UML类图中5中关系的辨析（修订）
?? Figure 1. UML offers five different types of class relationship
? 要理解这5中关系就不能简单的把它们和具体的语言实现相关联，UML介于问题域和解域之间，因而兼有这两者的概念。虽然类图是静态的，但类之间的这5种关系却是动静兼有的（也就是编译时和运行时混合的）。
??? 问题域、解域混合，编译时、运行时混合是这5种关系的特点。
??? 在UML规范中对这种关系的定义非常模糊，充斥这也许、可能这类字眼。《UML Distilled》中甚至说“聚合是完全没有意义的。因此，我建议，你在自己的的图中略去聚合。”
??? 我个人也认为Aggregation 是较难判断的一类关系，很难确切定义它。它的上面association和下面composition的定义都很明确，大概可理解为介于两者之间，但偏composition。
Composition组合:is part of
??? Composition用于表示“整体-部分”关系。这种关系是“非共享”的，也就是说，在任何时间，“部分”只能包含在一个“整体”中。二者的生命线总是相连的，如果整体被销毁了，部分也就不存在了。
??? 这里的“部分”不一定是多个。
??? 这里要提醒一下，这里的“非共享”关系指的是实例，而不是类。就是说一个类的不同实例可以属于不同的拥有者，但一个特定的实例只能有一个拥有者。
Generalization 泛化(Otherwise Known as Inheritance): is a type of
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以上5类关系依赖性由弱到强。
比较常用的是association/composition/generalization
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以下是相关概念的英文表述：
Dependency between classes means that one class uses, or has knowledge of, another class. It is typically a transient relationship, meaning a dependent class briefly interacts with the target class but typically doesn't retain a relationship with it for any real length of time.
A dependency implies only that objects of a class can work together.
The dependency relationship is often used when you have a class that is providing a set of general-purpose utility functions, such as in Java's regular expression (java.util.regex ) and mathematics (java.math ) packages. Classes depend on the java.util.regex and java.math classes to use the utilities that those classes offer.
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Associations are stronger than dependencies and typically indicate that one class retains a relationship to another class over an extended period of time. The lifelines of two objects linked by associations are probably not tied together (meaning one can be destroyed without necessarily destroying the other).
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参考文献
1.O'Reilly.Learning.UML.2.0.Apr.2006
2.O'Reilly.UML.2.0.in.a.Nutshell.Jun.2005
3.UML Distilled 3rd edition
4.道法自然：面向对象实践指南
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ps:本想收藏,可是想到收藏和记录总是有些不太一样,所以重新作为文章发出.
