Introduction To UML

Unified Modeling Language (UML) is a modeling language that helps us to represent how the software is structured, and how different modules, classes and object interact with each other, and the relation between them.

UML is frequently used in association with object-oriented design, but it has a much broader scope.

In UML, we can define the structure and behavior of a system, and we can visualize the model or parts of it through diagrams. There are two types of diagrams.

• Structural Diagrams
• Behavior Diagrams

Structure diagrams are used to represent the structure of a system. There are many types of structure diagrams, but we are only interested in class diagrams. object, package, and component diagrams are similar to class diagrams.

Behavior diagrams are used to describe the behavior of a system. Interaction diagrams are a subset of behavior diagrams and are used to describe the flow of control and data among different components of a system. Among the behavior diagrams, the sequence diagram is used extensively in object-oriented design.

Class diagrams are the type of diagrams used most in object-oriented design and development stages. They are a type of structure diagram, and are used to illustrate the structure of classes and the relations among them.

classDiagram
  class Vehicle {
    +String number
    +drive()
  }

  class Car {
    +drive()
  }

  Vehicle <|-- Car

Class diagrams are useful for describing how the classes are structured in an application. Most of the time, just looking at the structure can be enough to be able to understand how the classes interact, but sometimes this is not enough. For those cases, we can use behavior and interaction diagrams, of which the sequence diagram is used to describe class and object interaction.

Example of Sequence Diagram

sequenceDiagram
  objectA->>+objectB: getUser()
  objectB-->>-objectA: user

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Class Relations #

In Object-Oriented Programming different notations are used to represent the relation between the classes entities.

Few are listed below

• Generalization
• Realization
• Dependency
• Association
  • Aggregation
  • Composition

Read more about UML Conventions

Generalization #

Convention used to specify the inheritance is called generalization. Generalization is also called as is a relation.

Generalization Example

classDiagram
  Vehicle <|-- Car
  Vehicle <|-- Truck

Realization #

Convention used to specify the implementation of an interface.

Realization Example

classDiagram
  Vehicle <|.. Car
  Vehicle <|.. Truck

Dependency #

When a class is dependent of another class or abstract class or interface one or the otherway, it is represented with dependency notation. The dependency can be a passing dependent object as parameter to the method of a class.

Dependency Example

classDiagram
  Vehicle <.. Car

Association #

Associate represents the relations between two class entities.

There are two types of entities.

• Aggregation
• Composition

Association Example

classDiagram
  Driver <-- Car

Aggregation #

When a class ClassA has an instance of another class ClassB, and classB can exist without classA then the relation is represented as aggregation. Aggregation is considered to be the has a relationship while inheritance is considered to be the is a relationship.

Aggregation Example

classDiagram
  Department o-- Teacher

From the above class diagram, Department class has Teacher and Teacher can exist without Department.

Composition #

When a class ClassA has an instance of another class ClassB, and classB can not exist without classA then the relation is represented as aggregation.

Composition Example

classDiagram
  House *-- Room

From the above class diagram, House class has Room and Room can not exist without House.

Read more about UML Conventions