A key analysis skill is to be able to examine specific instances of things and to generalise their properties into a class.  A class is thus a generalisation or template which describes the general characteristics of things.  For example, the class 'person' can be defined as having the properties of name, address, date of birth etc.  Specific instances of 'person' will each have their own values for these properties, e.g. a name such as 'Paul Smith', an address such as '26 West Street' and a date of birth such as '7 August 1982'.

Definition of association

An association describes the fact that two object classes can be have a link between object instances within those classes.  Such links can be one-to-one, one-to-many or many-to-many.

Identification of classes

Determining appropriate classes is a key skill.  A number of techniques can be used to determine classes, including:

• noun phrase analysis 
• classes hidden in attributes of other classes
• classes in many-to-many associations
• classes in instances, where you need to distinguish between the general concept (e.g. book) and its instances (e.g. book copy)
• classes in events in which it is difficult to model all events for a given class and leads to another role for the class

The use of messages to manage control of an object oriented system 

Objects and their classes are said to be 'encapsulations', i.e. they are self-contained units which act with a degree of autonomy (independence) and are not heavily dependent upon other objects.  However objects cannot exist in isolation and must work co-operatively with other objects in order to deliver the objectives of the system.  Co-operation is achieved through the passing of 'messages' between objects'.  One object will send a message and another will receive it.  The receiving object can then act upon the message, with the result of sending its own messages either back to the original transmitting object or to other objects.