【JAVA基础篇教学】第十四篇：Java中设计模式

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

interface Shape {
    void draw();
}
 
class Rectangle implements Shape {
    @Override
    public void draw() {
        System.out.println("Drawing rectangle");
    }
}
 
class Circle implements Shape {
    @Override
    public void draw() {
        System.out.println("Drawing circle");
    }
}
 
class ShapeFactory {
    public Shape getShape(String shapeType) {
        if (shapeType == null) {
            return null;
        }
        if (shapeType.equalsIgnoreCase("RECTANGLE")) {
            return new Rectangle();
        } else if (shapeType.equalsIgnoreCase("CIRCLE")) {
            return new Circle();
        }
        return null;
    }
}

import java.util.ArrayList;
import java.util.List;
 
interface Observer {
    void update(String message);
}
 
class ConcreteObserver implements Observer {
    private String name;
 
    public ConcreteObserver(String name) {
        this.name = name;
    }
 
    @Override
    public void update(String message) {
        System.out.println(name + " received message: " + message);
    }
}
 
interface Subject {
    void attach(Observer observer);
 
    void detach(Observer observer);
 
    void notifyObservers(String message);
}
 
class ConcreteSubject implements Subject {
    private List<Observer> observers = new ArrayList<>();
 
    @Override
    public void attach(Observer observer) {
        observers.add(observer);
    }
 
    @Override
    public void detach(Observer observer) {
        observers.remove(observer);
    }
 
    @Override
    public void notifyObservers(String message) {
        for (Observer observer : observers) {
            observer.update(message);
        }
    }
}