How are design patterns implemented in the java framework?

Design patterns are used in Java frameworks to solve common programming problems, including: Strategy pattern: allows dynamic selection of algorithms, separating algorithm implementation and usage objects. Observer pattern: Define a one-to-many dependency relationship and notify observers when the subject status changes. Factory method pattern: Provides an interface to create product objects and delays the creation of specific implementations. Singleton pattern: Ensure that a class has only one instance and limit the instantiation of the class.

Design Patterns in Java Framework

In Java framework, design patterns play a vital role for Solve common programming problems. These patterns improve code flexibility, maintainability, and reusability. This article will explore some popular design patterns in Java frameworks and illustrate their implementation through practical cases.

Strategy Pattern

Strategy pattern defines a set of algorithms that allows dynamic selection of algorithms. The purpose is to separate the algorithm implementation from the objects using the algorithm.

Implementation in Java Framework:

java.util.Comparator The interface is an example of the Strategy pattern. It defines an algorithm for comparison operations, and the actual comparison behavior is implemented by a specific comparator class (such as StringComparator).

Observer Pattern

The Observer pattern defines a one-to-many dependency relationship, in which changes in the state of one object (topic) will automatically notify multiple objects ( observer).

Implementation in Java framework:

java.util.Observable and java.util.Observer classes implement Observer pattern. Observable represents the subject, while Observer represents the observer. When the topic's state changes, it notifies registered observers.

Factory Method Pattern

The Factory Method pattern provides an interface for creating product objects while deferring the actual implementation of creating the product.

Implementation in Java Framework:

java.sql.DriverManager class uses factory method pattern to create a database connection. It returns a Connection object by calling the getConnection() method, which is responsible for creating a specific connection for the specified database.

Singleton pattern

The singleton pattern ensures that there is only one instance of a class. It achieves this goal by restricting the instantiation of the class.

Implementation in Java Framework:

java.lang.Runtime class is an example of the Singleton pattern. It encapsulates the information and functionality of the current Java virtual machine.

Practical case:

The following is an example of using the observer pattern, which can be implemented in the logging framework:

// 主题接口
interface LogSubject {
    void attach(LogObserver observer);
    void detach(LogObserver observer);
    void notifyObservers(LogEntry logEntry);
}

// 具体主题（日志记录器）
class LoggerSubject implements LogSubject {
    private List<LogObserver> observers = new ArrayList<>();

    @Override
    public void attach(LogObserver observer) {
        observers.add(observer);
    }

    @Override
    public void detach(LogObserver observer) {
        observers.remove(observer);
    }

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

    public void log(String message) {
        LogEntry logEntry = new LogEntry(message);
        notifyObservers(logEntry);
    }
}

// 观察者接口
interface LogObserver {
    void update(LogEntry logEntry);
}

// 具体观察者（控制台日志记录器）
class ConsoleLoggerObserver implements LogObserver {
    @Override
    public void update(LogEntry logEntry) {
        System.out.println(logEntry.getMessage());
    }
}

// 使用该模式
LoggerSubject loggerSubject = new LoggerSubject();
ConsoleLoggerObserver consoleLoggerObserver = new ConsoleLoggerObserver();
loggerSubject.attach(consoleLoggerObserver);

loggerSubject.log("Hello World!"); // 将消息打印到控制台

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