Clean Architecture & Design Patterns in Practice · Урок

Паттерны наблюдателя и стратегии

Реализуйте наблюдатель для управления зависимостями, а стратегию — для взаимозаменяемых алгоритмов.

«Паттерны наблюдателя и стратегии» — бесплатный урок Clean Architecture & Design Patterns in Practice на CoddyKit. Это урок 1 из 4. Ты можешь прочитать весь урок бесплатно ниже — а потом практиковать его прямо в браузере с встроенным редактором кода и ИИ-репетитором 24/7. Это часть пути обучения Clean Architecture & Design Patterns in Practice, и твой прогресс синхронизируется между веб-версией и приложением CoddyKit. Курс Clean Architecture & Design Patterns in Practice содержит 4 уроков всего.

Части этого урока еще не переведены и отображаются на английском.

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!

Часто задаваемые вопросы

Урок «Паттерны наблюдателя и стратегии» бесплатный?

Да — полный текст урока «Паттерны наблюдателя и стратегии» бесплатно доступен здесь в веб-версии. Чтобы практиковать его интерактивно (встроенный редактор кода и ИИ-репетитор 24/7) и разблокировать остальной курс Clean Architecture & Design Patterns in Practice, подпишись на CoddyKit PRO. Курс Clean Architecture & Design Patterns in Practice содержит 4 уроков всего.

Чему я научусь в уроке «Паттерны наблюдателя и стратегии»?

Реализуйте наблюдатель для управления зависимостями, а стратегию — для взаимозаменяемых алгоритмов. Ты практикуешь Clean Architecture & Design Patterns in Practice с помощью реального кода, который запускаешь прямо в браузере, и ИИ-репетитор 24/7 отвечает на твои вопросы во время урока.

Нужен ли мне опыт, чтобы начать Clean Architecture & Design Patterns in Practice?

Предыдущий опыт не требуется. Clean Architecture & Design Patterns in Practice на CoddyKit структурирован для всех уровней — от новичков до продвинутых, поэтому ты можешь начать отсюда или с самого начала и учиться в своем темпе. Это урок 1 из 4.

Сколько времени занимает урок «Паттерны наблюдателя и стратегии»?

Большинство уроков CoddyKit занимают около 5–10 минут. Каждый из них компактный и интерактивный, поэтому ты постоянно делаешь прогресс и продолжаешь с того же места в веб-версии и приложении.

Можно ли писать и запускать код в этом уроке Clean Architecture & Design Patterns in Practice?

Да. Каждый урок Clean Architecture & Design Patterns in Practice включает встроенный редактор кода, поэтому ты пишешь и запускаешь реальный код прямо в браузере и получаешь моментальную обратную связь от AI — локальная установка не требуется.

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