1. DESIGN PATTENS - OBSERVER PATTERN
By,
Srinivas Reddy.S

2. A Simple Requirement Display Boards Websites www.JAVA9S.com
Mobile apps

3. A Simple Requirement.. Stock Market
An application in the server monitors and gathers information related to Stock prices.
The changed stock prices should be informed to different parties like stock market display boards, mobile apps, websites etc.,

4. stockPricesChanged()
StockUpdate
getStockPrices()
stockPricesChanged()
observable
MobileDisplay
Update( )
observers
StockBoardDisplay
Update( )
WebDisplay
Update( )

5. stockPricesChanged()
mobileDisplay.update(stockUpdates);
stockBoardDisplay.update(stockUpdates);
webdisplay.update(stockUpdates); }
What if there are few more observers to be added?
Violates the OO principles –
Less dependency
Less number of changes

6. Stock Update StockBoard One to Many Relationship Mobile WEBSITE OTHER

7. OBSERVER PATTERN
OBSERVER PATTERN defines one to many relationship between the objects.
When the state of the Observable object changes, all the observers will notified and updated automatically.

8. Design – Observer pattern
Collection of
Observers
<<interface>>
Observable
registerObserver()
removeObserver()
notifyObservers()
<<interface>>
Observer
update()
ObservableImplementation
registerObserver(){...}
removeObserver(){...}
notifyObservers(){....}
ObserverImplementation
update(){...}
//Other methods of observer
Instance
variable

9. Design – Observer pattern - example
<<interface>>
Observable
registerObserver()
removeObserver()
notifyObservers()
<<interface>>
Observer
update()
<<interface>>
Display
display()
StockUpdate
registerObserver(){...}
removeObserver(){...}
notifyObservers(){....}
MobileDisplay
update(){...}
//Other methods of observer

10. Pull and Push model Stock Update Push model StockBoard Pull model
Update(Observable o, Map stocks)
StockBoard
Pull model
update()
*The observer should have a reference to Observable

11. Code – Example

12. Java's build in Observer pattern
java.util.Observable
java.util.Observer
A concrete class
An Interface
Issues:
Observable is a concrete class and due to
which the class which has the state will not have a chance
to have inheritance of its own.
Well know usage:
Java Swing
Java RMI

13. Object Oriented Principles applied
Loose coupling between the Observer and Observable.
Programming to Interfaces.
Examples of Observer Pattern
Swing event listeners
Java RMI

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