Relation between classes

The duality of relations below corresponds to the duality `inheritance' and `delegation' we have just discussed.

• IS-Relation
  This relation has to do with inheritance: A dog is an animal. A car is a vehicle. Because a dog is animal it has all the features of animal and of course on top of that its own.
• HAS-Relation
  In the system using delegation described above we have seen that an object must have an instance of its super-class in its attributes in order to be able to delegate an unknown message to a higher level.

  Of course an object may have instances of several super-classes making it easy to implement multiple inheritance.