ObserverとStrategyパターン
依存関係の管理にObserverを、交換可能なアルゴリズムにStrategyを実装します。
「ObserverとStrategyパターン」はCoddyKit上の無料Clean Architecture & Design Patterns in Practiceレッスンです。 これはレッスン1/4です。 下記で完全なレッスンを無料で読むことができます。その後、ブラウザ内の組み込みコードエディタと24時間対応のAIチューターでハンズオン演習できます。 これはClean Architecture & Design Patterns in Practice学習パスの一部であり、ウェブとCoddyKitアプリ全体で進捗が同期されます。 Clean Architecture & Design Patterns in Practiceコースには全4レッスンが含まれています。
このレッスンの一部はまだ翻訳されておらず、英語で表示されています。
Behavioral Patterns: Actions & Interactions
Welcome to the world of Behavioral Design Patterns! These patterns focus on how objects interact and communicate, helping distribute responsibilities effectively.
They describe common communication patterns between objects, making your system more flexible and manageable.
Observer Pattern: Notifying Changes
First up is the Observer Pattern. Imagine you subscribe to a newsletter – when a new edition is out, you get notified! This pattern works similarly.
- It defines a one-to-many dependency between objects.
- When one object (the Subject) changes state, all its dependents (Observers) are notified and updated automatically.
It's great for event handling and distributed systems.
Observer Components: The Subject
The Subject (also known as Observable) is the object being watched. It has two main jobs:
- Maintain a list of its observers.
- Provide methods to attach (add) or detach (remove) observers.
- Notify all attached observers when its state changes.
Think of it as the news publisher.
Observer Components: The Observer
The Observer is any object that wants to be notified of changes in the Subject. It defines an update() method.
- When the Subject notifies, it calls each Observer's
update()method. - This method allows the Observer to react to the change, perhaps by fetching new data or updating its display.
These are your newsletter subscribers!
Observer Pattern: Publisher & Subscribers
Let's see the Observer pattern in action with a simple news publisher and readers. The NewsPublisher is our Subject, and NewsReaders are our Observers.
Try running the code to see how readers get updates!
import java.util.ArrayList;
import java.util.List;
// Observer Interface
interface Subscriber {
void update(String news);
}
// Subject Class
class NewsPublisher {
private List<Subscriber> subscribers = new ArrayList<>();
private String latestNews;
public void addSubscriber(Subscriber s) {
subscribers.add(s);
}
public void removeSubscriber(Subscriber s) {
subscribers.remove(s);
}
public void publishNews(String news) {
this.latestNews = news;
notifySubscribers();
}
private void notifySubscribers() {
for (Subscriber s : subscribers) {
s.update(latestNews);
}
}
}
// Concrete Observer
class NewsReader implements Subscriber {
private String name;
public NewsReader(String name) {
this.name = name;
}
@Override
public void update(String news) {
System.out.println(name + " received: " + news);
}
}
public class Main {
public static void main(String[] args) {
NewsPublisher publisher = new NewsPublisher();
NewsReader reader1 = new NewsReader("Alice");
NewsReader reader2 = new NewsReader("Bob");
publisher.addSubscriber(reader1);
publisher.addSubscriber(reader2);
publisher.publishNews("Breaking: New programming course released!");
System.out.println("\nAlice unsubscribes...");
publisher.removeSubscriber(reader1);
publisher.publishNews("Update: Course enrollment extended!");
}
}Strategy Pattern: Choosing Algorithms
Next, we have the Strategy Pattern. This pattern allows you to define a family of algorithms, encapsulate each one, and make them interchangeable.
It lets the algorithm vary independently from clients that use it. Think of choosing different payment methods (credit card, PayPal, bank transfer) at checkout.
Strategy Components: The Strategy
The Strategy is an interface or abstract class that declares a common operation for all supported algorithms.
- Each specific algorithm (e.g.,
CreditCardPayment,PayPalPayment) implements this interface. - This ensures they all have the same method signature, making them interchangeable.
Strategy Components: The Context
The Context is the class that holds a reference to a Strategy object. It defines an interface for clients to interact with.
- The Context does not know which concrete strategy it is using.
- It delegates the execution of the algorithm to its current Strategy object.
This keeps the Context simple and flexible, as it doesn't need to change when new strategies are added.
Strategy Pattern: Payment Methods
Here's a practical example of the Strategy pattern. We have different payment strategies (Credit Card, PayPal) and a ShoppingCart as our Context.
Notice how the ShoppingCart can easily switch between payment methods without changing its own core logic.
interface PaymentStrategy {
void pay(int amount);
}
class CreditCardPayment implements PaymentStrategy {
private String cardNumber;
public CreditCardPayment(String card) { this.cardNumber = card; }
@Override
public void pay(int amount) {
System.out.println("Paid " + amount + " using Credit Card " + cardNumber);
}
}
class PayPalPayment implements PaymentStrategy {
private String email;
public PayPalPayment(String email) { this.email = email; }
@Override
public void pay(int amount) {
System.out.println("Paid " + amount + " using PayPal account: " + email);
}
}
class ShoppingCart {
private PaymentStrategy strategy;
public void setPaymentStrategy(PaymentStrategy strategy) {
this.strategy = strategy;
}
public void checkout(int amount) {
if (strategy == null) {
System.out.println("Please set a payment strategy first.");
return;
}
strategy.pay(amount);
}
}
public class Main {
public static void main(String[] args) {
ShoppingCart cart = new ShoppingCart();
// Pay with Credit Card
cart.setPaymentStrategy(new CreditCardPayment("1234-5678-9012-3456"));
cart.checkout(100);
// Pay with PayPal
cart.setPaymentStrategy(new PayPalPayment("user@example.com"));
cart.checkout(50);
}
}Quick Check: Observer or Strategy?
Consider a stock trading application. It needs to calculate trading fees using different methods (e.g., a flat fee, a percentage of trade value, or a tiered system) based on the user's account type. The application should be able to easily switch between these methods.
Recap: Notifying & Swapping
Great job! You've explored two powerful behavioral design patterns:
- Observer Pattern: Manages one-to-many dependencies, where a Subject notifies multiple Observers of state changes. It promotes loose coupling between components.
- Strategy Pattern: Allows you to define a family of algorithms, encapsulate each one, and make them interchangeable. It helps in swapping algorithms at runtime, promoting flexibility and clean code.
These patterns help make your code more modular, flexible, and easier to maintain!
よくある質問
「ObserverとStrategyパターン」レッスンは無料ですか？
はい。「ObserverとStrategyパターン」の完全なテキストはこのウェブで無料で読めます。インタラクティブに演習し（組み込みコードエディタと24時間対応のAIチューター）、Clean Architecture & Design Patterns in Practiceコースの残りをアンロックするには、CoddyKit PROにアップグレードしてください。 Clean Architecture & Design Patterns in Practiceコースには全4レッスンが含まれています。
「ObserverとStrategyパターン」で何を学びますか？
依存関係の管理にObserverを、交換可能なアルゴリズムにStrategyを実装します。 ブラウザで直接実行するハンズオンコードでClean Architecture & Design Patterns in Practiceを演習し、24時間対応のAIチューターがレッスンを進める中での質問に答えます。
Clean Architecture & Design Patterns in Practiceを始めるのに経験は必要ですか？
事前経験は必要ありません。CoddyKitのClean Architecture & Design Patterns in Practiceは初級者から上級者向けに構成されているため、ここから始めるか最初から始めて、自分のペースで進むことができます。 これはレッスン1/4です。
「ObserverとStrategyパターン」レッスンにはどのくらい時間がかかりますか？
ほとんどのCoddyKitレッスンは約5～10分かかります。各レッスンはコンパクトでインタラクティブなので、着実に進歩し、ウェブとアプリ全体で正確に前回の場所から再開できます。
このClean Architecture & Design Patterns in Practiceレッスンでコードを書いて実行できますか？
はい。すべてのClean Architecture & Design Patterns in Practiceレッスンに組み込みコードエディタが含まれているため、ブラウザでリアルコードを書いて実行し、即座のAIフィードバックを取得できます。ローカル設定は不要です。
このコースのすべてのレッスン
- ObserverとStrategyパターン
- CommandとIteratorパターン
- Template MethodとStateパターン
- MediatorとChain of Responsibility