observer

Observer Pattern

The Observer Pattern is a behavioral design pattern that establishes a one-to-many dependency between objects. When one object (the subject) changes state, all its dependents (observers) are notified and updated automatically.

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📖 What is the Observer Pattern?

The Observer Pattern allows objects to observe and react to changes in another object. The subject maintains a list of observers and notifies them whenever there is a change in its state.

Key features:

1. One-to-Many Relationship: Updates all observers when the subject changes.
2. Decoupling: Reduces coupling between the subject and observers.
3. Dynamic Updates: Observers can dynamically register and unregister.

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🤔 Why Use the Observer Pattern?

1. Real-Time Updates: Keep multiple objects in sync with a central subject.
2. Flexibility: Observers can be added or removed dynamically.
3. Decoupled Design: Promotes loose coupling between the subject and its observers.

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🔧 Implementation

The implementation of the Observer Pattern can be found in:

• Subject.java: Defines the subject interface.
• Observer.java: Defines the observer interface.
• WeatherData.java: Concrete subject implementation.
• CurrentConditionsDisplay.java, StatisticsDisplay.java, ForecastDisplay.java: Concrete observer implementations.
• WeatherStation.java: Main class to demonstrate the Observer Pattern.

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🛠️ Example Usage

To see the Observer Pattern in action, refer to the WeatherStation.java file. It demonstrates how weather data is updated and displayed in real-time.

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📊 UML Diagram

classDiagram
    class Subject {
        -observers : Observer
        +Attach()
        +Detach()
        +Notify()
    }

    class Observer {
        +Update()
    }

    class ConcreteSubject {
        -state
        +GetState()
    }

    class ConcreteObserver {
        -subject : ConcreteSubject
        +Update()
    }

    Subject <|-- ConcreteSubject
    Subject o--> Observer
    Observer <|-- ConcreteObserver
    ConcreteSubject --> ConcreteObserver

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Note

If the UML above is not rendering correctly, you can view the diagram from the observer_uml.png file.

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📝 Key Takeaways

• The Observer Pattern is ideal for real-time updates between objects.
• It reduces coupling and increases flexibility in your codebase.
• Use it when you need to maintain consistency between related objects.

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