设计模式-结构型-享元模式(Flyweight)

概述
运用共享技术有效地支持大量细粒度的对象。

适用情形
    当都具备下列情况时，使用Flyweight模式：
    1.一个应用程序使用了大量的对象。
    2.完全由于使用大量的对象，造成很大的存储开销。
    3.对象的大多数状态都可变为外部状态。
    4.如果删除对象的外部状态，那么可以用相对较少的共享对象取代很多组对象。
    5.应用程序不依赖于对象标识。由于Flyweight对象可以被共享，对于概念上明显有别的对象，标识测试将返回真值。
参与者
    1.Flyweight      描述一个接口，通过这个接口flyweight可以接受并作用于外部状态。
    2.ConcreteFlyweight
      实现Flyweight接口，并为内部状态（如果有的话）增加存储空间。      ConcreteFlyweight对象必须是可共享的。它所存储的状态必须是内部的；即，它必须独立于ConcreteFlyweight对象的场景。
    3.UnsharedConcreteFlyweight      并非所有的Flyweight子类都需要被共享。Flyweight接口使共享成为可能，但它并不强制共享。      在Flyweight对象结构的某些层次，UnsharedConcreteFlyweight对象通常将ConcreteFlyweight对象作为子节点。
    4.FlyweightFactory      创建并管理flyweight对象。      确保合理地共享flyweight。当用户请求一个flyweight时，FlyweightFactory对象提供一个已创建的实例或者创建一个（如果不存在的话）。
结构图

示例代码：
package com.sql9.structured;

import java.util.ArrayList;
import java.util.List;

abstract class Flyweight
{
    abstract public void doOperation(int extrinsicState);
}

class UnsharedConcreteFlyweight extends Flyweight
{
    @Override
    public void doOperation(int extrinsicState)
    {
    }
}

class ConcreteEvenFlyweight extends Flyweight
{
    @Override
    public void doOperation(int extrinsicState)
    {
        System.out.println("In ConcreteEvenFlyweight.DoOperation: " + extrinsicState);
    }
}

class ConcreteUnevenFlyweight extends Flyweight
{
    @Override
    public void doOperation(int extrinsicState)
    {
        System.out.println("In ConcreteUnevenFlyweight.DoOperation: " + extrinsicState);
    }
}

class FlyweightFactory
{
    private List<Flyweight> pool = new ArrayList<Flyweight>();

    // the flyweightfactory can crete all entries in the pool at startup
    // (if the pool is small, and it is likely all will be used), or as
    // needed, if the pool si large and it is likely some will never be used
    public FlyweightFactory()
    {
        pool.add(new ConcreteEvenFlyweight());
        pool.add(new ConcreteUnevenFlyweight());
    }

    public Flyweight getFlyweight(int key)
    {
        // here we would determine if the flyweight identified by key
        // exists, and if so return it. If not, we would create it.
        // As in this demo we have implementation all the possible
        // flyweights we wish to use, we retrun the suitable one.
        int i = key % 2;
        return((Flyweight)pool.get(i));
    }
}

public class FlyweightTest {

    public static void main(String[] args) {
        int[] data = {1,2,3,4,5,6,7,8};

        FlyweightFactory f = new FlyweightFactory();

        int extrinsicState = 3;
        for (int i : data)
        {
            Flyweight flyweight = f.getFlyweight(i);
            flyweight.doOperation(extrinsicState);
        }
    }
}

结果
In ConcreteUnevenFlyweight.DoOperation: 3
In ConcreteEvenFlyweight.DoOperation: 3
In ConcreteUnevenFlyweight.DoOperation: 3
In ConcreteEvenFlyweight.DoOperation: 3
In ConcreteUnevenFlyweight.DoOperation: 3
In ConcreteEvenFlyweight.DoOperation: 3
In ConcreteUnevenFlyweight.DoOperation: 3
In ConcreteEvenFlyweight.DoOperation: 3

补充：软件开发 , Java ,