1. OO Design - Observer Pattern
Object-Oriented Programming

2. The Weather Monitoring application
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OO Design Pattern

3. WeatherData API
Our job: to implement measurementsChanged() so that it updates displays
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OO Design Pattern

4. Our job
to implement measurementsChanged() so that it updates the displays:
Given:
Getter methods available for temperature, humidity, and pressure
measurementsChanged() is called each time new weather data is available
We need to implement three displays that update every time WeatherData has new data
Can add new displays and remove displays
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OO Design Pattern

5. First draft solution
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6. What’s wrong? We are coding to concrete implementation, not interfaces
For every new display element, we need to alter code.
We have no way to add (or remove) display elements at run time
The display elements don’t implement a common interface.
We haven’t encapsulated the part that changes
We are violating encapsulation of the WeatherData class.
What’s wrong?
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OO Design Pattern

7. What’s wrong? We are coding to concrete implementation, not interfaces
For every new display element, we need to alter code.
We have no way to add (or remove) display elements at run time
The display elements don’t implement a common interface.
We haven’t encapsulated the part that changes
We are violating encapsulation of the WeatherData class.
What’s wrong?
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OO Design Pattern

8. The Observer Pattern How newspaper or magazine subscriptions work?
Subjects + Observers = ObserverPattern
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OO Design Pattern

9. The Observer Pattern
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10. The Observer Pattern
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11. The Observer Pattern
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OO Design Pattern

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OO Design Pattern

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15. The Observer Pattern defines a one-to-many dependency between objects so that when one object changes state, all of its dependents are notified and updated automatically
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16. In Class Diagram
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17. Benefits of the Observer Pattern
Subjects and observers can interact but have very little knowledge of each other.
The only thing the subject knows about an observer is that it implements the Observer interface
We can add new observers at any time.
We never need to modify the subject to add new type of observers
We can reuse subjects or observers independently of each other
Changes to either the subject or an observer will not affect the other
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OO Design Pattern

19. Back to your task
to implement measurementsChanged() so that it updates the displays:
Given:
Getter methods available for temperature, humidity, and pressure
measurementsChanged() is called each time new weather data is available
We need to implement three displays that update every time WeatherData has new data
Can add new displays and remove displays
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OO Design Pattern

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OO Design Pattern

21. Homework Reimplement using java.util.Observable
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