decorator design pattern violating is-a relationship <p>I have recently started studying about decorator design pattern but I have a query. Decorators implement the same interface as the Component they are trying to decorate. Doesn't this violate the is-a relationship. Moreover since the decorator has the component (through composition) , why is it really required for the decorator to implement the same component interface which the concrete component implements.?</p> <p>Going through the decorator design pattern on Headfirst, it gives me a feeling that decorators can directly implement the component. There is no need to have an abstract class / interface for decorator. </p> <p>I am worries that this could really be a dumb question but help would allow me to have a strong foundation. </p>