Class Diagram in Software Engineering

A class diagram is one type of UML diagram that represents the static view of a software system. A class diagram is used to visualize, describe, and document various aspects of a system, and also to create executable software code. It illustrates the types of objects present in a system and the different types of relationships that exist between them.

It demonstrates the attributes, classes, functions, and relationships to give an overview of the software system. It organizes class names, attributes, and functions in a separate designed section that aids in software development. 

Class diagrams in software engineering are the only UML diagrams that can be mapped directly with object-oriented methods. It is widely used in modeling and constructing object-oriented systems. Its importance in software development lies in:

  • It analyzes and designs the static view of the application.
  • It describes the major responsibilities of the system.
  • Component and deployment diagrams can be built on it.
  • It integrates forward and reverse engineering.

Components of a Class Diagram

Classes and Objects

Classes are a template or blueprint of an object. In simple words, it is a template for creating objects and implementing behavior in a system. A class in UML represents an object or a group of objects that have similar structure and behavior. They’re represented as a rectangle that includes rows:

  1. Name: The first row in a class shape.
  2. Attribute: The second row in a class shape and each attribute of a class is represented in a separate line.
  3. Methods: The third row in a class shape, also known as operations. Each operation on a method is represented in a list format on its own line.

Objects are instances of a class or classes. Objects can be introduced to a class diagram to represent either concrete or prototype instances.

Attributes and Operations

An attribute is a property of the class which describes the object it is modeling. It is situated just below the name section in a class diagram.

E.g, attributes can be name, address, contact, date of birth, etc.

The attributes are generally written with the accessibility(public, private, protected, and package) of an attribute of a class. Attributes must have a meaningful name describing the use of it in a class.



An associate class is a part of an association relation between two other classes. By building association relationships with the association class additional information can be obtained.

For example – the relationship between a company and an employee is work. An association can be classified into three types-

  1. Directed association: This is a one-directional relationship that ensures the control flow from one classifier to another. Like the above example, employees work for a company, but the vice-versa can’t happen.
  2. Bidirectional association: Both classes are aware of each other and how they are related to each other.


It is a unique kind of association that interacts with the aggregate and its constituent pieces as a whole. A complex object is built up by merging different objects together. While the object’s integrity is preserved, the control object controls the combined object’s responses.

For example, a class bank is made up of a number of account holders. In aggregation, the contained classes are never totally dependent on the lifecycle of the container. Here, the class bank will remain even if an account holder is not available.


It is a form of aggregation which represents the whole part of the relationship. This means in a composition relationship a child will never exist without a parent. For example, House(parent) and Room(child), so a room won’t exist without a house.


It is a property in which a child or subclass class derives functionalities of a parent or superclass. For example, a class “Dog” would inherit all of the methods like (eat(), drink()) of the parent class “Animal” in addition to the specific methods(bark(), breed(), color()).

Benefits of Using Class Diagrams

1. Improved Communication among Team Members

Class diagrams are a simple representation of the overview and the schematics within the overall system. They use standardized symbols and notation that make it easy for developers to understand the relationships between different classes and objects, and to communicate these relationships to other team members, clients, and businesspeople. This improves communication among developers and other stakeholders.

2. Better Understanding of the System

Any data model, no matter how simple or complex can be demonstrated with the help of class diagrams to gain insights. It aids in better understanding 

by providing a basic overview of the schematics of an application. The needs and requirements of the system are clearly and visually represented. The diagrams are easy to understand without any technical knowledge.

3. Easier Maintenance and Modification of Code

Before studying the actual code, class diagrams provide an overview of how the application is structured. This can easily reduce the maintenance and code modification time. It helps to draw detailed charts that emphasize how to implement a specific code and programs can be written according to described structure. Class diagrams help in creating pre-plans which ease the programming process.

Steps involved in creating a Class Diagram

The class diagram is a graphical representation of the static view of the system and represents the various aspects of the application. It is quite simple to create a class diagram from scratch. Follow the steps below:

  1. Open a blank document or a template
  2. Click on “Insert” and select shapes from “Shapes”.
  3. Use a text box to highlight classes, name them, and fill out the components.
  4. For better understanding interconnect the rectangles with straight lines.

Tools used for creating class diagrams

Tools help you to design a specific type of diagram. There are a large number of tools available. A few of the widely used tools are-

  1. Lucidchart: The platform is interactive and versatile. It helps create hassle-free class diagrams tailored according to needs.

  2. Creately: The tool aids in simple class diagrams with real-time collaboration features.

  3. Miro: It is an excellent intuitive tool that lets one design and communicate technical plans using standard symbols and notions.

  4. Edraw Max: The platform offers a plethora of templates with easy user interface.

  5. yUML: It is an online tool exclusively designed for class diagrams and can be integrated with trackers, blogs, and forums to make them visually attractive.

  6. Cacoo: It is popularly known for its simple and secure file management capability. The software is mainly popular amongst product development, marketing, and project management teams to build and share professional UML diagrams.

Best Practices for Creating Class Diagrams

1. Keep it Simple and Clear

  • Keep class diagrams as simple and straightforward as possible. 
  • Each element and the relationships among them should be clearly identified. 
  • Avoid using more than two levels of hierarchy or too many abstract classes that increase complexity.
  • Attributes and methods of each class should be clearly identified.

2. Use Consistent Naming Conventions

  • The name of the class diagram should be meaningful to describe the aspect of the system.
  • Standardized symbols should be used to make class diagrams more sensible.

3. Avoid Overcrowding the Diagram

  • Unnecessary properties can make the diagram complicated and crowded. It can be tougher for the audience to focus on what is important. So, you should specify a minimum number of properties for every class.
  • Whenever there is a need to define some aspect of the diagram, user notes should be included. 
  • Keep the relationships as short as possible without losing their purpose.


Class diagrams are the most important UML diagrams used for software application development. They are the building blocks of UML. A class diagram allows analysis and design of the static view of the application. Class, Attributes, and Relationships are the essential components of class diagrams. It is useful to map object-oriented programming languages like Java, C++, etc.

Drawing UML class diagrams is a lot easier and simpler with standard symbols and various tools available. These tools offer templates, easy drawing features, and real-time collaboration. Class diagrams help to understand the system better and easier code maintenance easier.

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