Class Diagram for Software Design

INTRODUCTION

 CLASS DIAGRAM

•       A class diagram shows the existence of classes and their relationships in the logical view of a system

•       UML modeling elements in class diagrams are:

–      Classes, their structure and behavior.

–      relationships components among the classes like association, aggregation, composition, dependency and inheritance

–      Multiplicity and navigation indicators

–      Role names or labels.

Basic two types  of classes

Concrete classes

•       A concrete class is a class that is instantiable;  that is it can have different instances.

•       Only concrete classes may be leaf classes in the inheritance tree.

Abstract classes

•        is a class that has no direct instance but whose descendants classes have direct instances.

•       can define the protocol for an operation without supplying a corresponding method we call this as an abstract operation.

operation defines the form of operation, for which each concrete subclass should provide its own implementation

Types of Relationships between classes

•       Association

•       Aggregation

•       Composition

•       Inheritance

•       Dependency

•       Instantiation

ASSOCIATION

      These are the most general type of relationship:

•       It denotes a semantic  connection between two classes

•        It shows BI directional connection between two classes

•       It is a weak coupling as associated classes remain somewhat independent of each other

•       Example:

AGGREGATION

      This  is a special type of association

•       The association with label “contains” or “is part of” is an aggregation  è  represents “has a “ relationship

•       It is used when one object logically/physically contains other

•       The container is called as aggregate 

•       It has a diamond at its end

•       The components of aggregate can be shared with others

•       It expresses a whole - part relationships, like …

     COMPOSITION

      This  is a strong form of aggregation 

•       It expresses the stronger coupling between the classes

•       The owner is explicitly responsible for creation and deletion of the part

•       Any deletion of whole is considered to cascade its part

•        The aggregate has a filled diamond at its end

INHERITANCE

•       The inheritance relationship helps in managing the complexity by ordering objects within trees of classes with increasing levels of abstraction. Notation used is solid line with arrowhead, shown below.

•       Generalization and specialization are points of view that are based on inheritance hierarchies.

DEPENDENCY

•       Dependency is semantic connection between dependent and independent model elements.

•       This association is unidirectional and is shown with dotted arrowhead line.

•       In the following example it shows the dependency relationship between client and server.

•       The client avails services provided by server so it should have semantic knowledge of server.

•       The server need not know about client.

INSTANTATION

      This relationship is defined between parameterized class and actual class.

•       Parameterized class is also referred as generic class.

•       A parameterized class can’t have instances unless we first instantiated it

                Example:

      Cardinality or Multiplicity

      Definition: Number of instances of each class involved in the dialogue is specified by cardinality.

•       Common multiplicity values:

•       Symbol                 Meaning

•       1                                              One and only one

•       0..1                                         Zero or one

•       M…N                     From M to N (natural integer)

•       0..*                                        From zero to any positive integer

•       1..*                                        From one to any positive integer

•       Also thought can be given about navigability to every applicable relationship.

 source:Arun Timalishna(ioe lecturer)