Relationship

Introduction to Relationship

Relationship  provide a pathway for  communication  between objects. There are mainly  five types of relationship in UML: that exist among  classes. They are as follows:

  • Association
  • Aggregation / composition
  • Dependency
  • Generalization /specialization
  • Realization

Before going in detail of these relations  let us  see some important  terms  related to  them:

1. Multiplicity: It is defined as the number of objects  of a class that participates  in a relation. Or in more easy words multiplicity is the number of instances of one class related to one instance of the  other class.

2. Navigation: It  shows  direction of a relation . it will be  more clear in the examples that are presented later.

  • Association

Association is a bidirectional  structural relationship that describes  connection  among the classes. If the association is between two  classes then it is called  binary association. It  is represented by a solid line. There can be  association even among more than two  classes, but  it is very  rare. Even  an association can circle  back to the  same class.

2. Aggregation / Composition

Aggregation is another  type of  structural  relationship, that  connects two  classes but in a stronger way. It  is in fact a type of association  where  one of the class participating in a relation,  participates as  a whole   while  the other  participates  as a part. In simpler words it is  the structural  inclusion of one class by another.

3. Dependecy

A dependency relationship is a weaker form of relationship  than the  above  discussed ones. A dependency states that  a change  in specification of one  class my  effect another  class that uses the first  one but not  vice versa. Most often dependencies  are used in context of classes to shown that  one class used another  class  as an  argument in  signature  of an operation. It  is shown  in fig  8.7,

4. Generalization /specialization

It comes from  the property of inheritance of OOPS generalization is nothing  but deriving a class out of  a parent class having  some  inherited property ( form the  parent  class) and some  new  property of the derived class. The term generalization is for  the inheritance in the  bottom  to up direction. i. e. ,from parent class to the derived class while  another relation  is specialization which is in direction  form the parent  class  to  the derived class. Both  concepts  are shown in fig  8.8

5. Realization

Realization is very similar to inheritance or specialization the  only difference between them is that here a class is derived from an interface instead of a base  class. an interface is a class or rather an abstract class where the operations  are declared and not  defined or implemented. The derived class implements  the operation declared  by the interface and thus  the relation between  an interface and the  class that implements  the interface  is called  realization.

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