class relationships review

Classes, by themselves, aren't particularly useful. It's the relationships among classes that provide the foundation for the structure of a new system.

In UML2.0

1.Association: [Fields]
peer relationship, use the a straight line. An association is assumed to be bidirectional, which means that you can navigate from either class to the other one. However, you can specify that navigation can only occur from one class to another by using a feathered arrow.




2.Aggregation [fields]
it is a special kind of association―a "whole/part" relationship within which one or more classes are parts of a larger whole. A class can be aggregated to one or more other classes.An aggregation appears as a line with an open diamond at one end.







3. Composition [fields]
it is a "strong" form of aggregation. There are two differences between composition and regular aggregation, as follows:

• Within a composition relationship, the whole and the parts have coincident lifetimes. This means that if a particular instance of the whole is destroyed, so are the instances of the parts.
• A class can only belong to one composition relationship at a time as a part.

4. Generalization [class]
it refers to a relationship between a general class (the superclass or parent) and a more specific version of that class (the subclass or child). We can think of the subclass as being a "kind of" the superclass.






5.Dependency [parameters]
it is a "using" relationship within which a change in one thing (such as a class) may affect another thing (for instance, another class). The dependent element is called the client or source; the independent element is called the supplier or target.
A dependency involving two classes appears as a dashed line with a feathered arrow pointing at the supplier.

6. Association class [class]
it is a cross between an association and a class.