11.5 - Behavioral Design Patterns
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Observer Pattern
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Today, we will be discussing the Observer Pattern. Can anyone tell me what they think it does?
Isn’t it about objects watching other objects?
Exactly! It allows one object to notify multiple other objects about a state change. This is also known as a one-to-many dependency.
Can you give an example of where we might use this?
Sure! A common example is in event listeners in user interfaces; when an event occurs, all subscribed listeners are notified. Remember the acronym 'NEMO'—Notify, Event, Many Objects—to help you remember how the Observer Pattern works.
What if an observer doesn’t want to receive updates anymore?
Good question! Observers can unregister themselves from the subject when they no longer want to receive updates.
So, to summarize, the Observer Pattern allows one object to monitor changes and notify multiple dependents without tightly coupling them. Let’s move on!
Strategy Pattern
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Now, let’s talk about the Strategy Pattern. Does anyone know what this does?
Is it about choosing a strategy in a game or something?
Close! It’s about defining a family of algorithms, encapsulating each one, and making them interchangeable.
Why would we want to do that?
This approach allows algorithms to vary independently from clients that use them. For instance, suppose you have different sorting algorithms; you can choose which to use dynamically. The acronym 'FLEX' can help to remember this: 'Flexible Learning EXecution.'
Can we see a simple example of this?
Sure! Imagine a payment system where you could choose to pay via credit card or PayPal without altering the underlying payment processing framework.
In summary, the Strategy Pattern enhances flexibility and allows for easier management of algorithms.
Command Pattern
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Lastly, let’s look at the Command Pattern. What do you think this pattern is about?
Is it about giving commands to a robot?
In a way! The Command Pattern encapsulates a request as an object, allowing users to parameterize clients with different requests.
So, it can be used to queue requests?
Exactly! This is helpful for tracking operations, undoable actions, or even logging activity. You can remember this with 'LOG'—'Log Operations Generally.'
Are there any downsides to this pattern?
A potential downside is complexity; using this pattern can introduce additional classes. But the benefits often outweigh the costs when managing complex actions.
To summarize, the Command Pattern encapsulates requests, supports flexible execution, and can track actions effectively.
Introduction & Overview
Read summaries of the section's main ideas at different levels of detail.
Quick Overview
Standard
This section delves into behavioral design patterns, which illustrate how objects collaborate effectively. Key examples include the Observer, Strategy, and Command patterns, each serving distinct purposes to enhance system modularity and maintainability.
Detailed
Behavioral Design Patterns
Behavioral design patterns are essential for defining how objects interact and communicate in a software application. Unlike creational and structural patterns, which focus on object creation and composition respectively, behavioral patterns prioritize the responsibilities and interaction patterns of classes and objects.
Key Behavioral Patterns:
- Observer Pattern: Establishes a one-to-many dependency between objects so that when one object changes its state, all its dependents are notified. This pattern is crucial in event-driven programming, allowing objects to subscribe to events and react accordingly.
- Strategy Pattern: Encapsulates a family of algorithms, allowing for their interchangeability. This promotes flexibility, enabling clients to select the appropriate algorithm at runtime without modifying the context or algorithms themselves.
- Command Pattern: Encapsulates requests as objects, allowing for parameterization of clients with different requests. This is useful in scenarios requiring queuing of requests, logging operations, and support for undoable operations.
Incorporating these patterns into software design promotes cleaner architecture, making it more maintainable, scalable, and flexible.
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Observer Pattern
Chapter 1 of 3
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Chapter Content
11.5.1 Observer Pattern
Defines a one-to-many dependency between objects so that when one object changes state, all its dependents are notified.
interface Observer {
void update(String message);
}
class ConcreteObserver implements Observer {
public void update(String message) {
System.out.println("Message received: " + message);
}
}
class Subject {
private List observers = new ArrayList<>();
public void addObserver(Observer o) { observers.add(o); }
public void notifyObservers(String message) {
for (Observer o : observers) {
o.update(message);
}
}
}
Detailed Explanation
The Observer Pattern establishes a relationship where one object, called the Subject, maintains a list of its dependents, called Observers. When the state of the Subject changes, it calls the notifyObservers method to update all Observers automatically.
- Subject: This class is responsible for tracking the state change and notifying all Observers.
- Observer Interface: An interface that must be implemented by any concrete observer (like ConcreteObserver). This allows various observers to react to changes in the subject.
- ConcreteObserver: This is a specific type of Observer that reacts when it receives an update from the Subject.
Examples & Analogies
Imagine a weather station (Subject) that tracks temperature changes. News agencies (Observers) subscribe to updates on the weather data. When the temperature changes, the weather station notifies all news agencies, allowing them to report the latest temperature. Each agency can choose how to present this information.
Strategy Pattern
Chapter 2 of 3
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Chapter Content
11.5.2 Strategy Pattern
Defines a family of algorithms, encapsulates each one, and makes them interchangeable.
interface Strategy {
int execute(int a, int b);
}
class AddStrategy implements Strategy {
public int execute(int a, int b) {
return a + b;
}
}
class Context {
private Strategy strategy;
public Context(Strategy strategy) {
this.strategy = strategy;
}
public int executeStrategy(int a, int b) {
return strategy.execute(a, b);
}
}
Detailed Explanation
The Strategy Pattern helps define a strategy interface that encapsulates a family of algorithms so that they are interchangeable. The Context class holds a reference to the Strategy and invokes its algorithm without needing to know the specifics of how the algorithm is implemented.
1. Strategy Interface: This defines a method (e.g., execute) that different algorithms will implement.
2. Concrete Strategies (like AddStrategy): These are specific implementations of the Strategy interface that perform specific algorithms. In this case, an addition operation.
3. Context: This class uses a Strategy and can change the strategy as needed during runtime.
Examples & Analogies
Consider a navigation app that provides route calculations. The app may use different strategies for routes: the fastest route, the shortest distance, or scenic routes. By using the Strategy Pattern, the app can switch its navigation method based on user preferences without modifying the underlying logic.
Command Pattern
Chapter 3 of 3
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Chapter Content
11.5.3 Command Pattern
Encapsulates a request as an object, thereby letting users parameterize clients with different requests.
interface Command {
void execute();
}
class Light {
void turnOn() {
System.out.println("Light ON");
}
}
class LightOnCommand implements Command {
Light light;
public LightOnCommand(Light light) {
this.light = light;
}
public void execute() {
light.turnOn();
}
}
Detailed Explanation
The Command Pattern allows you to encapsulate a request (or action) as an object, thereby allowing you to parameterize clients with different requests. This pattern provides an object-oriented way to store information for a command.
1. Command Interface: This interface defines a method execute() that any command should implement.
2. Light Class: This represents the receiver that performs specific actions—in this case, turning a light on.
3. Concrete Command (LightOnCommand): This class implements the Command interface and holds a reference to the Light object, allowing it to call the correct method to execute the command.
Examples & Analogies
Think of a remote control (Receiver) and different actions (commands) it can perform, such as turning on the light. Each button corresponds to a Command object (e.g., LightOnCommand) that knows how to invoke the Light's turnOn method. Pressing the button sends a command to be executed, making the system flexible and allowing you to add new commands without changing the remote control's structure.
Key Concepts
-
Observer Pattern: A pattern that defines a one-to-many relationship allowing multiple observers to be notified of changes.
-
Strategy Pattern: A method of defining a family of algorithms allowing selection at runtime.
-
Command Pattern: A pattern that encapsulates a request as an object, enabling flexible execution and parameterization.
Examples & Applications
Observer Pattern: A news agency that notifies multiple subscribers about news updates.
Strategy Pattern: A sorting application that allows users to choose between various sorting algorithms.
Command Pattern: A remote control that can execute different commands like turning on/off devices.
Memory Aids
Interactive tools to help you remember key concepts
Rhymes
The Observer sees all that goes, observing changes as it flows.
Stories
Imagine a weather station sending alerts to multiple devices, this illustrates how observers keep in the loop.
Memory Tools
For the Strategy Pattern, think 'flow' - each algorithm can flow through dynamically.
Acronyms
Remember 'REC' for the Command Pattern
Request
Execute
Control.
Flash Cards
Glossary
- Observer Pattern
A behavioral design pattern that allows a subject to notify multiple observers about state changes.
- Strategy Pattern
A behavioral design pattern that defines a family of algorithms, encapsulates each one, and makes them interchangeable.
- Command Pattern
A behavioral design pattern that encapsulates a request as an object, allowing for parameterization of clients with different requests.
Reference links
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