关于对聚合和组合的理解

今天看了看

深入PHP++面向对象、模式与实践_第三版

其中看到“聚合”跟“组合”的概念，对此理解也比较模糊
文中是这样说的：

理解上面文本中的话，我个人理解是成这样：

首先是聚合

    <?php
/**
 * Created by PhpStorm.
 * ClassName: AggregationA
 * User: JAN
 * Date: 14-9-12
 * Time: 下午12:18
 * Link: http://blog.segmentfault.com/janhuang
 */

namespace DesignPattern\SimpleExample\Aggregation;

class AggregationA
{
    protected $aggregation = array();

    public function __construct()
    {
        array_push($this->aggregation, new AggregationB());
        array_push($this->aggregation, new AggregationC());
    }

    public function addAggregation(AggregationInterface $aggregation)
    {
        array_push($this->aggregation, $aggregation);

        return $this;
    }

    public function getAggregationList()
    {
        return $this->aggregation;
    }
} 

    <?php
/**
 * Created by PhpStorm.
 * ClassName: AggregationB
 * User: JAN
 * Date: 14-9-12
 * Time: 下午12:18
 * Link: http://blog.segmentfault.com/janhuang
 */

namespace DesignPattern\SimpleExample\Aggregation;

class AggregationB implements AggregationInterface
{
    public function getName()
    {
        return $this;
    }
} 

    <?php
/**
 * Created by PhpStorm.
 * ClassName: AggregationC
 * User: JAN
 * Date: 14-9-12
 * Time: 下午2:34
 * Link: http://blog.segmentfault.com/janhuang
 */

namespace DesignPattern\SimpleExample\Aggregation;

class AggregationC implements AggregationInterface
{
    public function getName()
    {
        return $this;
    }
} 

    <?php
/**
 * Created by PhpStorm.
 * ClassName: AggregationInterface
 * User: JAN
 * Date: 14-9-12
 * Time: 下午2:38
 * Link: http://blog.segmentfault.com/janhuang
 */

namespace DesignPattern\SimpleExample\Aggregation;

interface AggregationInterface
{
    public function getName();
} 

类内聚了两个类：分别是agregationB, agregationC 分别都实现了AgregationInterface, 而且在实力AgregationA的时候构造时实例化B和C。

再次是组合：

A类

    <?php
/**
 * Created by PhpStorm.
 * ClassName: CompositionA
 * User: JAN
 * Date: 14-9-12
 * Time: 下午12:25
 * Link: http://blog.segmentfault.com/janhuang
 */

namespace DesignPattern\SimpleExample\Composition;

class CompositionA
{
    private $compositionB = null;

    private $compositionC = null;

    public function setCompositionB(CompositionB $compositionB)
    {
        $this->compositionB = $compositionB;

        return $this;
    }

    public function getCompositionB()
    {
        return $this->compositionB;
    }

    public function setCompositionC(CompositionC $compositionC)
    {
        $this->compositionC = $compositionC;

        return $this;
    }

    public function getCompositionC()
    {
        return $this->compositionC;
    }
}

B类

    <?php
/**
 * Created by PhpStorm.
 * ClassName: CompositionB
 * User: JAN
 * Date: 14-9-12
 * Time: 下午12:25
 * Link: http://blog.segmentfault.com/janhuang
 */

namespace DesignPattern\SimpleExample\Composition;

class CompositionB implements CompositionObjectInterface
{
    public function getName()
    {
        return $this;
    }
} 

C类

    <?php
/**
 * Created by PhpStorm.
 * ClassName: CompositionC
 * User: JAN
 * Date: 14-9-12
 * Time: 下午2:42
 * Link: http://blog.segmentfault.com/janhuang
 */

namespace DesignPattern\SimpleExample\Composition;

class CompositionC implements CompositionObjectInterface
{
    public function getName()
    {
        return $this;
    }
} 

接口

    <?php
/**
 * Created by PhpStorm.
 * ClassName: CompositionObjectInterface
 * User: JAN
 * Date: 14-9-12
 * Time: 下午12:29
 * Link: http://blog.segmentfault.com/janhuang
 */

namespace DesignPattern\SimpleExample\Composition;

interface CompositionObjectInterface
{
    public function getName();
} 

在实例化CompositionA 的时候并不会直接实例化其他类，这里只是清楚限定了CompositionB类作为参数类型传入setCompotionB方法，这里理解的组合就是：有提供组合的方法提供外界指定的类进行自由组合，并且使用组合当中的方法。

以上仅是个人的理解，不知道这对类的关系有没有帮助，最近在学习设计模式，概念有点模糊，还请各路英雄指点。感谢