java23種設計模式十(連載)

10、享元模式

        享元模式（Flyweight Pattern）顧名思義，就是多個對象共享一個對象，即共用一個內存地址，在實際應用中，採用享元模式的好處就是能夠大大節約內存空間，提升系統的運行效率。享元模式常常會出如今工廠模式當中，下面是具體的實現代碼：

首先建立一個享元類：

public abstract class Flyweight {

	public abstract void operation();
}

/**
 * 具體的享元類
 * @author Lynn
 *
 */
public class ConcreteFlyweight extends Flyweight {

	private String str;
	public ConcreteFlyweight(String str){
		this.str = str;
	}
	
	@Override
	public void operation() {
		System.out.println(str);
	}

}

而後建立一個工廠類，用於實體化享元類：

import java.util.Hashtable;

public class FlyweightFactory {

	private Hashtable<Object, Flyweight> flyweights = new Hashtable<>();
	public Flyweight getFlyweight(Object obj){
		Flyweight flyweight = flyweights.get(obj);
		if(null == flyweight){
			flyweight = new ConcreteFlyweight((String)obj);
			flyweights.put(obj, flyweight);
		}
		return flyweight;
	}
	
	public int getFlyweightSize(){
		return flyweights.size();
	}
}

這裏採用hashtable保存每一個享元對象，以便下次直接使用，下面是測試方法：

public class Client {

	FlyweightFactory factory = new FlyweightFactory();   
	Flyweight fly1;  
	Flyweight fly2;  
	Flyweight fly3;  
	Flyweight fly4;  
	Flyweight fly5;  
	Flyweight fly6;  

	public Client(){  
		fly1 = factory.getFlyweight("1");  
		fly2 = factory.getFlyweight("2");  
		fly3 = factory.getFlyweight("1");  
		fly4 = factory.getFlyweight("1");  
		fly5 = factory.getFlyweight("1");  
		fly6 = factory.getFlyweight("1");  
	}  
	public void showFlyweight(){  
		fly1.operation();  
		fly2.operation();  
		fly3.operation();  
		fly4.operation();  
		fly5.operation();  
		fly6.operation();  
		int size = factory.getFlyweightSize();  
		System.out.println("size = " + size);  
	}  
	public static void main(String[] args){  
		Client client = new Client();  
		client.showFlyweight();  
	}  
}

這樣實際上只建立了兩個對象，key 爲1和key爲2的

1
2
1
1
1
1
size = 2

在java中，String就是一個典型的採用享元模式的例子，String是final的，不能被繼承的，它也是不可變的，每次建立字符串對象都會保存到對象池中，下次使用到直接從對象池中取出字符串，而不會從新構建對象，下面的例子你們應該不陌生，

String a = "abc";

String b = "abc";

System.out.println(a==b);

「=」是比較的內存地址，這個例子當中打印爲true，說明他們的內存地址同樣，也就是都指向一個地址，即共享了一個對象。