掌握Java设计模式的23种武器（全）：深入解析与实战示例

一、创建型模式

1. 单例模式 (Singleton Pattern)

2. 工厂模式 (Factory Pattern)

3. 抽象工厂模式 (Abstract Factory Pattern)

4. 建造者模式 (Builder Pattern)

5. 原型模式 (Prototype Pattern)

二、结构型模式

6. 适配器模式 (Adapter Pattern)

7. 桥接模式 (Bridge Pattern)

8. 组合模式 (Composite Pattern)

9. 装饰器模式 (Decorator Pattern)

10. 外观模式 (Facade Pattern)

11. 享元模式 (Flyweight Pattern)

12. 代理模式 (Proxy Pattern)

三、行为型模式

13. 责任链模式 (Chain of Responsibility Pattern)

14. 命令模式 (Command Pattern)

15. 解释器模式 (Interpreter Pattern)

16. 迭代器模式 (Iterator Pattern)

17. 中介者模式 (Mediator Pattern)

18. 备忘录模式 (Memento Pattern)

19. 观察者模式 (Observer Pattern)

20. 状态模式 (State Pattern)

21. 空对象模式 (Null Object Pattern)

22. 策略模式 (Strategy Pattern)

23. 模板模式 (Template Pattern)

24. 访问者模式 (Visitor Pattern)

一、创建型模式

1. 单例模式 (Singleton Pattern)

类比：如同系统中的打印机管理器，无论你调用多少次获取打印机管理器实例的方法，都返回同一个实例。

示例代码：

public class Singleton {// 创建 SingleObject 的一个对象private static Singleton instance = new Singleton();// 让构造函数为 private，这样该类就不会被实例化private Singleton() {}// 获取唯一可用的对象public static Singleton getInstance() {return instance;}public void showMessage() {System.out.println("Hello World!");}
}public class SingletonPatternDemo {public static void main(String[] args) {// 不合法的构造函数// 编译时错误：构造函数 Singleton() 是不可见的// Singleton object = new Singleton();// 获取唯一可用的对象Singleton object = Singleton.getInstance();// 显示消息object.showMessage();}
}

2. 工厂模式 (Factory Pattern)

类比：如同汽车工厂，根据不同的订单生产不同类型的汽车。

示例代码：

interface Shape {void draw();
}class Rectangle implements Shape {public void draw() {System.out.println("Inside Rectangle::draw() method.");}
}class Square implements Shape {public void draw() {System.out.println("Inside Square::draw() method.");}
}class Circle implements Shape {public void draw() {System.out.println("Inside Circle::draw() method.");}
}class ShapeFactory {// 使用 getShape 方法获取形状类型的对象public Shape getShape(String shapeType) {if (shapeType == null) {return null;}if (shapeType.equalsIgnoreCase("CIRCLE")) {return new Circle();} else if (shapeType.equalsIgnoreCase("RECTANGLE")) {return new Rectangle();} else if (shapeType.equalsIgnoreCase("SQUARE")) {return new Square();}return null;}
}public class FactoryPatternDemo {public static void main(String[] args) {ShapeFactory shapeFactory = new ShapeFactory();// 获取 Circle 对象，并调用它的 draw 方法Shape shape1 = shapeFactory.getShape("CIRCLE");shape1.draw();// 获取 Rectangle 对象，并调用它的 draw 方法Shape shape2 = shapeFactory.getShape("RECTANGLE");shape2.draw();// 获取 Square 对象，并调用它的 draw 方法Shape shape3 = shapeFactory.getShape("SQUARE");shape3.draw();}
}

3. 抽象工厂模式 (Abstract Factory Pattern)

类比：如同一个提供家电的工厂，有不同的产品系列，如电视、冰箱、洗衣机，每个系列都有具体的产品。

示例代码：

interface Shape {void draw();
}class Rectangle implements Shape {public void draw() {System.out.println("Inside Rectangle::draw() method.");}
}class Square implements Shape {public void draw() {System.out.println("Inside Square::draw() method.");}
}class Circle implements Shape {public void draw() {System.out.println("Inside Circle::draw() method.");}
}abstract class AbstractFactory {abstract Shape getShape(String shapeType);
}class ShapeFactory extends AbstractFactory {public Shape getShape(String shapeType) {if (shapeType == null) {return null;}if (shapeType.equalsIgnoreCase("CIRCLE")) {return new Circle();} else if (shapeType.equalsIgnoreCase("RECTANGLE")) {return new Rectangle();} else if (shapeType.equalsIgnoreCase("SQUARE")) {return new Square();}return null;}
}class FactoryProducer {public static AbstractFactory getFactory(String choice) {if (choice.equalsIgnoreCase("SHAPE")) {return new ShapeFactory();}return null;}
}public class AbstractFactoryPatternDemo {public static void main(String[] args) {// 获取形状工厂AbstractFactory shapeFactory = FactoryProducer.getFactory("SHAPE");// 获取 Circle 对象，并调用它的 draw 方法Shape shape1 = shapeFactory.getShape("CIRCLE");shape1.draw();// 获取 Rectangle 对象，并调用它的 draw 方法Shape shape2 = shapeFactory.getShape("RECTANGLE");shape2.draw();// 获取 Square 对象，并调用它的 draw 方法Shape shape3 = shapeFactory.getShape("SQUARE");shape3.draw();}
}

4. 建造者模式 (Builder Pattern)

类比：如同建造房屋，有不同的步骤如打地基、建墙、装屋顶，每一步都可以有不同的选择。

示例代码：

class Meal {private List<Item> items = new ArrayList<Item>();public void addItem(Item item) {items.add(item);}public float getCost() {float cost = 0.0f;for (Item item : items) {cost += item.price();}return cost;}public void showItems() {for (Item item : items) {System.out.print("Item : " + item.name());System.out.print(", Packing : " + item.packing().pack());System.out.println(", Price : " + item.price());}}
}interface Item {String name();Packing packing();float price();
}interface Packing {String pack();
}class Wrapper implements Packing {public String pack() {return "Wrapper";}
}class Bottle implements Packing {public String pack() {return "Bottle";}
}abstract class Burger implements Item {public Packing packing() {return new Wrapper();}public abstract float price();
}abstract class ColdDrink implements Item {public Packing packing() {return new Bottle();}public abstract float price();
}class VegBurger extends Burger {public float price() {return 25.0f;}public String name() {return "Veg Burger";}
}class ChickenBurger extends Burger {public float price() {return 50.5f;}public String name() {return "Chicken Burger";}
}class Coke extends ColdDrink {public float price() {return 30.0f;}public String name() {return "Coke";}
}class Pepsi extends ColdDrink {public float price() {return 35.0f;}public String name() {return "Pepsi";}
}class MealBuilder {public Meal prepareVegMeal() {Meal meal = new Meal();meal.addItem(new VegBurger());meal.addItem(new Coke());return meal;}public Meal prepareNonVegMeal() {Meal meal = new Meal();meal.addItem(new ChickenBurger());meal.addItem(new Pepsi());return meal;}
}public class BuilderPatternDemo {public static void main(String[] args) {MealBuilder mealBuilder = new MealBuilder();Meal vegMeal = mealBuilder.prepareVegMeal();System.out.println("Veg Meal");vegMeal.showItems();System.out.println("Total Cost: " + vegMeal.getCost());Meal nonVegMeal = mealBuilder.prepareNonVegMeal();System.out.println("\nNon-Veg Meal");nonVegMeal.showItems();System.out.println("Total Cost: " + nonVegMeal.getCost());}
}

5. 原型模式 (Prototype Pattern)

类比：如同剪贴板中的复制功能，可以复制现有的对象并创建新的对象。

示例代码：

abstract class Shape implements Cloneable {private String id;protected String type;abstract void draw();public String getType() {return type;}public String getId() {return id;}public void setId(String id) {this.id = id;}public Object clone() {Object clone = null;try {clone = super.clone();} catch (CloneNotSupportedException e) {e.printStackTrace();}return clone;}
}class Rectangle extends Shape {public Rectangle() {type = "Rectangle";}public void draw() {System.out.println("Inside Rectangle::draw() method.");}
}class Square extends Shape {public Square() {type = "Square";}public void draw() {System.out.println("Inside Square::draw() method.");}
}class Circle extends Shape {public Circle() {type = "Circle";}public void draw() {System.out.println("Inside Circle::draw() method.");}
}class ShapeCache {private static Hashtable<String, Shape> shapeMap = new Hashtable<String, Shape>();public static Shape getShape(String shapeId) {Shape cachedShape = shapeMap.get(shapeId);return (Shape) cachedShape.clone();}public static void loadCache() {Circle circle = new Circle();circle.setId("1");shapeMap.put(circle.getId(), circle);Square square = new Square();square.setId("2");shapeMap.put(square.getId(), square);Rectangle rectangle = new Rectangle();rectangle.setId("3");shapeMap.put(rectangle.getId(), rectangle);}
}public class PrototypePatternDemo {public static void main(String[] args) {ShapeCache.loadCache();Shape clonedShape = (Shape) ShapeCache.getShape("1");System.out.println("Shape : " + clonedShape.getType());Shape clonedShape2 = (Shape) ShapeCache.getShape("2");System.out.println("Shape : " + clonedShape2.getType());Shape clonedShape3 = (Shape) ShapeCache.getShape("3");System.out.println("Shape : " + clonedShape3.getType());}
}

二、结构型模式

6. 适配器模式 (Adapter Pattern)

类比：如同电源适配器，可以将一种类型的插头转换为另一种类型的插座。

示例代码：

interface MediaPlayer {void play(String audioType, String fileName);
}interface AdvancedMediaPlayer {void playVlc(String fileName);void playMp4(String fileName);
}class VlcPlayer implements AdvancedMediaPlayer {public void playVlc(String fileName) {System.out.println("Playing vlc file. Name: " + fileName);}public void playMp4(String fileName) {// 什么也不做}
}class Mp4Player implements AdvancedMediaPlayer {public void playVlc(String fileName) {// 什么也不做}public void playMp4(String fileName) {System.out.println("Playing mp4 file. Name: " + fileName);}
}class MediaAdapter implements MediaPlayer {AdvancedMediaPlayer advancedMusicPlayer;public MediaAdapter(String audioType) {if (audioType.equalsIgnoreCase("vlc")) {advancedMusicPlayer = new VlcPlayer();} else if (audioType.equalsIgnoreCase("mp4")) {advancedMusicPlayer = new Mp4Player();}}public void play(String audioType, String fileName) {if (audioType.equalsIgnoreCase("vlc")) {advancedMusicPlayer.playVlc(fileName);} else if (audioType.equalsIgnoreCase("mp4")) {advancedMusicPlayer.playMp4(fileName);}}
}class AudioPlayer implements MediaPlayer {MediaAdapter mediaAdapter;public void play(String audioType, String fileName) {// 内置支持 mp3 播放if (audioType.equalsIgnoreCase("mp3")) {System.out.println("Playing mp3 file. Name: " + fileName);}// mediaAdapter 提供其他格式支持else if (audioType.equalsIgnoreCase("vlc") || audioType.equalsIgnoreCase("mp4")) {mediaAdapter = new MediaAdapter(audioType);mediaAdapter.play(audioType, fileName);} else {System.out.println("Invalid media. " + audioType + " format not supported");}}
}public class AdapterPatternDemo {public static void main(String[] args) {AudioPlayer audioPlayer = new AudioPlayer();audioPlayer.play("mp3", "beyond the horizon.mp3");audioPlayer.play("mp4", "alone.mp4");audioPlayer.play("vlc", "far far away.vlc");audioPlayer.play("avi", "mind me.avi");}
}

7. 桥接模式 (Bridge Pattern)

类比：如同不同品牌的遥控器控制不同类型的电视，遥控器和电视之间通过桥接模式实现。

示例代码：

interface DrawAPI {void drawCircle(int radius, int x, int y);
}class RedCircle implements DrawAPI {public void drawCircle(int radius, int x, int y) {System.out.println("Drawing Circle[ color: red, radius: " + radius + ", x: " + x + ", " + y + "]");}
}class GreenCircle implements DrawAPI {public void drawCircle(int radius, int x, int y) {System.out.println("Drawing Circle[ color: green, radius: " + radius + ", x: " + x + ", " + y + "]");}
}abstract class Shape {protected DrawAPI drawAPI;protected Shape(DrawAPI drawAPI) {this.drawAPI = drawAPI;}public abstract void draw();
}class Circle extends Shape {private int x, y, radius;public Circle(int x, int y, int radius, DrawAPI drawAPI) {super(drawAPI);this.x = x;this.y = y;this.radius = radius;}public void draw() {drawAPI.drawCircle(radius, x, y);}
}public class BridgePatternDemo {public static void main(String[] args) {Shape redCircle = new Circle(100, 100, 10, new RedCircle());Shape greenCircle = new Circle(100, 100, 10, new GreenCircle());redCircle.draw();greenCircle.draw();}
}

8. 组合模式 (Composite Pattern)

类比：如同一个文件系统，文件夹可以包含文件或其他文件夹，文件和文件夹通过组合模式形成树形结构。

示例代码：

import java.util.ArrayList;
import java.util.List;class Employee {private String name;private String dept;private int salary;private List<Employee> subordinates;// 构造函数public Employee(String name, String dept, int salary) {this.name = name;this.dept = dept;this.salary = salary;subordinates = new ArrayList<Employee>();}public void add(Employee e) {subordinates.add(e);}public void remove(Employee e) {subordinates.remove(e);}public List<Employee> getSubordinates() {return subordinates;}public String toString() {return ("Employee :[ Name : " + name + ", dept : " + dept + ", salary :" + salary + " ]");}
}public class CompositePatternDemo {public static void main(String[] args) {Employee CEO = new Employee("John", "CEO", 30000);Employee headSales = new Employee("Robert", "Head Sales", 20000);Employee headMarketing = new Employee("Michel", "Head Marketing", 20000);Employee clerk1 = new Employee("Laura", "Marketing", 10000);Employee clerk2 = new Employee("Bob", "Marketing", 10000);Employee salesExecutive1 = new Employee("Richard", "Sales", 10000);Employee salesExecutive2 = new Employee("Rob", "Sales", 10000);CEO.add(headSales);CEO.add(headMarketing);headSales.add(salesExecutive1);headSales.add(salesExecutive2);headMarketing.add(clerk1);headMarketing.add(clerk2);// 打印所有员工System.out.println(CEO);for (Employee headEmployee : CEO.getSubordinates()) {System.out.println(headEmployee);for (Employee employee : headEmployee.getSubordinates()) {System.out.println(employee);}}}
}

9. 装饰器模式 (Decorator Pattern)

类比：如同在一件衣服上添加不同的装饰物，例如纽扣、拉链，可以在不改变衣服结构的前提下添加额外功能。

示例代码：

interface Shape {void draw();
}class Rectangle implements Shape {public void draw() {System.out.println("Shape: Rectangle");}
}class Circle implements Shape {public void draw() {System.out.println("Shape: Circle");}
}abstract class ShapeDecorator implements Shape {protected Shape decoratedShape;public ShapeDecorator(Shape decoratedShape) {this.decoratedShape = decoratedShape;}public void draw() {decoratedShape.draw();}
}class RedShapeDecorator extends ShapeDecorator {public RedShapeDecorator(Shape decoratedShape) {super(decoratedShape);}@Overridepublic void draw() {decoratedShape.draw();setRedBorder(decoratedShape);}private void setRedBorder(Shape decoratedShape) {System.out.println("Border Color: Red");}
}public class DecoratorPatternDemo {public static void main(String[] args) {Shape circle = new Circle();Shape redCircle = new RedShapeDecorator(new Circle());Shape redRectangle = new RedShapeDecorator(new Rectangle());System.out.println("Circle with normal border");circle.draw();System.out.println("\nCircle of red border");redCircle.draw();System.out.println("\nRectangle of red border");redRectangle.draw();}
}

10. 外观模式 (Facade Pattern)

类比：如同电影院售票系统，购票过程复杂但对外只提供简单的购票接口。

示例代码：

class ShapeMaker {private Shape circle;private Shape rectangle;private Shape square;public ShapeMaker() {circle = new Circle();rectangle = new Rectangle();square = new Square();}public void drawCircle() {circle.draw();}public void drawRectangle() {rectangle.draw();}public void drawSquare() {square.draw();}
}public class FacadePatternDemo {public static void main(String[] args) {ShapeMaker shapeMaker = new ShapeMaker();shapeMaker.drawCircle();shapeMaker.drawRectangle();shapeMaker.drawSquare();}
}

11. 享元模式 (Flyweight Pattern)

类比：如同在文本编辑器中，多个相同的字符只存储一份节省内存。

示例代码：

import java.util.HashMap;interface Shape {void draw();
}class Circle implements Shape {private String color;private int x;private int y;private int radius;public Circle(String color) {this.color = color;}public void setX(int x) {this.x = x;}public void setY(int y) {this.y = y;}public void setRadius(int radius) {this.radius = radius;}public void draw() {System.out.println("Circle: Draw() [Color : " + color + ", x : " + x + ", y :" + y + ", radius :" + radius);}
}class ShapeFactory {private static final HashMap<String, Shape> circleMap = new HashMap<>();public static Shape getCircle(String color) {Circle circle = (Circle) circleMap.get(color);if (circle == null) {circle = new Circle(color);circleMap.put(color, circle);System.out.println("Creating circle of color : " + color);}return circle;}
}public class FlyweightPatternDemo {private static final String colors[] = {"Red", "Green", "Blue", "White", "Black"};public static void main(String[] args) {for (int i = 0; i < 20; ++i) {Circle circle = (Circle) ShapeFactory.getCircle(getRandomColor());circle.setX(getRandomX());circle.setY(getRandomY());circle.setRadius(100);circle.draw();}}private static String getRandomColor() {return colors[(int) (Math.random() * colors.length)];}private static int getRandomX() {return (int) (Math.random() * 100);}private static int getRandomY() {return (int) (Math.random() * 100);}
}

12. 代理模式 (Proxy Pattern)

类比：如同明星请经纪人代理处理事务，经纪人是代理模式的实例。

示例代码：

interface Image {void display();
}class RealImage implements Image {private String fileName;public RealImage(String fileName) {this.fileName = fileName;loadFromDisk(fileName);}private void loadFromDisk(String fileName) {System.out.println("Loading " + fileName);}public void display() {System.out.println("Displaying " + fileName);}
}class ProxyImage implements Image {private RealImage realImage;private String fileName;public ProxyImage(String fileName) {this.fileName = fileName;}public void display() {if (realImage == null) {realImage = new RealImage(fileName);}realImage.display();}
}public class ProxyPatternDemo {public static void main(String[] args) {Image image = new ProxyImage("test_10mb.jpg");// 图像将从磁盘加载image.display();System.out.println("");// 图像不需要从磁盘加载image.display();}
}

三、行为型模式

13. 责任链模式 (Chain of Responsibility Pattern)

类比：如同企业内部的审批流程，申请需要层层递交直到满足条件。

示例代码：

abstract class AbstractLogger {public static int INFO = 1;public static int DEBUG = 2;public static int ERROR = 3;protected int level;// 责任链中的下一个元素protected AbstractLogger nextLogger;public void setNextLogger(AbstractLogger nextLogger) {this.nextLogger = nextLogger;}public void logMessage(int level, String message) {if (this.level <= level) {write(message);}if (nextLogger != null) {nextLogger.logMessage(level, message);}}abstract protected void write(String message);
}class ConsoleLogger extends AbstractLogger {public ConsoleLogger(int level) {this.level = level;}protected void write(String message) {System.out.println("Standard Console::Logger: " + message);}
}class ErrorLogger extends AbstractLogger {public ErrorLogger(int level) {this.level = level;}protected void write(String message) {System.out.println("Error Console::Logger: " + message);}
}class FileLogger extends AbstractLogger {public FileLogger(int level) {this.level = level;}protected void write(String message) {System.out.println("File::Logger: " + message);}
}public class ChainPatternDemo {private static AbstractLogger getChainOfLoggers() {AbstractLogger errorLogger = new ErrorLogger(AbstractLogger.ERROR);AbstractLogger fileLogger = new FileLogger(AbstractLogger.DEBUG);AbstractLogger consoleLogger = new ConsoleLogger(AbstractLogger.INFO);errorLogger.setNextLogger(fileLogger);fileLogger.setNextLogger(consoleLogger);return errorLogger;}public static void main(String[] args) {AbstractLogger loggerChain = getChainOfLoggers();loggerChain.logMessage(AbstractLogger.INFO, "This is an information.");loggerChain.logMessage(AbstractLogger.DEBUG, "This is a debug level information.");loggerChain.logMessage(AbstractLogger.ERROR, "This is an error information.");}
}

14. 命令模式 (Command Pattern)

类比：如同电灯开关，通过开关可以控制电灯的打开和关闭，开关就是命令模式的实例。

示例代码：

interface Order {void execute();
}class Stock {private String name = "ABC";private int quantity = 10;public void buy() {System.out.println("Stock [ Name: " + name + ", Quantity: " + quantity + " ] bought");}public void sell() {System.out.println("Stock [ Name: " + name + ", Quantity: " + quantity + " ] sold");}
}class BuyStock implements Order {private Stock abcStock;public BuyStock(Stock abcStock) {this.abcStock = abcStock;}public void execute() {abcStock.buy();}
}class SellStock implements Order {private Stock abcStock;public SellStock(Stock abcStock) {this.abcStock = abcStock;}public void execute() {abcStock.sell();}
}class Broker {private List<Order> orderList = new ArrayList<Order>();public void takeOrder(Order order) {orderList.add(order);}public void placeOrders() {for (Order order : orderList) {order.execute();}orderList.clear();}
}public class CommandPatternDemo {public static void main(String[] args) {Stock abcStock = new Stock();BuyStock buyStockOrder = new BuyStock(abcStock);SellStock sellStockOrder = new SellStock(abcStock);Broker broker = new Broker();broker.takeOrder(buyStockOrder);broker.takeOrder(sellStockOrder);broker.placeOrders();}
}

15. 解释器模式 (Interpreter Pattern)

类比：如同编程语言的解释器，将源代码解释为机器可以理解的指令。

示例代码：

interface Expression {boolean interpret(String context);
}class TerminalExpression implements Expression {private String data;public TerminalExpression(String data) {this.data = data;}public boolean interpret(String context) {if (context.contains(data)) {return true;}return false;}
}class OrExpression implements Expression {private Expression expr1;private Expression expr2;public OrExpression(Expression expr1, Expression expr2) {this.expr1 = expr1;this.expr2 = expr2;}public boolean interpret(String context) {return expr1.interpret(context) || expr2.interpret(context);}
}class AndExpression implements Expression {private Expression expr1;private Expression expr2;public AndExpression(Expression expr1, Expression expr2) {this.expr1 = expr1;this.expr2 = expr2;}public boolean interpret(String context) {return expr1.interpret(context) && expr2.interpret(context);}
}public class InterpreterPatternDemo {// 规则：Robert 和 John 是男性public static Expression getMaleExpression() {Expression robert = new TerminalExpression("Robert");Expression john = new TerminalExpression("John");return new OrExpression(robert, john);}// 规则：Julie 是一个已婚女性public static Expression getMarriedWomanExpression() {Expression julie = new TerminalExpression("Julie");Expression married = new TerminalExpression("Married");return new AndExpression(julie, married);}public static void main(String[] args) {Expression isMale = getMaleExpression();Expression isMarriedWoman = getMarriedWomanExpression();System.out.println("John is male? " + isMale.interpret("John"));System.out.println("Julie is a married woman? " + isMarriedWoman.interpret("Married Julie"));}
}

16. 迭代器模式 (Iterator Pattern)

类比：如同书架上的书，通过迭代器可以逐本取出书本。

示例代码：

interface Iterator {boolean hasNext();Object next();
}interface Container {Iterator getIterator();
}class NameRepository implements Container {public String names[] = {"Robert", "John", "Julie", "Lora"};public Iterator getIterator() {return new NameIterator();}private class NameIterator implements Iterator {int index;public boolean hasNext() {if (index < names.length) {return true;}return false;}public Object next() {if (this.hasNext()) {return names[index++];}return null;}}
}public class IteratorPatternDemo {public static void main(String[] args) {NameRepository namesRepository = new NameRepository();for (Iterator iter = namesRepository.getIterator(); iter.hasNext();) {String name = (String) iter.next();System.out.println("Name : " + name);}}
}

17. 中介者模式 (Mediator Pattern)

类比：如同机场的塔台，塔台作为中介者协调各飞机的起降。

示例代码：

class User {private String name;public String getName() {return name;}public void setName(String name) {this.name = name;}public User(String name) {this.name = name;}public void sendMessage(String message) {ChatRoom.showMessage(this, message);}
}class ChatRoom {public static void showMessage(User user, String message) {System.out.println(new Date().toString() + " [" + user.getName() + "] : " + message);}
}public class MediatorPatternDemo {public static void main(String[] args) {User robert = new User("Robert");User john = new User("John");robert.sendMessage("Hi! John!");john.sendMessage("Hello! Robert!");}
}

18. 备忘录模式 (Memento Pattern)

类比：如同游戏存档，可以在某个节点保存游戏状态并在需要时恢复。

示例代码：

class Memento {private String state;public Memento(String state) {this.state = state;}public String getState() {return state;}
}class Originator {private String state;public void setState(String state) {this.state = state;}public String getState() {return state;}public Memento saveStateToMemento() {return new Memento(state);}public void getStateFromMemento(Memento memento) {state = memento.getState();}
}class CareTaker {private List<Memento> mementoList = new ArrayList<Memento>();public void add(Memento state) {mementoList.add(state);}public Memento get(int index) {return mementoList.get(index);}
}public class MementoPatternDemo {public static void main(String[] args) {Originator originator = new Originator();CareTaker careTaker = new CareTaker();originator.setState("State #1");originator.setState("State #2");careTaker.add(originator.saveStateToMemento());originator.setState("State #3");careTaker.add(originator.saveStateToMemento());originator.setState("State #4");System.out.println("Current State: " + originator.getState());originator.getStateFromMemento(careTaker.get(0));System.out.println("First saved State: " + originator.getState());originator.getStateFromMemento(careTaker.get(1));System.out.println("Second saved State: " + originator.getState());}
}

19. 观察者模式 (Observer Pattern)

类比：如同报纸订阅者，报社有新报纸出版时会通知订阅者。

示例代码：

import java.util.ArrayList;
import java.util.List;class Subject {private List<Observer> observers = new ArrayList<Observer>();private int state;public int getState() {return state;}public void setState(int state) {this.state = state;notifyAllObservers();}public void attach(Observer observer) {observers.add(observer);}public void notifyAllObservers() {for (Observer observer : observers) {observer.update();}}
}abstract class Observer {protected Subject subject;public abstract void update();
}class BinaryObserver extends Observer {public BinaryObserver(Subject subject) {this.subject = subject;this.subject.attach(this);}public void update() {System.out.println("Binary String: " + Integer.toBinaryString(subject.getState()));}
}class OctalObserver extends Observer {public OctalObserver(Subject subject) {this.subject = subject;this.subject.attach(this);}public void update() {System.out.println("Octal String: " + Integer.toOctalString(subject.getState()));}
}class HexaObserver extends Observer {public HexaObserver(Subject subject) {this.subject = subject;this.subject.attach(this);}public void update() {System.out.println("Hex String: " + Integer.toHexString(subject.getState()).toUpperCase());}
}public class ObserverPatternDemo {public static void main(String[] args) {Subject subject = new Subject();new HexaObserver(subject);new OctalObserver(subject);new BinaryObserver(subject);System.out.println("First state change: 15");subject.setState(15);System.out.println("Second state change: 10");subject.setState(10);}
}

20. 状态模式 (State Pattern)

类比：如同电梯的状态，可以是上升、下降、静止，通过状态模式来管理状态转换。

示例代码：

interface State {void doAction(Context context);
}class StartState implements State {public void doAction(Context context) {System.out.println("Player is in start state");context.setState(this);}public String toString() {return "Start State";}
}class StopState implements State {public void doAction(Context context) {System.out.println("Player is in stop state");context.setState(this);}public String toString() {return "Stop State";}
}class Context {private State state;public Context() {state = null;}public void setState(State state) {this.state = state;}public State getState() {return state;}
}public class StatePatternDemo {public static void main(String[] args) {Context context = new Context();StartState startState = new StartState();startState.doAction(context);System.out.println(context.getState().toString());StopState stopState = new StopState();stopState.doAction(context);System.out.println(context.getState().toString());}
}

21. 策略模式 (Strategy Pattern)

类比：如同商场的促销策略，商场可以根据不同的节日选择不同的促销方案。

示例代码：

interface Strategy {int doOperation(int num1, int num2);
}class OperationAdd implements Strategy {public int doOperation(int num1, int num2) {return num1 + num2;}
}class OperationSubtract implements Strategy {public int doOperation(int num1, int num2) {return num1 - num2;}
}class OperationMultiply implements Strategy {public int doOperation(int num1, int num2) {return num1 * num2;}
}class Context {private Strategy strategy;public Context(Strategy strategy) {this.strategy = strategy;}public int executeStrategy(int num1, int num2) {return strategy.doOperation(num1, num2);}
}public class StrategyPatternDemo {public static void main(String[] args) {Context context = new Context(new OperationAdd());System.out.println("10 + 5 = " + context.executeStrategy(10, 5));context = new Context(new OperationSubtract());System.out.println("10 - 5 = " + context.executeStrategy(10, 5));context = new Context(new OperationMultiply());System.out.println("10 * 5 = " + context.executeStrategy(10, 5));}
}

22. 模板模式 (Template Pattern)

类比：如同烹饪菜谱，每道菜都有固定的流程，通过模板模式定义模板方法和步骤。

示例代码：

abstract class Game {abstract void initialize();abstract void startPlay();abstract void endPlay();// 模板方法public final void play() {initialize();startPlay();endPlay();}
}class Cricket extends Game {void initialize() {System.out.println("Cricket Game Initialized! Start playing.");}void startPlay() {System.out.println("Cricket Game Started. Enjoy the game!");}void endPlay() {System.out.println("Cricket Game Finished!");}
}class Football extends Game {void initialize() {System.out.println("Football Game Initialized! Start playing.");}void startPlay() {System.out.println("Football Game Started. Enjoy the game!");}void endPlay() {System.out.println("Football Game Finished!");}
}public class TemplatePatternDemo {public static void main(String[] args) {Game game = new Cricket();game.play();System.out.println();game = new Football();game.play();}
}

23. 访问者模式 (Visitor Pattern)

类比：如同博物馆导览员，可以根据游客身份提供不同的讲解服务。

示例代码：

interface ComputerPart {void accept(ComputerPartVisitor computerPartVisitor);
}class Keyboard implements ComputerPart {public void accept(ComputerPartVisitor computerPartVisitor) {computerPartVisitor.visit(this);}
}class Monitor implements ComputerPart {public void accept(ComputerPartVisitor computerPartVisitor) {computerPartVisitor.visit(this);}
}class Mouse implements ComputerPart {public void accept(ComputerPartVisitor computerPartVisitor) {computerPartVisitor.visit(this);}
}class Computer implements ComputerPart {ComputerPart[] parts;public Computer() {parts = new ComputerPart[]{new Mouse(), new Keyboard(), new Monitor()};}public void accept(ComputerPartVisitor computerPartVisitor) {for (int i = 0; i < parts.length; i++) {parts[i].accept(computerPartVisitor);}computerPartVisitor.visit(this);}
}interface ComputerPartVisitor {void visit(Computer computer);void visit(Mouse mouse);void visit(Keyboard keyboard);void visit(Monitor monitor);
}class ComputerPartDisplayVisitor implements ComputerPartVisitor {public void visit(Computer computer) {System.out.println("Displaying Computer.");}public void visit(Mouse mouse) {System.out.println("Displaying Mouse.");}public void visit(Keyboard keyboard) {System.out.println("Displaying Keyboard.");}public void visit(Monitor monitor) {System.out.println("Displaying Monitor.");}
}public class VisitorPatternDemo {public static void main(String[] args) {ComputerPart computer = new Computer();computer.accept(new ComputerPartDisplayVisitor());}
}