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3. 3
 Name: Singleton
 Intent: 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 one
instance 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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5. 5

6. 6
 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

8. 8
 We want to create a remote connection to a
server / database system and assure that only one
connection is present.
 Apply Singleton pattern

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Class RemoteConnection{
private static RemoteConnetion remoteConn;
private RemoteConnection(){…} //private Constructor
public static RemoteConnection getInstance(){
if(remoteConn == null){
remoteConn = new RemoteConnection();
}
return remoteConn;
}
}

10. In a university….when a student changes his
address…pass this information to:
Exams department
transport department
What's the solution?
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Different Objects
Different Interfaces
Observers
Observable

12. 12

13.  Name: Observer
 Intent: 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.
13

14.  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
14

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16. 16

17. public interface myObserver{
public void update(Student s);
}
17

18. class ExamDept implements myObserver{
public void update(Student s){
System.out.println("Student Updated in Exam Dept");
System.out.println("New Address: "+ s.getAddress());
}
}
18

19. class TransportDept implements myObserver{
public void update(Student s){
System.out.println("Student Updated in TD");
System.out.println("New Address:
"+s.getAddress());
}
}
19

20. 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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21. 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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23. 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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