What is Class Diagram

What is Class Diagram?

10-13 minutes

In software engineering, a class diagram in the Unified Modeling Language (UML) is a type of static structure diagram that describes the structure of a system by showing the system’s classes, their attributes, operations (or methods), and the relationships among objects.

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Purpose of Class Diagrams

1. Shows static structure of classifiers in a system
2. Diagram provides a basic notation for other structure diagrams prescribed by UML
3. Helpful for developers and other team members too
4. Business Analysts can use class diagrams to model systems from a business perspective

A UML class diagram is made up of:

• A set of classes and
• A set of relationships between classes

What is a Class

A description of a group of objects all with similar roles in the system, which consists of:

Structural features (attributes) define what objects of the class “know”

• Represent the state of an object of the class
• Are descriptions of the structural or static features of a class

Behavioral features (operations) define what objects of the class “can do”

• Define the way in which objects may interact
• Operations are descriptions of behavioral or dynamic features of a class

Class Notation

A class notation consists of three parts:

1. Class Name

• The name of the class appears in the first partition.

2. Class Attributes

• Attributes are shown in the second partition.
• The attribute type is shown after the colon.
• Attributes map onto member variables (data members) in code.

3. Class Operations (Methods)

• Operations are shown in the third partition. They are services the class provides.
• The return type of a method is shown after the colon at the end of the method signature.
• The return type of method parameters is shown after the colon following the parameter name.
• Operations map onto class methods in code

The graphical representation of the class – My Class as shown above:

• My Class has 3 attributes and 3 operations
• Parameter p3 of op2 is of type int
• op2 returns a float op3 returns a pointer (denoted by a *) to Class6

Class Relationships

A class may be involved in one or more relationships with other classes. A relationship can be one of the following types: (Refer to the figure on the right for the graphical representation of relationships).