UML associations

I'm trying to read UML Class diagrams used to document design patterns. I understand aggregation and composition - and aggregation uses the open diamond and composition uses the filled diamond.

I've read somewhere that a dotted line implies instantiation - i.e. if one class instantiates another then you use a dotted line from the instantiator to the instantiated. First, wouldn't composition use this as well as I thought composition implied ownership?

Second, I've seen lines drawn between classes which are not aggregate or composition - i.e. there's no diamnad at all. Here's an example where they are documenting the observer pattern:

http://www.dofactory.com/Patterns/PatternObserver.aspx

they show a line from subj开发者_JAVA百科ect to observer without any diamond. What's all this about?

thanks

Ray

A UML association is used to convey a relationship between two or more classifiers (e.g. classes). Compositions and aggregations are specialized forms of association.

Associations are often used in class diagrams to indicate that instances of one class can have references to instances of another class. For example, in the diagram to which you link, the association between ConcreteObserver and ConcreteSubject implies that a ConcreteObserver instance can have a reference to a ConcreteSubject instance (note the directionality of the association).

Dotted line implies dependency. There are many different types of dependency, one of them being instantiation. As said before, associations are a way to describe relationship between a classs having a property of another class. When it comes to relationships between classes in classs diagram, it is good to start from most concrete and go to more general. So start with composition, aggregation, association and dependency.