an arrow to other object.
Writes message name and arguments above arrow.

Screenshot from 2014-02-20 05:45:28






Different arrow heads for different calls

Screenshot from 2014-02-20 05:45:46

Screenshot from 2014-02-20 05:46:13

Lifetime of the objects.

Creation :
Arrow with ‘new’ written above it.
Object created after start of the scenario appears loweer than others.

Deletion :
An X at the bottom of the object’s lifeline.
Java doesn’t explicitly delete objects. They fall out of scope and are garbage-collected.

Indicating method calls.
Thick box over the object’s life line is drawn when the object’s method is on the stack.
– Object may be running it’s code.
– It is on the stack waiting for another object’s methods to finish..

Nested activations : To indicate the recursion.

Selection and loops

  • Frame is denoted by a  box around a part of the diagram indicates if or a loop.
  • loop  is denoted by (loop) [condition or items to loop over]
  • if only is denoted by (opt) [condition]
  • if / else is denoted by (alt) [condition]
    • – (else)
    • – if / else is separated by dashed lines

Screenshot from 2014-02-20 05:47:51

Reference :


Written by Namal Fernando

February 20, 2014 at 5:21 am

Posted in Sequencial Diagrams, UML

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