in these slides i have explained the Observer design pattern. slides includes the complete definition, explanation and then implementation with code examples.

1. Observer Design Pattern
From Definition to Implementation
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2. Contents / Agenda
• Definition
• Diagram
• Sequence Diagram
• Examples
• Advantages and Usage
• Implementation – UML Class diagram
• Implementation – Step 1 to 3
• Test and Result
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3. Definition
1. The observer pattern is a software design pattern in which an object, called
the subject, maintains a list of its dependents, called observers, and notifies
them automatically of any state changes, usually by calling one of their
methods.
2. There are two typical models for Observer based on the way data is passed.
It can be a Push model or a Pull model. In the push model, data is pushed
along with the notification. An example for this is mail delivery, magazine
subscriptions etc. In the pull model, the client will be notified about what is
happening. But, it is always the responsibility of the client to check whether
the relevant data is there or not. An example for this is RSS feeds.
Whenever there is a change in the website, and if you have subscribed for
the feed, you will get a notification. But, it is up to you to go back and see if
you are interested.
• It comes under the “Behavioral Design Patterns”.
• It is also known as Dependents, Publish-Subscribe, Model-View.
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4. Diagram
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5. Sequence Diagram
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6. Examples
1. Company updates all its shareholders for any decision they make here Company is
Subject and Shareholders are Observers, any change in policy of company and Company
notifies all its Shareholders or Observer.
2. A typical example for the Observer pattern is the conventional mail delivery system.
Whenever you have a mail, the postman will come and knock at your door with the mail.
Just imagine if you have to go to the post office every day and check whether there are
any mails. It would have been a really inefficient system as each individual needs to go to
the post office periodically. This is eliminated in real life by introducing a mail delivery
mechanism.
3. It is the same in a GUI system . Your business logic, in this case, the cassette player, will
update all the GUI components periodically, just like a post man who comes to your
house with mail. It would have been inefficient for the total system to implement a
periodic check by each widget to get the progress.
4. We read newspaper. We get the daily news by news paper. Here we are the observer and
the news paper is observable. Observer Design Pattern keeps the data of its entire
listener and whenever there is any change it notifies to its listener.
5. Speaking clock is another example of Observer Design Pattern. Speaking clock notifies
when it finishes one hour.
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7. Advantages and Usage
• Benefits:
1. Objects are loosely coupled with each other but still communicates properly.
2. Observers can be added/removed at any time.
3. Subject and observer can belong to different abstraction layers.
4. Subject does not need to know the concrete class of an observer, just that each
observer implements the update interface.
5. Observers can be added without modifying the subject.
6. Note that: there is java.util.Observable Class and java.util.Observer Interface
available in java library to implement this pattern.
• Usage:
1. It is used when there is one to many relationship between objects such as if one
object is modified, its dependent objects are to be notified automatically.
2. When an abstraction has two aspects, one dependent on the other. Encapsulating
these aspects in separate objects lets you vary and reuse them independently.
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8. Implementation - UML
This is a example where three
observers are attached to a
subject through abstract observer
class. If any changes happened in
subject, it will notify all observers
about its changes.
For Example, number base
converter observers are attached
to a subject containing a integer
value. If we change the integer in
test, all observer will be changing
new value of integer to their
respective base.
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9. Step 1 : Subject
List: to keep track of attached observers.
State: current state
attach: to attach a observer with subject
notifyAllObservers: to send message to
all attached observers.
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10. Step 2 : Observer (abstract)
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11. Step 3 : Observers
There are our three observer concrete classes that
inherits the abstract observer class and updates
there function with respect to new state of subject.
Note all these observer are attached to the subject.
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12. Step 4 : Test
• We can easily test our pattern by following code.
Following are the two test in the code:
First,
the state of subject is set to 15, means
every observer will print number 15 after
converting it in its respective base.
Second,
the state of subject is set to 10, observer
observe this and print this number in
there respective base without any extra
effort. Our notify method notifies all
observers that state of subject has been
changed.
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13. Step 5 : Result
• Here is a clear result of our test.
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