Java设计模式：从单例到观察者

设计模式是解决特定问题的通用解决方案。在Java编程中，设计模式可以帮助我们编写更灵活、可维护和可扩展的代码。本文将介绍几种常见的Java设计模式，包括单例、工厂、策略、装饰器和观察者模式。

1. 单例模式

单例模式是一种创建型模式，它确保一个类只有一个实例，并提供一个全局访问点。

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

2. 工厂模式

工厂模式是一种创建型模式，它提供了一种接口来创建对象，而不是直接使用new关键字。

public interface Animal {void makeSound();
}public class Dog implements Animal {public void makeSound() {System.out.println("Woof!");}
}public class AnimalFactory {public static Animal createAnimal(String type) {if (type.equals("dog")) {return new Dog();} else {// Handle other animal typesreturn null;}}
}Animal animal = AnimalFactory.createAnimal("dog");
animal.makeSound();

3. 策略模式

策略模式是一种行为型模式，它定义了一系列算法，并将每个算法封装在一个可互换的对象中。

public interface SortingStrategy {void sort(int[] arr);
}public class BubbleSort implements SortingStrategy {public void sort(int[] arr) {// Bubble sort implementation}
}public class SelectionSort implements SortingStrategy {public void sort(int[] arr) {// Selection sort implementation}
}public class Sorter {private SortingStrategy strategy;public Sorter(SortingStrategy strategy) {this.strategy = strategy;}public void setStrategy(SortingStrategy strategy) {this.strategy = strategy;}public void sort(int[] arr) {strategy.sort(arr);}
}Sorter sorter = new Sorter(new BubbleSort());
sorter.sort(new int[]{5, 2, 8, 1});

4. 装饰器模式

装饰器模式是一种结构型模式，它动态地给一个对象添加一些额外的功能，而不影响该对象的原有功能。

public abstract class Beverage {protected String description = "Unknown Beverage";public String getDescription() {return description;}public abstract double cost();
}public class HouseBlend extends Beverage {public HouseBlend() {description = "House Blend Coffee";}public double cost() {return 0.89;}
}public abstract class CondimentDecorator extends Beverage {protected Beverage beverage;public CondimentDecorator(Beverage beverage) {this.beverage = beverage;}public abstract String getDescription();
}public class Mocha extends CondimentDecorator {public Mocha(Beverage beverage) {super(beverage);}public String getDescription() {return beverage.getDescription() + ", Mocha";}public double cost() {return beverage.cost() + 0.20;}
}Beverage beverage = new HouseBlend();
beverage = new Mocha(beverage);
System.out.println(beverage.getDescription() + " $" + beverage.cost());

5. 观察者模式

观察者模式是一种行为型模式，它定义了对象之间的一种一对多的依赖关系，当一个对象的状态发生改变时，所有依赖于它的对象都会受到通知并自动更新。

public interface Observer {void update(String message);
}public class ConcreteObserver implements Observer {public void update(String message) {System.out.println("Received message: " + message);}
}public interface Subject {void registerObserver(Observer observer);void removeObserver(Observer observer);void notifyObservers(String message);
}public class ConcreteSubject implements Subject {private List<Observer> observers = new ArrayList<>();public void registerObserver(Observer observer) {observers.add(observer);}public void removeObserver(Observer observer) {observers.remove(observer);}public void notifyObservers(String message) {for (Observer observer : observers) {observer.update(message);}}
}ConcreteSubject subject = new ConcreteSubject();
ConcreteObserver observer = new ConcreteObserver();
subject.registerObserver(observer);
subject.notifyObservers("Hello, world!");