Observer & singleton pattern

Observer Pattern&

Singleton Pattern

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Singleton Pattern(Creational)

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Singleton PatternName: SingletonIntent: Ensure a class has only one instance

and provide a global point of access to it Problem: How can we guarantee that one and

only oneinstance of a class can be created? Solution: Define a class with a private

constructor. The class constructs a single instance of itself Supply a static method that returns a reference to the single instance.

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Singleton: Basic Structure

Singleton

new_instance : Singleton

getInstance() : SingletonSingleton()

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Singleton sequence diagram

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Singleton – Example A status bar ….It could be implemented as a

Singleton object, allowing only one instance and a focal point for updates.

One file system, one window manager, one printer spooler, one Test engine, one Input/Output socket , Windows Registry etc.

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Class Singleton {

private static Singleton uniqueInstance = null;private Singleton( ) { .. } // private constructor

public static Singleton getInstance( ) {if (uniqueInstance == null)

uniqueInstance = new Singleton(); // call constructor

return uniqueInstance;}

}

Singleton: Basic Implementation

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Case Study

We want to create a remote connection to a server / database system and assure that only one connection is present.

Apply Singleton pattern

RemoteConnection

remoteConn : RemoteConnection

getInstance() : RemoteConnectionRemoteConnection()

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Implementation: RemoteConnection

Class RemoteConnection{private static RemoteConnetion remoteConn;private RemoteConnection(){…} //private

Constructor

public static RemoteConnection getInstance(){if(remoteConn == null){

remoteConn = new RemoteConnection(); } return remoteConn;}

}

In a university….when a student changes his address…pass this information to:Exams departmenttransport department

What's the solution?

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A case study…

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TransportDept

Student

changeAdd()

ExamDept

Different Objects

Different Interfaces

Observers

Observable

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Observer Pattern

Name: ObserverIntent: Define a one-to-many dependency

between objects so that when one object changes state, all its dependents are notified and updated automatically.

Problem: You need to notify a varying list of objects that an event has occurred.

Solution: delegate the responsibility for monitoring an event to a central object.

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Observer Pattern (Behavioral)

Step 1: Make the Observers behave in the same way

Step 2: Have the observers register themselves

Step 3: Notify the observers when the event occurs

Step 4: Get the information from the observable

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Applying the Observer

ConcreteObserver

Observer

Update(s : Subject)

<<<Interface>>>Subject

ObserversList : Vector

register(Obs : myObserver)unRegister(Obs : myObserver)notify()

Notify lets all observers know that event has occured

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Basic Structure

TransportDept ExamDept

myObserver

update(s : Student)

<<<Interface>>>

Student

address : StringmyObs : Vector

register(Obs : myObserver)unRegister(Obs : myObserver)notifyObs()changeAddress()getAddress()

Composition

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Applying observer to case study

public interface myObserver{public void update(Student s);

}

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Implementation: Observer interface

class ExamDept implements myObserver{public void update(Student s){

System.out.println("Student Updated in Exam Dept");

System.out.println("New Address: "+ s.getAddress());}

}

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Implementation: ExamDept

class TransportDept implements myObserver{public void update(Student s){

System.out.println("Student Updated in TD");

System.out.println("New Address: "+s.getAddress());}

}

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Implementation: TransportDept

class Student{private String address;private Vector myObs;

public Student(){ myObs = new Vector(); address = "Rawalpindi"; }

public void register(myObserver obs){ myObs.addElement(obs); }

public void unRegister(myObserver obs){ myObs.remove(obs); }

public void notifyObs(){Enumeration e = myObs.elements();while(e.hasMoreElements()){((myObserver)e.nextElement()).update(this); } }

public void changeAddress(){ address = "Islamabad"; notifyObs(); }

}

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Implementation: Student

class Main{public static void main(String[]args){

Student s = new Student();TransportDept td = new TransportDept();ExamDept ed = new ExamDept();

System.out.println("Present Address:" + s.getAddress());s.register(td);s.register(ed);s.changeAddress();

System.out.println("******Unregister Exam Dept*******");s.unRegister(ed);s.changeAddress();

}}

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Implementation: Main

TransportDept ExamDept

myObserver

update(s : Student)

<<<Interface>>>

Student

address : StringmyObs : Vector

register(Obs : myObserver)unRegister(Obs : myObserver)notifyObs()changeAddress()getAddress()

Composition

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Applying observer to case study

Suppose you are working on an MDI (Multiple Documents Interface) Form that has several child. You need to notify all the child about the changes that occur in MDI form (e.g title changed).

Apply Observer pattern to solve this problem and draw the corresponding class diagram.

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Exercise

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The End