如何理解這6種常見設(shè)計模式？

package FactoryMethod;public class FactoryPattern{public static void main(String[] args){        Factory factory = new ConcreteFactoryA();        Product product = factory.createProduct();        product.use();    }}//抽象產(chǎn)品：提供了產(chǎn)品的接口interface Product{public void use();}//具體產(chǎn)品A：實現(xiàn)抽象產(chǎn)品中的抽象方法class ConcreteProductA implements Product{public void use(){        System.out.println("具體產(chǎn)品A顯示...");    }}//具體產(chǎn)品B：實現(xiàn)抽象產(chǎn)品中的抽象方法class ConcreteProductB implements Product{public void use(){        System.out.println("具體產(chǎn)品B顯示...");    }}//抽象工廠：提供了廠品的生成方法interface Factory{public Product createProduct();}//具體工廠A：實現(xiàn)了廠品的生成方法class ConcreteFactoryA implements AbstractFactory{public Product createProduct(){        System.out.println("具體工廠A生成-->具體產(chǎn)品A.");return new ConcreteProductA();    }}//具體工廠B：實現(xiàn)了廠品的生成方法class ConcreteFactoryB implements AbstractFactory{public Product createProduct(){        System.out.println("具體工廠B生成-->具體產(chǎn)品B.");return new ConcreteProductB();    }}

public class Singleton{private static volatile Singleton instance=null;    //保證 instance 在所有線程中同步private Singleton(){}    //private 避免類在外部被實例化public static synchronized Singleton getInstance(){//getInstance 方法前加同步if(instance == null)        {            instance = new Singleton();        }return instance;    }}

package decorator;public class DecoratorPattern{public static void main(String[] args){        Component component = new ConcreteComponent();        component.operation();        System.out.println("---------------------------------");        Component decorator = new ConcreteDecorator(component);        decorator.operation();    }}//抽象構(gòu)件角色interface  Component{public void operation();}//具體構(gòu)件角色class ConcreteComponent implements Component{public ConcreteComponent(){        System.out.println("創(chuàng)建具體構(gòu)件角色");           }   public void operation(){        System.out.println("調(diào)用具體構(gòu)件角色的方法operation()");               }}//抽象裝飾角色class Decorator implements Component{private Component component;   public Decorator(Component component){this.component=component;    }   public void operation(){        component.operation();    }}//具體裝飾角色class ConcreteDecorator extends Decorator{public ConcreteDecorator(Component component){super(component);    }   public void operation(){super.operation();        addBehavior();    }public void addBehavior(){        System.out.println("為具體構(gòu)件角色增加額外的功能addBehavior()");               }}

package strategy;public class StrategyPattern{public static void main(String[] args){        Context context = new Context();        Strategy strategyA = new ConcreteStrategyA();        context.setStrategy(strategyA);        context.algorithm();        System.out.println("-----------------");        Strategy strategyB = new ConcreteStrategyB();        context.setStrategy(strategyB);        context.algorithm();    }}//抽象策略類interface Strategy{   public void algorithm();    //策略方法}//具體策略類Aclass ConcreteStrategyA implements Strategy{public void algorithm(){        System.out.println("具體策略A的策略方法被訪問！");    }}//具體策略類Bclass ConcreteStrategyB implements Strategy{public void algorithm(){      System.out.println("具體策略B的策略方法被訪問！");  }}//環(huán)境類class Context{private Strategy strategy;public Strategy getStrategy(){return strategy;    }public void setStrategy(Strategy strategy){this.strategy=strategy;    }public void algorithm(){        strategy.algorithm();    }}

package proxy;public class ProxyPattern{public static void main(String[] args){        Proxy proxy = new Proxy();        proxy.request();    }}//抽象主題interface Subject{void request();}//真實主題class RealSubject implements Subject{public void request(){        System.out.println("訪問真實主題方法...");    }}//代理class Proxy implements Subject{private RealSubject realSubject;public void request(){if (realSubject==null)        {            realSubject=new RealSubject();        }        preRequest();        realSubject.request();        afterRequest();    }public void preRequest(){        System.out.println("訪問真實主題之前的預(yù)處理。");    }public void afterRequest(){        System.out.println("訪問真實主題之后的后續(xù)處理。");    }}

package observer;import java.util.*;public class ObserverPattern{public static void main(String[] args)    {        Subject subject = new ConcreteSubject();        Observer obsA = new ConcreteObserverA();        Observer obsb = new ConcreteObserverB();        subject.add(obsA);        subject.add(obsB);        subject.setState(0);    }}//抽象目標(biāo)abstract class Subject{protected List observerList = new ArrayList();   //增加觀察者方法public void add(Observer observer)    {        observers.add(observer);    }    //刪除觀察者方法public void remove(Observer observer)    {        observers.remove(observer);    }   public abstract void notify(); //通知觀察者方法}//具體目標(biāo)class ConcreteSubject extends Subject{private Integer state;public void setState(Integer state){this.state = state;
// 狀態(tài)改變通知觀察者        notify();    }public void notify()    {        System.out.println("具體目標(biāo)狀態(tài)發(fā)生改變...");        System.out.println("--------------");       
for(Observer obs:observers)        {            obs.process();        }
    }          }//抽象觀察者interface Observer{void process(); //具體的處理}//具體觀察者Aclass ConcreteObserverA implements Observer{public void process()    {        System.out.println("具體觀察者A處理！");    }}//具體觀察者Bclass ConcreteObserverB implements Observer{public void process()    {        System.out.println("具體觀察者B處理！");    }}