What are the application scenarios of design patterns in the Java framework?

In the Java framework, design patterns are widely used in the following scenarios: Singleton pattern: limits the number of class instances to one, and is suitable for situations where global access to objects is required. Observer pattern: Define a one-to-many dependency relationship. When the state of the subject object changes, all observer objects are notified and updated.

Application scenarios of design patterns in Java framework

Introduction

Design patterns Is a standard way to reuse good practices and solutions in programming. In the Java framework, design patterns are widely used. They provide common, reusable components and simplify application development and maintenance.

Single-case mode

Single-case mode limits the number of instances of a class to one. This pattern can be used when global access to an object is required, such as a log service or database connection.

Practical case:

// Singleton.java
public class Singleton {
    private static Singleton instance;

    private Singleton() { }

    public static Singleton getInstance() {
        if (instance == null) {
            instance = new Singleton();
        }
        return instance;
    }
}

// Client.java
public class Client {
    public static void main(String[] args) {
        Singleton singleton1 = Singleton.getInstance();
        Singleton singleton2 = Singleton.getInstance();

        System.out.println(singleton1 == singleton2); // true
    }
}

Observer pattern

The observer pattern defines a one-to-many dependency relationship , when the state of an object (subject) changes, it notifies and updates all dependent objects (observers).

Practical case:

// Subject.java
public interface Subject {
    void addObserver(Observer observer);
    void removeObserver(Observer observer);
    void notifyObservers();
}

// ConcreteSubject.java
public class ConcreteSubject implements Subject {
    private List<Observer> observers = new ArrayList<>();

    @Override
    public void addObserver(Observer observer) {
        observers.add(observer);
    }

    @Override
    public void removeObserver(Observer observer) {
        observers.remove(observer);
    }

    @Override
    public void notifyObservers() {
        for (Observer observer : observers) {
            observer.update();
        }
    }
}

// Observer.java
public interface Observer {
    void update();
}

// ConcreteObserver.java
public class ConcreteObserver implements Observer {
    @Override
    public void update() {
        System.out.println("Observer updated!");
    }
}

// Client.java
public class Client {
    public static void main(String[] args) {
        ConcreteSubject subject = new ConcreteSubject();
        ConcreteObserver observer1 = new ConcreteObserver();
        ConcreteObserver observer2 = new ConcreteObserver();

        subject.addObserver(observer1);
        subject.addObserver(observer2);

        subject.notifyObservers();
    }
}

Other commonly used design patterns:

• Factory method pattern: Creating objects Factory interface
• Proxy mode: Provides alternatives or enhancements to other objects
• Strategy mode: Allows algorithm exchange without modifying client code

Conclusion

Design patterns are an integral part of the Java framework. They provide universal solutions that make applications easier to develop, maintain, and extend. By understanding and applying design patterns, developers can create more robust and flexible applications.

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