设计模式——享元模式

享元模式是什么？

运用共享技术有效地支持大量细粒度的对象

享元模式解决什么问题？

如淘宝中每个店家的网站有首页、分类、会员等

public class WebSite {
    private String type = "";

    public WebSite(String type) {
        this.type= type;
    }

    public void show() {
        System.out.println("网站类型: " + type);
    }
}

当使用时

 WebSite A = new WebSite("首页");
 A.show();
 
 WebSite B = new WebSite("首页");
 B.show();

实际上不可能为每个商家都新建一个网站实例，不同网站之间只有数据不同

享元模式实现

抽象WebSite，展示的时候传入数据加载

abstract class WebSite {
    abstract void show(User user);
}

class concreteWebSite extends WebSite {
    private String type = "";

    public concreteWebSite(String name) {
        this.type = name;
    }

    @Override
    void show(User user) {
        System.out.println("网站类型: " + type + ", 用户:" + user.getName());
    }
}

class User {
    private String name;

    public User(String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }
}

创建工厂维护WebSite，第一次创建时存起来，此后返回已创建的对象

class WebSiteFactory {
    private Hashtable flyweights = new Hashtable<>();

    public WebSite getWebSite(String key) {
        if (!flyweights.contains(key)) {
            flyweights.put(key, new concreteWebSite(key));
        }
        return flyweights.get(key);
    }

    public int getWebSiteCount() {
        return flyweights.size();
    }
}

当使用时，从工厂中取出WebSite，为其设置数据

WebSiteFactory webSiteFactory = new WebSiteFactory();
WebSite A = webSiteFactory.getWebSite("首页");
A.show(new User("A"));

WebSite B = webSiteFactory.getWebSite("首页");
B.show(new User("B"));

WebSite C = webSiteFactory.getWebSite("分类");
C.show(new User("C"));

System.out.println("网站数量: "+webSiteFactory.getWebSiteCount());

String的实现就是运用了享元模式

String s1 = new String("s");
String s2 = new String("s");
System.out.println("s1==s2: " + (s1 == s2));
System.out.println("s1.equals(s2): " + s1.equals(s2));

String s3 = "s";
String s4 = "s";
System.out.println("s3 == s4: " + (s3 == s4));
System.out.println("s3.equals(s4): " + s3.equals(s4));

打印如下，当使用new时，创建的对象地址不一样，当直接赋值时，s4的引用指向字符串"s"

s1==s2: false
s1.equals(s2): true
s3 == s4: true
s3.equals(s4): true