1. Slide design: Dr. Mark L. Hornick
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4/27/2017
Week 4, Day 1: Observer
Don't sit in front row
Review Schedule
Return Quiz 2 (I have it with me)
For next time: Write member variable names on arrows in UML diagrams; reuse code;
Review patterns
Review coupling vs. cohesion
The Observer Pattern
Muddiest Points
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Slide design: Dr. Mark L. Hornick
Content: Dr. Hornick
Errors: Dr. Yoder
Dr. Yoder

2. Patterns Muddiest Point
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4/27/2017
Patterns Muddiest Point
We really didn't cover any sorting algorithms and what each's benefits are
Can we do an example???
Related to the quiz for below question
Do we need to implement code versions of the strategy and factory method patterns?? If so how extensive and will it be similar to how we implemented in class?
Implementing the methods needed for the factory method pattern.
Is there a flowchart/checklist to determine which pattern is more appropriate to use?
Choosing the correct design pattern
What are Coupling and Cohesion and is it better to have high or low coupling or cohesion?
On a quiz/test would you ask us to draw the uml of a particular general pattern, or would you just ask us to write the code?
Why would we use Factory Pattern?
How do you know when to use what pattern to solve a problem?
Diagrams
High/Low Cohesion/Coupling
Class Diagram for Factory Design Pattern
Nothing
Dr. Yoder

3. In your assigned groups, discuss
Strategy, Singleton, Factory Method, multithreading
Brainstorm what you've learned
Pick three most important things to know about that pattern/subject to share with rest of the class
3 minutes
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This exercise recommended by Dr. Hasker

4. Coupling vs. Cohesion
Module 4 shows what we are trying to avoid: low cohesion and high coupling
(more circles==lower)
(more lines==higher)
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Dr. Mark L. Hornick

5. Motivating application: Microsoft Word
How to update toolbars every time user clicks somewhere different in the document? [Demo]
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Dr. Mark L. Hornick

6. Solution 1: (What can be improved?)
public void onClick(ClickEvent e) { if(cursorInBoldText()) { boldButton.setHighlight(); } styleDialog.setStyle(getCurrentCursorStyle()); if(selection.isActive()) { copyButton.setActive(); } /*… etc. … */
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Dr. Yoder

7. Observer Pattern Context
A system contains objects exhibiting:
One-to-many dependency between objects
One object changes state
All dependents are notified and updated automatically
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Slide content originally by Dr. Hornick

8. What are we trying to achieve with the Observer Pattern ?
Separation of software subsystems
Separation between GUI & Domain objects
Loosely-coupled classes to …
Avoid editing code in multiple places
Increase reusability
Increase understanding
Avoid polling
A generic/elegant way for the classes to communicate
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Slide content originally by Dr. Hornick

9. Key components in the Observer Pattern
Subject
Subject has dependent observers.
Observer(s)
When the state of the subject changes, each dependent observer is notified.
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Slide content originally by Dr. Hornick

10. Slide content originally by Dr. Hornick
Generic Subject class
class SubjectClass implements Subject {
public SubjectClass();
public void attach(Observer obs);
public void detach(Observer obs);
public void notifyObservers();
private ArrayList <Observer> observers; }
Subject::attach(Observer* pObserver) {
m_hObservers.push_back(pObserver); }
Subject::detach(Observer* pObserver)
m_hObservers.remove(pObserver);
Subject::notify()
Vector<Observer*>::iterator m_ppObserver;
for (m_ppObserver = m_hObservers.begin();m_ppObserver = m_hObservers.end(); ++m_ppObserver)
(*m_ppObserver)->update();
Note: Some texts define a notify() instead of notifyObservers() method. However, Java’s Object class already has a notify() method, which we don’t want to override.
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Slide content originally by Dr. Hornick

11. Slide content originally by Dr. Hornick
Generic Observer
class ObserverClass implements Observer {
public ObserverClass();
public void update(???); }
What is the appropriate argument for the update() method?
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Slide content originally by Dr. Hornick

12. Basic class relationships
Subject
attach():void detach():void notifyObservers():void
-observers
Observer
update(???):void
0..*
SubjectClass
ObserverClass1
ObserverClass2
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Slide content originally by Dr. Hornick

13. Collaborations between objects in the Observer pattern
s:SubjectClass
o1:ObserverClass1
o2:ObserverClass2
attach()
attach()
notifyObservers()
update(???)
getContextSpecificInfo()
update(???)
getContextSpecificInfo()
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Slide content originally by Dr. Hornick

14. LinearSubject example
[write notes on back page, see code online]
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Dr. Mark L. Hornick

15. Weather Program example
class WeatherData implements Subject {
//private data attributes
List<Observer> observers; ...
public WeatherData(){…}
public void getTemp() {…}
public int getWindSpeed() {…}
public void attach(Observer obs) {…}
public void detach(Observer obs) {…}
public void notifyObservers() {…}
... }
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Slide content originally by Dr. Hornick

16. Slide content originally by Dr. Hornick
Example (contd.)
public void acquireDataFromSensors() {
// acquire updated weather data ……
notifyObservers(); // notify observers }
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Slide content originally by Dr. Hornick

17. Slide content originally by Dr. Hornick
Example (contd.)
class mainDisplay extends Observer {
public mainDisplay (WeatherData wd){...}
public void update(???) {...}
public void updateDisplayUI() {...} }
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Slide content originally by Dr. Hornick

18. Slide content originally by Dr. Hornick
Example (contd.)
public mainDisplay(WeatherData wd) {
Subject wdSubject = wd;
wdSubject.attach(this); }
// What do we pass to update()?
public void update(???)
// How do we get data from the Subject?
updateDisplayUI(???); // mainDisplay class method
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Slide content originally by Dr. Hornick

19. Implementation Questions
What should be the arguments of the update method? Should we send the Subject as the argument?
Should each instance of the Observer store the “concrete subject” as a data attribute, or just an Interface reference?
Can Subject be an abstract class instead of an Interface?
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Slide content originally by Dr. Hornick

20. Consequences (positive)
Coupling between Subject and Observers:
Subject knows it has a list of Observers, but not specific classes
Each Observer conforms to the simple interface of the abstract Observer Interface.
Hence, coupling is
Minimal
Abstract
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Slide content originally by Dr. Hornick

21. Consequences (positive)
Cohesion is increased from single-class implementation
State management and display/response are separated
E.g. GUI innards separated from “your code”
E.g. Web access separated from display
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Slide content originally by Dr. Hornick

22. Consequences (negative)
Broadcast communication
Notification is broadcast to all interested objects.
Observers can be added/removed at any time.
Observer decides when it needs to be notified.
Unexpected updates
Observers have no knowledge
Of each other’s presence.
About the cost of “state change of subject”
Cascade of updates.
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Slide content originally by Dr. Hornick

23. https://www.polleverywhere.com/free_text_polls/zIp2tH2IVWjXv4H SE-2811
4/27/2017
Muddiest Point
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Dr. Mark L. Hornick
Dr. Yoder

24. Threading Muddiest Point [To be revisited in a future lecture]
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4/27/2017
Threading Muddiest Point [To be revisited in a future lecture]
All lambda examples the same code?
Lambda examples
When would you use a Lambda expression over an anonymous class and vice versa?
Lambda vs. Anon inner syntax
The Lambda is a Java thing, not an intelliJ thing right?
Java 8
whens the next quiz?
Quiz
Agree
More examples of using lambdas (didn't get exposure in software dev 2)
Effectively Final?
"Effectively Final"
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Dr. Mark L. Hornick
Dr. Yoder