Object Relationships
http://cnx.rice.edu/content/m11709/latest/
"Is-a" or "Inheritance"
"Is-a" or "inheritance" (sometimes also called "generalization") relationships capture a hierarchal relationship between classes of objects. For instance, a "fruit" is a generalization of "apple", "orange", "mango" and many others. We say that fruit is an abstraction of apple, orange, etc. Conversely, we can say that since apples are fruit (i.e. an apple "is-a" fruit), that they inherit all the properties common to all fruit, such as being a fleshy container for the seed of a plant.
"Has-a" or "Composition"
"Has-a" or "composition" (sometimes referred to as an "associative") relationships capture the notion that one object has a distinct and persistant communication relationship with another object. for instance, we can say a car "has-a" motor. The car and the motor are not related in a hierarchal manner, but instead we need to be able to express that this pair of objects has a particular working relationship. The car gives gas to the motor and the the motor will propel the car along. Compositional relationships can be one-way, where one object can, in a persistant manner, talk to (i.e. call methods of) a second object but the second object cannot, in a persistent manner, talk back to the first object. Compositional relationships can also be two-way, where both objects can talk to each other in a persistent manner. - posted by Saravanan Jayachandran @ 11:00 PM Permanent Link