CS 537 Group 3 Report: Activity Diagrams and

Observer Design Pattern

Anna DeghdzunyanXuan YangKeenan KnaurJohn Hurley

Part I: Activity Diagrams

Introduction(I)

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

•Activity diagrams focus on the action sequence of execution and the conditions that trigger or guard those actions.

•Activity diagrams focus on the activity’s internal actions, not on the external interfaces

Introduction(II)•Activity diagrams have similarities to

flowcharts▫But flowcharts notation does not support

parallel behavior.▫Business managers may prefer activity

diagrams over flowcharts, because they are more understandable for non-technical people.

•An activity diagram is a special case of state chart diagram in which states are actions.

Introduction(III)

•An activity diagram shows flow control within a system.

HandleIncident

ArchiveIncident

DocumentIncident

Activity Diagram Elements

•Initial node•Activity final node•Action•Flow/edge•Fork•Join•Decision•Merge•Synch

Basic Elements—Action(I)

•Action in Activity Diagram Elements’ official UML name is action state.

•Distinction between action and activity▫Action state refers to it as action▫Use term activity only refer to the whole

task being modeled by the activity diagram

Basic Elements—Action(II)

•The rounded rectangle represents an action that occurs.

•E.g., Customer calls ticket office :

A sample action that is part of an activity diagram

Customer Calls Ticket Office

Basic Elements--Initial state

•The filled circle is the staring point of the diagrams.An initial node isn’t required.

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

First Action To DO

Basic Elements--Initial state(II)

•Initial state can indicate only ONE action.

Incorrect rendering of an initial state within an activity diagram. The initial state can indicate only ONE action

Action 1

Action 2

Basic Elements—Flow/edge

•The arrow on the diagram. There is a subtle difference between flows and edges.

Basic Elements—Final node

•The filled circle with a border is the ending point.An activity diagram can have zero or more

activity final nodes

First Action To DO

Decision

Get Drink For Customer

Make Sure Customer Is At least 21 Years

Old

Customer Orders Drink

Merge

Customer Orders Drink

Make Sure Customer Is At least 21 Years

Old

Get Drink For

Customer

Tell Customer To Order A Non Alcoholic Drink

Synch

•A thick, solid line, allowing two or more action sequences to proceed in parallel

Action 1

Fork•Synch with one flow going into it and

several leaving it.•Denotes the beginning of parallel actions.

Receive Order

Verify Customer Has

Available Credit

Verify Order Products Are

In Stock

Join•Synch with several flows entering and one

leaving.•All incoming flows must reach it before

processing may continue. This denotes the end of parallel processing.Verify Order

Products Are In Stock

Verify Customer Has

Available Credit

Accept Order

Signals

• An Activity diagram can have a clearly defined start point, 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 receives an event from an outside process.

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

Signals•Activity diagrams can show signals sent or

received•For example, we can send a message and

then wait for a reply before we can continue.

•Basically, the signals are flow triggers.

Send signal Time signal Accept signal

Flow•Connection between 2 actions• Simple flow - arrow - from a node to another

•Flow with Exception

Flows (cont.)

•Flow with objects

•Flow with pins - similar to flows with objects - data needed and data

produced

Flows (cont.)

•Decision flows -labeled

•Connectors - does the same job as a

simple arrow

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, nothing happens until all the tokens appear at the join; then a token is produced on the outward flow.

Tokens

Join Specification• Boolean expression using the names of the

incoming edges to specify the conditions under which the join will emit a token.

• Evaluated whenever a new token is offered on any incoming edge.

• Default - "and”

Expansion Region

• Structured activity region that executes multiple times corresponding to elements of an input collection.

• Example: The hotels may be

booked independently and concurrently with each other and with booking the flight.

Advanced Notation

•Conditional threads

• Nested activity diagrams

• Partitions

Conditional threads

•One of a set of concurrent threads is conditional.

• Example: Frequent-flyer

member? Award the

passenger frequent flyer miles.

Nested Activity Diagram

•diagram refers to an external one that uses more abstraction

Partitions •The contents of an

activity diagram may be organized into partitions

•Does not have a formal semantic interpretation

•May represent organizational unit

Dimensional Partition

Dimensional Partition

Dimensional Partition

Multidimensional Hierarchical Diagram

Activity vs. Sequence Diagrams

•Activity diagrams give focus to the workflow•Sequence diagrams give focus to the

handling of business entities.

•Activity diagram with partitions focuses on how you divide responsibilities onto classes

•The sequence diagram helps you understand how objects interact and in what sequence.

When to Use Activity Diagrams

•Describe a behavior which contains parallel activities

Or

•Show how behaviors in several use-cases interact.

Specification standards

•No need for documenting the Activity diagrams

beyond diagram itself.

• However, most UML tools provide in-built documentation capturing and printing

capabilities for the Activity diagram and its elements.

UML Activity diagram editor

Questions on Activity Diagrams

Part II: Observer Design Pattern

Also Known As:Dependents, Publish-Subscribe,and confusingly, Model-View

Introduction to Observer• A Gang of Four design pattern, one of the

patterns discussed in Design Patterns: Elements of Reusable Object-Oriented Software by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides.

• Observer defines a one-to-many or many-to-many dependency between objects.

• When the state of one object changes, then all the other objects that are dependent on that object are updated automatically.

• Used for event handling where consistency between objects is necessary, e.g. Swing Framework for GUI development.

General Example• Suppose you have some

data that can be displayed by a table, a bar graph or a pie chart.

• Changes to the underlying data should be reflected in all three of the displays

• This is where the Observer Design Pattern comes in handy.

Motivation for Using•Maintaining consistency between related

objects is necessary when a system contains a collection of cooperating classes

•This consistency shouldn’t be accomplished through tightly coupling the classes since this reduces the reusability of those tightly coupled classes.

•Needs to be scalable. There should also be no limit on the number of objects that depend on one or more other objects.

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

• 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,O’Reilly 2004

Example of the Problem:

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

Three Major Aspects of Observer

1. 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

Generalized Structure

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

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

of its Observers by calling their update() methods

Generalized Structure (cont.)

•Observer▫Interface for ConcreteObserver objects▫Requires an update method

•ConcreteObserver▫This is the actual object that is observing

the state of the ConcreteSubject.▫The state that it maintains should always

be consistent with the state of its Subject.▫Implements update() method.

Two Ways to Implement Updates• The Push Model

▫ Subject sends all of the necessary information about any of its changes to all 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 to pull all 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

General Implementation

•Subjects can track Observers through ArrayLists or other data structures.

•Observers can track multiple Subjects and get different data from each.

•Pull model uses an update method that takes a reference to Subject as a parameter.

•The Subject should trigger updates when its state changes.

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();}

Simple Example: Swing Button With Listeners

•Swing JButtons are Subjects; Listeners are Observers

•JButton extends AbstractButton, an abstract class that requires methods to add and remove listeners, as well as several types of notify() methods

•ActionListener requires that implementers have actionPerformed() method (update())

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

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 Design Patterns,O’Reilly 2004, p. 73

Implementation in Java

•Java has built-in support for Observer

• java.util.Observable class can be extended by a Subject

• java.util.Observer interface can be implemented by a class that wants to observe a Subject

UML Diagram for Observable/Observer Classes

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.

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 the Observer objects respectively from the list of Observers kept by the Subject

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 the clearChanged() method. When given an arg as a parameter in the function call, the Observer knows which attribute of the Subject has changed otherwise the Observer can be notified without specifying an arg.

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 that can be passed to tell the Observer which attribute of the Subject has changed.

Limitations of Built-In Implementation•Observable is a class, not an interface

▫Can’t add its behavior to a concrete class that subclasses something else.

▫Since there is no Observable interface, you can’t create an impl that works with Observer but doesn’t subclass Observable.

•Can’t compose another class that has an Observable since setChanged() is protected

Questions on Observer Pattern

Benefits of the Observer Pattern

• Minimal coupling between the Subject and Observer Objects

• Many Observers can be added to a Subject without having 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 one implements the Observer interface

Trouble Spots• Cascading notifications if Observers update

their own clients or if they can also make changes to the Subject

• Repeated notifications when sequences of changes occur.

• “Dangling references” to Subjects or Observers when either type are manually deleted in non-garbage collected environments. Need to notify Observers when Subjects are deleted and vice-versa.

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

• Careful not to push irrelevant information on observers with push model.

• If update() fails, the Observer won’t know that it missed potentially important information

ReferencesBooks:• Freeman, Freeman, Sierra, and Bates, Head First Design Patterns,O’Reilly 2004, p. 73• Gamma, Helm, Johnson, and Vlissides, Design Patterns: Elements of Reusable Object-Oriented Software,

Addison-Wesley 1995.

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• http://en.wikibooks.org/wiki/Java_Programming/Design_Patterns#Observer• http://www.wohnklo.de/patterns/observer.html