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前言

Java 中常用的设计模式有很多种，其实平常用到的还比较少，但是还是有必要了解一下，可以按照实际情况运用到我们的代码中。按照类型可以基本分解为，创建型模式、结构型模式和行为型模式。

创建型模式 (Creational Patterns)

1. 工厂方法模式 (Factory Method Pattern):

定义一个创建对象的接口，但是将实际的创建工作交给子类来实现。
考虑一个简单的工厂方法，生产不同类型的形状对象：

  interface Shape {void draw();}class Circle implements Shape {@Overridepublic void draw() {System.out.println("Inside Circle::draw() method.");}}class Rectangle implements Shape {@Overridepublic void draw() {System.out.println("Inside Rectangle::draw() method.");}}class ShapeFactory {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();}return null;}}

2. 抽象工厂模式 (Abstract Factory Pattern)

提供一个创建一系列相关或相互依赖对象的接口，而无需指定它们具体的类。
考虑一个抽象工厂，生产不同类型的形状和颜色对象的组合：

   interface Shape {void draw();}interface Color {void fill();}class Rectangle implements Shape {@Overridepublic void draw() {System.out.println("Inside Rectangle::draw() method.");}}class Blue implements Color {@Overridepublic void fill() {System.out.println("Inside Blue::fill() method.");}}abstract class AbstractFactory {abstract Shape getShape(String shapeType);abstract Color getColor(String colorType);}class ShapeFactory extends AbstractFactory {@OverrideShape getShape(String shapeType) {if (shapeType.equalsIgnoreCase("RECTANGLE")) {return new Rectangle();}return null;}@OverrideColor getColor(String colorType) {return null;}}class ColorFactory extends AbstractFactory {@OverrideShape getShape(String shapeType) {return null;}@OverrideColor getColor(String colorType) {if (colorType.equalsIgnoreCase("BLUE")) {return new Blue();}return null;}}

3. 建造者模式 (Builder Pattern)

将一个复杂对象的构建与其表示分离，使得同样的构建过程可以创建不同的表示。
考虑一个简单的食物构建器，可以构建汉堡和冷饮：

    class Burger {private String size;private boolean cheese;private boolean pepperoni;private boolean lettuce;public Burger(BurgerBuilder builder) {this.size = builder.size;this.cheese = builder.cheese;this.pepperoni = builder.pepperoni;this.lettuce = builder.lettuce;}static class BurgerBuilder {private String size;private boolean cheese;private boolean pepperoni;private boolean lettuce;public BurgerBuilder(String size) {this.size = size;}public BurgerBuilder cheese(boolean cheese) {this.cheese = cheese;return this;}public BurgerBuilder pepperoni(boolean pepperoni) {this.pepperoni = pepperoni;return this;}public BurgerBuilder lettuce(boolean lettuce) {this.lettuce = lettuce;return this;}public Burger build() {return new Burger(this);}}}

4. 单例模式 (Singleton Pattern)

确保一个类只有一个实例，并提供一个全局访问点。
实现一个线程安全的懒汉式单例：

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

 

结构型模式 (Structural Patterns)

1.适配器模式 (Adapter Pattern)

将一个类的接口转换成客户端希望的另一个接口。
使用适配器让美国插头可以插入英国插座：

 interface UKPlug {void provideElectricity();}class UKSocket implements UKPlug {@Overridepublic void provideElectricity() {System.out.println("Providing electricity with UK plug.");}}interface USPlug {void supplyElectricity();}class USPlugConnector implements USPlug {@Overridepublic void supplyElectricity() {System.out.println("Supplying electricity with US plug.");}}class USPlugAdapter implements UKPlug {private USPlug plug;public USPlugAdapter(USPlug plug) {this.plug = plug;}@Overridepublic void provideElectricity() {plug.supplyElectricity();}}

2. 桥接模式 (Bridge Pattern)

将抽象部分与它的实现部分分离，使它们可以独立地变化。
将不同颜色的笔与不同类型的绘图设备连接起来：

  interface DrawAPI {void draw();}abstract class Shape {protected DrawAPI drawAPI;protected Shape(DrawAPI drawAPI) {this.drawAPI = drawAPI;}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;}@Overridevoid draw() {drawAPI.draw();}}

3. 组合模式 (Composite Pattern)

将对象组合成树形结构以表示“部分-整体”的层次结构。
用组合模式表示员工的层级结构：

     import java.util.ArrayList;import java.util.List;class Employee {private String name;private String dept;private int salary;private List<Employee> subordinates;public Employee(String name, String dept, int sal) {this.name = name;this.dept = dept;this.salary = sal;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;}@Overridepublic String toString() {return ("Employee :[ Name : " + name + ", dept : " + dept + ", salary :" + salary + " ]");}}

行为型模式 (Behavioral Patterns)

1. 观察者模式 (Observer Pattern)

定义对象间的一对多依赖关系，当一个对象状态发生改变时，它的所有依赖者都会收到通知并自动更新。
实现一个简单的气象站，当气象数据发生变化时，通知所有注册的观察者：

  import java.util.ArrayList;import java.util.List;interface Observer {void update(float temperature, float humidity, float pressure);}interface Subject {void registerObserver(Observer observer);void removeObserver(Observer observer);void notifyObservers();}class WeatherData implements Subject {private List<Observer> observers;private float temperature;private float humidity;private float pressure;public WeatherData() {observers = new ArrayList<>();}public void registerObserver(Observer observer) {observers.add(observer);}public void removeObserver(Observer observer) {observers.remove(observer);}public void notifyObservers() {for (Observer observer : observers) {observer.update(temperature, humidity, pressure);}}public void measurementsChanged() {notifyObservers();}public void setMeasurements(float temperature, float humidity, float pressure) {this.temperature = temperature;this.humidity = humidity;this.pressure = pressure;measurementsChanged();}}

2. 状态模式 (State Pattern)

允许对象在内部状态改变时改变它的行为。
使用状态模式实现一个简单的电梯控制系统：
 

   interface State {void doAction(Context context);}class Context {private State state;public void setState(State state) {this.state = state;}public State getState() {return state;}}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";}}

3. 策略模式 (Strategy Pattern)

定义一系列算法，将每一个算法封装起来，并使它们可以互相替换。
实现一个简单的排序器，可以根据不同的策略选择不同的排序算法：
 

   import java.util.List;interface SortStrategy {<T extends Comparable<T>> void sort(List<T> items);}class BubbleSortStrategy implements SortStrategy {public <T extends Comparable<T>> void sort(List<T> items) {// 实现冒泡排序算法}}class QuickSortStrategy implements SortStrategy {public <T extends Comparable<T>> void sort(List<T> items) {// 实现快速排序算法}}class Sorter {private SortStrategy strategy;public Sorter(SortStrategy strategy) {this.strategy = strategy;}public <T extends Comparable<T>> void sort(List<T> items) {strategy.sort(items);}}

这只是一些常见的设计模式，Java一共有23种设计模式，实际掌握几种设计模式就可以帮助你更好地组织代码、提高代码的可维护性和可扩展性。