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I am trying to understand UML diagram describing Decorator Pattern at link below
http://www.dofactory.com/Patterns/PatternDecorator.aspx
I don’t understand why there is a “Aggregation” relation between Decorator and Component.
I believe it should be composition as Decorator cannot exist without the base component.
Composition is stronger that aggregation, it usually means that the object takes ownership of its components. This is not the case in this situation because a decorator doesn’t own a decorated object. Moreover you could remove the decorator without a need to remove the decorated object as well.
In practice the line between aggregation and composition can be blurry and often it doesn’t make much difference whether you choose one or the other, especially if you treat you diagrams as sketches.