Generalization and Specialization

Generalisation is an abstraction for sharing similarities among classes while preserving their differences. For example, each piece of equipment has a manufacturer weight and cost . Pumps also have suction pressure and flow rate. Tank also have volume and pressure we would like to define equipment features just once and then add details for pump, tank and other equipment types.

Generalisation is a relationship between a class and one or more refined versions of it. The class being refined is called superclass and each refined version is called a subclass. For example equipment is the superclass of pump and tank. Attributes and operations common to a group of subclasses are attached to the superclass and shared by each subclass. Each subclass is set to inherit the feature of its superclass, for example, pump inherites attributes manufacturer, weight and cost from equipment. Generalisation is sometimes called the "is-a" relationship because each instance of a subclass is a instance of the superclass as well.

Each subclass not only inherits the features of its superclass but as its own specific attributes and operations as well , for example pump adds attributes flow rate, which is not shared by other kind of equipment.

Figure shows an equipment generalisation. Each piece of equipment is a pump, heat exchanger, tank or another type of equipment. There are several kinds of pumps: Centrifugal, diaphragm, and plunger. There are several kinds of tanks: floating roof, pressurized and spherical. Pump type and tank type both define second level generalisation class is down to the third level.

Specialization: Specialization is the process of defining a set of subclasses of an entity type; this entity type is called the superclass of the specialization. The set of classes that forms specialisation is defined on the basis of some distinguishing characteristic of the entities in the superclass. For example:the set of subclasses {SECRATARY, ENGINEER, TECHNICIAN} is a specialization of a superclass EMPLOYEE that distinguishes among employee entities based on the job type of each employee entity. We may have several specializations of the same entity type based on different distinguishing characteristics. For example, another specialization of the EMPLOYEE entity type the set of subclasses {SALARIED_EMPLOYEE, HOURLY EMPLOYEE} this specialization  distinguish among employees based on the method of pay.

The above figure shows few entity instances that belong to subclasses of {SECRETARY, ENGINEER,  TECHNICIAN} specialiZation. An entity that belongs to a subclass represents the same real-world entity as the entity connected to it in the EMPLOYEE superclass, even the same entity is shown twice, for example e1 is shown in both EMPLOYEE and SECRETARY, As figure suggest a superclass/subclass relationship such as EMPLOYEE/ SECRETARY somewhat resembles 1:1 relationship at instance-level. The main difference in is that 1:1 relationship two distinct entities are relative where as in superclass/subclass relationship the entity in the subclass in the same real-world entity as the entity in the superclass but is playing as a specialized role for example, and EMPLOYEE specialized in the role of SECRETARY, or an EMPLOYEE specialized in the role of TECHNICIAN.
In summary, the specialization process allow us to do the following:

• Define a set of subclass of an entity type
• Establish additional specific attributes with each subclass
• Establish additional specific relationship types between each subclass and other entity types or other classes