historyThe first of this model was the semantic data model (SDM) developed by M. Hammer and D. Mcleon and published in 1981 in their database description with SDM : A Semantic Database Model
Since SDM is an object- oriented data model (OODM). In turn the OODM becomes the basis for the object oriented database model with the manage of through an object oriented database management system (OODBMS)
Objects that are similar in characteristics are grouped in class.
Class: is a collection of similar objects with shared structure ( attributes) and behavior (method) Method: represents areal word action such as finding a selected person’s name, changing person’s name or printing a persons address.
Classes are organized in class hierarchy. The class hierarchy resembles an up side down tree in which each class has only one parent.