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

1. public class FootballPlayer {   
2.   
3.     public int name;   
4.   
5.     public int number;   
6.        
7.     FootballTeam footballTeam;   
8. }  

java 代码

1. public class FootballTeam {   
2.   
3.     public String name;   
4.   
5.     public int numPlayers;   
6.   
7.     public String homeCity;   
8.   
9.     Collection<FootballPlayer> footballPlayer;   
10.   
11.     FootballLeague footballLeague;   
12. }  

java 代码

1. public class FootballLeague {   
2.   
3.     public String name;   
4.   
5.     public int numTeams;   
6.   
7.     Collection<FootballTeam> footballTeam;   
8. }  

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

1. public class FootballPlayer {   
2.   
3.     /**  
4.      * @model.uin <code>design:node:::hnimszew7l00y4-ezsg1t:-tdsqjuew7kty3j-7fwj74</code>  
5.      */  
6.     public int name;   
7.   
8.     /**  
9.      * @model.uin <code>design:node:::2plt7ew7l4dei-q4hyxp:-tdsqjuew7kty3j-7fwj74</code>  
10.      */  
11.     public int number;   
12.   
13.     /**  
14.      * @model.uin <code>design:link:::ieje0rew8ip8hf-3ho9xj:-2urp5jew7l1xaj1vaegh</code>  
15.      */  
16.     FootballLeague footballLeague;   
17.   
18.     /**  
19.      * @model.uin <code>design:node:::-s99ai1ew7l1xajmc7cgz</code>  
20.      */  
21.     FootballTeam footballTeam;   
22. }   
23.   
24. public class FootballTeam {   
25.   
26.     /**  
27.      * @model.uin <code>design:node:::hnimszew7lak9j-67pmjw:-s99ai1ew7l1xajmc7cgz</code>  
28.      */  
29.     public String name;   
30.   
31.     /**  
32.      * @model.uin <code>design:node:::hnimszew7lb0vy-dlg5p6:-s99ai1ew7l1xajmc7cgz</code>  
33.      */  
34.     public int numPlayers;   
35.   
36.     /**  
37.      * @model.uin <code>design:node:::hnimszew7lbej9s8soby:-s99ai1ew7l1xajmc7cgz</code>  
38.      */  
39.     public String homeCity;   
40.   
41.     /**  
42.      * @model.uin <code>design:link:::ieje0rew8ip4w3-eo406e:-2urp5jew7l1xaj1vaegh</code>  
43.      */  
44.     FootballPlayer footballPlayer;   
45.   
46.     /**  
47.      * @model.uin <code>design:link:::ieje0rew8ip8hf-3ho9xj:-2urp5jew7l1xaj1vaegh</code>  
48.      */  
49.     FootballLeague footballLeague;   
50. }   
51.   
52. public class FootballLeague {   
53.   
54.     /**  
55.      * @model.uin <code>design:node:::hnimszew7l0j2az6d4si:-tdsqjuew7ku7zj-apfzse</code>  
56.      */  
57.     public String name;   
58.   
59.     /**  
60.      * @model.uin <code>design:node:::hnimszew7l0vjz-ahe69h:-tdsqjuew7ku7zj-apfzse</code>  
61.      */  
62.     public int numTeams;   
63.   
64.     /**  
65.      * @model.uin <code>design:link:::ieje0rew8ip4w3-eo406e:-2urp5jew7l1xaj1vaegh</code>  
66.      */  
67.     Collection<FootballPlayer> footballPlayer;   
68.   
69.     /**  
70.      * @model.uin <code>design:node:::-s99ai1ew7l1xajmc7cgz</code>  
71.      */  
72.     Collection<FootballTeam> footballTeam;   
73. }  

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。

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。