对association class的疑问

今天看association class怎么看好像都不是很明白。

摘自UML2.0 in a Nutshell

2.6.6. Association Classes

Often the relationship between two elements isn't a simple structural connection. For example, a football player may be associated with a league by virtue of being on a team. If the association between two elements is complex, you can represent the connection using an association class. An association class is an association that has a name and attributes, like a normal class. You show an association class like a regular class with a dashed line connecting it to the association it represents. Figure 2-29 shows a football player's relationships to a league.

 

Figure 2-29. Example association class

 

When translated into code, relationships with association classes often result in three classes: one for each end of the association and one for the association class itself. There may or may not be a direct link between the association ends; the implementation may require you to traverse through the association class to get to the opposite end of the link. In other words, FootballPlayer may not have a direct reference to FootballLeague but may have a reference to FootballTeam instead. FootballTeam would then have a reference to FootballLeague. How the relationships are constructed is a matter of implementation choices; however, the fundamental concept of an association class is unchanged.

按照我的理解以上例子其实看成是FootballPlayer和FootballTeam的Aggregation relationship和FootballTeam和FootballLeague的Aggregation relationship.只不过用Association Classes可以更好的表达FootballPlayer和FootballLeague的间接关系。但这两种表达方法最后的代码表达应该都是一样的。

[代码表现]（我认为）

java 代码

public class FootballPlayer {

    public int name;

    public int number;

    FootballTeam footballTeam;
}

java 代码

public class FootballTeam {

    public String name;

    public int numPlayers;

    public String homeCity;

    Collection<FootballPlayer> footballPlayer;

    FootballLeague footballLeague;
}

java 代码

public class FootballLeague {

    public String name;

    public int numTeams;

    Collection<FootballTeam> footballTeam;
}

但用together生成的代码和我的想法又不一样

[代码表现]（together generation）

java 代码

public class FootballPlayer {

    /**
     * @model.uin <code>design:node:::hnimszew7l00y4-ezsg1t:-tdsqjuew7kty3j-7fwj74</code>
     */
    public int name;

    /**
     * @model.uin <code>design:node:::2plt7ew7l4dei-q4hyxp:-tdsqjuew7kty3j-7fwj74</code>
     */
    public int number;

    /**
     * @model.uin <code>design:link:::ieje0rew8ip8hf-3ho9xj:-2urp5jew7l1xaj1vaegh</code>
     */
    FootballLeague footballLeague;

    /**
     * @model.uin <code>design:node:::-s99ai1ew7l1xajmc7cgz</code>
     */
    FootballTeam footballTeam;
}

public class FootballTeam {

    /**
     * @model.uin <code>design:node:::hnimszew7lak9j-67pmjw:-s99ai1ew7l1xajmc7cgz</code>
     */
    public String name;

    /**
     * @model.uin <code>design:node:::hnimszew7lb0vy-dlg5p6:-s99ai1ew7l1xajmc7cgz</code>
     */
    public int numPlayers;

    /**
     * @model.uin <code>design:node:::hnimszew7lbej9s8soby:-s99ai1ew7l1xajmc7cgz</code>
     */
    public String homeCity;

    /**
     * @model.uin <code>design:link:::ieje0rew8ip4w3-eo406e:-2urp5jew7l1xaj1vaegh</code>
     */
    FootballPlayer footballPlayer;

    /**
     * @model.uin <code>design:link:::ieje0rew8ip8hf-3ho9xj:-2urp5jew7l1xaj1vaegh</code>
     */
    FootballLeague footballLeague;
}

public class FootballLeague {

    /**
     * @model.uin <code>design:node:::hnimszew7l0j2az6d4si:-tdsqjuew7ku7zj-apfzse</code>
     */
    public String name;

    /**
     * @model.uin <code>design:node:::hnimszew7l0vjz-ahe69h:-tdsqjuew7ku7zj-apfzse</code>
     */
    public int numTeams;

    /**
     * @model.uin <code>design:link:::ieje0rew8ip4w3-eo406e:-2urp5jew7l1xaj1vaegh</code>
     */
    Collection<FootballPlayer> footballPlayer;

    /**
     * @model.uin <code>design:node:::-s99ai1ew7l1xajmc7cgz</code>
     */
    Collection<FootballTeam> footballTeam;
}

所以我现在我都不清楚这个association class到底是怎么用的了。假如那位有任何思路请一起讨论下。

评论

1 楼 ocean 2006-12-28  

今天看了UML Distilled这本书的解释才好像明白了这个association class关系。
摘自（UML Distilled third）
Association Class
Association classes allow you to add attributes, operations, and other features to associations.
然后再看了下
http://www.javaworld.com.tw/jute/post/view?bid=33&id=92661&tpg=7&ppg=1&sty=1&age=0#92661网上的另一个评论。
以下是我的总结。
Association Class其实对于数据库中的多对多关系时是非常有用的。而这个association class就相当于数据库中的那个中间表的作用。