activitydiagram.ppt

7/30/2019 ActivityDiagram.ppt

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CS 537 Group 3 Report:Activity Diagrams and

Observer Design Pattern

Anna Deghdzunyan

Xuan Yang

Keenan Knaur

John Hurley


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Part I: Activity Diagrams


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Introduction(I)

Activity diagrams describe procedural logic,business process, and workflow.

Activity diagrams focus on the action sequenceof execution and the conditions that trigger or

guard those actions.

Activity diagrams focus on the activitys internalactions, not on the external interfaces


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Introduction(II)

Activity diagrams have similarities to flowcharts

But flowcharts notation does not support parallelbehavior.

Business managers may prefer activity diagramsover flowcharts, because they are moreunderstandable for non-technical people.

An activity diagram is a special case of statechart diagram in which states are actions.


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Introduction(III)

An activity diagram shows flow control within asystem.

HandleIncident

ArchiveIncident

DocumentIncident


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Activity Diagram Elements

Initial node

Activity final node

Action

Flow/edge

Fork

Join Decision

Merge

Synch


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Basic Elements

Action(I)

Action in Activity Diagram Elements officialUML name is action state.

Distinction between action and activity

Action state refers to it as action Use term activityonly refer to the whole task

being modeled by the activity diagram


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Basic Elements

Action(II)

The rounded rectangle represents an action thatoccurs.

E.g., Customer calls ticket office :

A sample action that is part of an activity diagram

Customer Calls TicketOffice


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Basic Elements--Initial state

The filled circle is the staring point of thediagrams.

An initial node isnt required.

The initial state shows the starting point for the action sequencewithin an activity diagram.

First Action To DO


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Basic Elements--Initial state(II)

Initial state can indicate onlyONE action.

Incorrect rendering of an initial state within anactivity diagram. The initial state can indicate onlyONE action

Action 1

Action 2


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Basic Elements

Flow/edge

The arrow on the diagram. There is a subtledifference between flows and edges.


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Basic Elements

Final node

The filled circle with a border is the ending point.

An activity diagram can have zero or more activityfinal nodes

First Action To DO


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Decision

A diamond with one flow entering and severalleaving.

[Drink contains

alcohol]

[else] Get Drink ForCustomer

Make Sure Customer IsAt least 21 Years Old

CustomerOrders Drink


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Merge

A diamond with several flows entering and oneleaving.

[customers age < 21]

[Drink containsalcohol]

[customers age >= 21]

[else]

Customer

Orders Drink

Make SureCustomer Is At

least 21 Years Old

Get Drink ForCustomer

Tell Customer ToOrder A Non

Alcoholic Drink


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Synch

A thick, solid line, allowing two or more actionsequences to proceed in parallel

Action 1


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Fork Synch with one flow going into it and several

leaving it.

Denotes the beginning of parallel actions.

Receive Order

Verify CustomerHas Available

Credit

Verify OrderProducts Are InStock


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Join Synch with several flows entering and one leaving.

All incoming flows must reach it before processingmay continue. This denotes the end of parallel

processing.Verify Order

Products Are InStock

Verify CustomerHas Available

Credit

Accept Order


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Signals

An Activity diagram can have a clearly defined startpoint, which corresponds to an invocation of a

program or routine. Activity diagram can also show response to signals.

A time signal occurs because of the passage of time(for example, each month end might trigger a signal.)

A real time signal indicates that the activity receivesan event from an outside process.

The activity listens for those signals, and thediagram defines how the activity reacts.


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Signals Activity diagrams can show signals sent or

received

For example, we can send a message and thenwait for a reply before we can continue.

Basically, the signals are flow triggers.

Send signal Time signal Accept signal


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Flow

Connection between 2 actions Simple flow

- arrow- from a node to another

Flow with Exception


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Flows (cont.)

Flow with objects

Flow with pins- similar to flows with objects

- data needed and data produced


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Flows (cont.)

Decision flows

-labeled

Connectors- does the same job as a simplearrow


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Tokens flow through the diagrams:

The initial node creates a token, executes, passes

the token to the next Fork produces a token on each of its outward

flows.

On a join, as each inbound token arrives, nothinghappens until all the tokens appear at the join;then a token is produced on the outward flow.

Tokens


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Join Specification Boolean expression using the names of the incoming

edges to specify the conditions under which the join willemit a token.

Evaluatedwhenever a new token is offered on anyincoming edge.

Default - "and


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Expansion Region

Structured activity regionthat executes multipletimes corresponding toelements of an input

collection.

Example:

The hotels may be bookedindependently andconcurrently with each otherand with booking the flight.


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Advanced Notation

Conditional threads

Nested activity diagrams

Partitions


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Conditional threads

One of a set ofconcurrent threads isconditional.

Example:

Frequent-flyermember?

Award the passengerfrequent flyer miles.


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Nested Activity Diagram

diagram refers to anexternal one that usesmore abstraction


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Partitions

The contents of an activity

diagram may be organizedintopartitions

Does not have a formalsemantic interpretation

May represent

organizational unit


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Dimensional Partition


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Multidimensional Hierarchical Diagram


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Activity vs. Sequence Diagrams

Activity diagrams give focus to theworkflow

Sequence diagrams give focus to the handling ofbusiness entities.

Activity diagram with partitions focuses on how youdivide responsibilities onto classes

The sequence diagram helps you understand howobjects interact and in what sequence.


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When to Use Activity Diagrams Describe a behavior which contains parallel

activities

Or

Show how behaviors in several use-cases interact.


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Specification standards

No need for documenting the Activity diagrams

beyond diagram itself.

However, most UML tools provide in-built

documentation capturing and printingcapabilities for the Activity diagram and itselements.

UML A ti it di dit


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UML Activity diagram editor


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Questions on Activity Diagrams


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Part II: Observer Design Pattern

Also Known As:

Dependents,

Publish-Subscribe,and confusingly, Model-View


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Introduction to Observer AGang of Four design pattern, one of the patterns

discussed inDesign Patterns: Elements of ReusableObject-Oriented Softwareby Erich Gamma, RichardHelm, Ralph Johnson, and John Vlissides.

Observer defines a one-to-many or many-to-manydependency between objects.

When the state of one object changes, then all the

other objects that are dependent on that object areupdated automatically. Used for event handling where consistency between

objects is necessary, e.g. Swing Framework for GUIdevelopment.


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General Example Suppose you have some data

that can be displayed by atable, a bar graph or a pie

chart.

Changes to the underlyingdata should be reflected in allthree of the displays

This is where the Observer

Design Pattern comes inhandy.


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Motivation for Using Maintaining consistency between related objects

is necessary when a system contains a collection

of cooperating classes This consistency shouldnt be accomplished

through tightly coupling the classes since thisreduces the reusability of those tightly coupled

classes. Needs to be scalable. There should also be no

limit on the number of objects that depend onone or more other objects.


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You are coding an app in which a weather station updates three objects, one that displayscurrent conditions, one that calcs statistics over time (up to date), and one that makes aforecast.

Here is the obvious approach:

public class WeatherData {[declarations and getters/setters omitted]

public void measurements changed(){float temp = getTemperature();float humidity = getHumidity();float pressure = getPressure();

currentConditionsDisplay.update(temp, humidity, pressure);

statisticsDisplay.update(temp, humidity, pressure);forecastDisplay.update(temp, humidity, pressure);

}}

Freeman, Freeman, Sierra, and Bates, Head First Design Patterns,OReilly 2004

Example of the Problem:


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public void measurementsChanged(){float temp = getTemperature();

float humidity = getHumidity();

float pressure = getPressure();

currentConditionsDisplay.update(temp, humidity, pressure);

statisticsDisplay.update(temp, humidity, pressure);

forecastDisplay.update(temp, humidity, pressure);

}

Problems:

Area that is likely to change is mixed with area that is not likely to change update() calls are coded to concrete objects, not types

Need to change code if the subscribers change

Observer addresses these problems

Problems With The Obvious Approach


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Three Major Aspects of Observer1. The Subject, which is the object being

observed

2. The Observer, which observes a Subject3. Relationship between 1 and 2: attach/detach

(or subscribe / unsubscribe) and update


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Generalized Structure


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Generalized Structure (cont.) Subject

Interface for ConcreteSubjects Requires implementations to provide at least the following

methods: subscribe / attach unsubscribe / detach notify all observers of state changes

ConcreteSubject

Implements the Subject interface Maintains direct or indirect references to one or moreConcreteObservers

Keeps track of its own state When its state changes it sends a notification to all of its

Observers by calling their update() methods


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Generalized Structure (cont.) Observer

Interface for ConcreteObserver objects

Requires an update method

ConcreteObserver

This is the actual object that is observing the stateof the ConcreteSubject.

The state that it maintains should always beconsistent with the state of its Subject.

Implements update() method.


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Two Ways to Implement Updates The Push Model

Subject sends all of the necessary information about any of its changes toall the Observers.

Pushes information to the Observer as parameter with the update()method.

Requires assumptions about what the Observers need to know. May need to allow for subscription to relevant changes only, but this

adds complexity The Pull Model

The Subject sends an indication to the Observer that a change has

occurred. Observers use public methods of Subject to query information they want It is up to the Observer topullall of the necessary information from the

Subject in order to effect any relevant changes. Subject requires fewer assumptions about what the observers want to

know


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General Implementation Subjects can track Observers through ArrayLists

or other data structures.

Observers can track multiple Subjects and getdifferent data from each.

Pull model uses an update method that takes a

reference to Subject as a parameter. The Subject should trigger updates when itsstate changes.


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General ImplementationMethods that change state may call a stateChanged() method:

public void notifyObservers(){for(Observer o: observers)

o.update();

}

public void stateChanged(){// do other things

notifyObservers();

// do whatever else still needs doing}

public void setMeasurements(arguments.) {// set instance variables first

stateChanged();}


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Simple Example: Swing Button With Listeners

Swing JButtons are Subjects; Listeners are Observers

JButton extends AbstractButton, an abstract class thatrequires methods to add and remove listeners, as wellas several types of notify() methods

ActionListener requires that implementers haveactionPerformed() method (update())

Can add as many listeners as you like to JButton, aslong as they implement ActionListener


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Familiar Example: Swing Button With Listenerspublic class SwingObserverExample{

Jframe frame;[stuff omitted]

public void go() {frame = new JFrame();JButton button = new JButton("Should I do it?");

button.addActionListener(new AngelListener());button.addActionListener(new DevilListener());frame.getContentPane().add(BorderLayout.CENTER, button);[frame property code omitted]

}

// using inner classes in this very simple exampleclass AngelListener implements ActionListener {

public void actionPerformed(ActionEvent event) {System.out.println("Don't do it");

}

}

class DevilListener implements ActionListener {public void actionPerformed(ActionEvent event) {

System.out.println("Come on, do it!");}

}}

When we click the button, both listeners are notified and take action.Freeman Freeman Sierra and Bates Head First Desi n Patterns OReill 200 .


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Implementation in Java Java has built-in support for Observer

java.util.Observable class can be extended bya Subject

java.util.Observer interface can beimplemented by a class that wants to observe aSubject


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UML Diagram for

Observable/Observer Classes


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Methods in java.util.Observable Observable()

Creates an Observable object (Subject) with no Observers initially setChanged()

Indicates that this Subject has changed in some way.

hasChanged() Returns True if the setChanged() method has been called more recently

than the clearChanged() method. Returns False if otherwise.

clearChanged() Indicates that this object is done notifying all of its observers of its most

recent changes. It is called automatically by notifyObservers() method countObservers()

Returns the number of objects that are Observing this Subject.


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Methods in java.util.Observable (cont.)

addObserver(Observer o)

Adds the passed Observer object to the list of

Observers kept by the Subject deleteObserver(Observer o) / deleteObservers()

Removes the passed Observer object or all of theObserver objects respectively from the list of

Observers kept by the Subject


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Methods in java.util.Observable (cont.)

notifyObservers(Object arg) / notifyObservers()

If this Subject has changed, this method notifies

all of its Observers and then calls theclearChanged() method. When given an arg as aparameter in the function call, the Observer knowswhich attribute of the Subject has changed

otherwise the Observer can be notified withoutspecifying an arg.


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Methods in java.util.Observer update(Observable o, Object arg)

Called when the Subject has changed. o is the

Subject in question, and arg is an argument thatcan be passed to tell the Observer which attributeof the Subject has changed.


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Limitations of Built-In Implementation

Observable is a class, not an interface

Cant add its behavior to a concrete class thatsubclasses something else.

Since there is no Observable interface, you cantcreate an impl that works with Observer butdoesnt subclass Observable.

Cant compose another class that has an

Observable since setChanged() is protected


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Questions on Observer Pattern


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Benefits of the Observer Pattern Minimal coupling between the Subject and Observer

Objects

Many Observers can be added to a Subject withouthaving to modify the Subject. Reuse of Subjects without needing to also reuse any

of their Observers. The opposite also holds true. The only thing a Subject needs to keep track of is its

list of Observers. The Subject does not need to know the concrete

classes of its Observers, only that each oneimplements the Observer interface


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Trouble Spots

Cascading notifications if Observers update their ownclients or if they can also make changes to the Subject Repeated notifications when sequences of changes

occur. Dangling references to Subjects or Observerswhen

either type are manually deleted in non-garbagecollected environments. Need to notify Observerswhen Subjects are deleted and vice-versa.

Subject state must be self-consistent before callingnotify(), especially with pull model.

Careful not to push irrelevant information onobservers with push model.

If update() fails, the Observer wont know that itmissed potentially important information

References


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ReferencesBooks:

Freeman, Freeman, Sierra, and Bates, Head First Design Patterns,OReilly 2004, p. 73

Gamma, Helm, Johnson, and Vlissides, Design Patterns: Elements of Reusable Object-Oriented Software, Addison-Wesley1995.

Websites:

Activity Diagrams:

http://www.agilemodeling.com/artifacts/activityDiagram.htm

http://computersciencesource.wordpress.com/2010/03/15/software-engineering-activity-diagrams/

http://www.cse.unt.edu/~rgoodrum/Teaching/2009/fall/4910/website%20files/handouts/umlbasicsp2_actdg.pdf

http://www.devx.com/ibm/Article/21615

http://edutechwiki.unige.ch/en/UML_activity_diagram http://www.sa-depot.com/?p=158

http://sourcemaking.com/uml/modeling-business-systems/external-view/activity-diagrams

http://www.uml-diagrams.org/activity-diagrams.html

Observer:

http://www.blackwasp.co.uk/Observer.aspx

http://www.cs.clemson.edu/~malloy/courses/patterns/observer.html

http://www.codeproject.com/KB/architecture/Observer_Design_Pattern.aspx

http://java.dzone.com/articles/observer-pattern?utm_source=am6_feedtweet&utm_medium=twitter&utm_campaign=toya256ForRSS

http://www.patterndepot.com/put/8/observer.pdf

http://sourcemaking.com/design_patterns/observer

http://userpages.umbc.edu/~tarr/dp/lectures/Observer.pdf

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