What is a class diagram in software engineering?

Class diagrams in Unified Modeling Language (UML) are a part of software engineering. It is used to map the type of a static structure diagram followed by modeling its attributes, classes, object relationships, and operations. Class diagrams are easy and simple to understand. 

The UML helps model structure in many ways, and one of the methods popular to understand them is to use a class diagram. Class diagrams are used in the documentation of the architecture of software by software engineers. It is a structure diagram type that is essential to modeling the system. UML is widely used in OOP or object-oriented programming, where classes and class diagrams are the building blocks. The components of class diagrams hence represent the classes to be programmed using the class diagram. It shows the main objects in a class and the interconnections and interactions between them and the UML classes.


UML class diagrams bring in many organizational benefits. Some of them are:

● They can represent the illustration of information systems and data models, making complex interactions simple to understand.

● They help understand the general schematic overview of a software application.

● They visually represent any specific system needs throughout the business while systematically disseminating information to all business levels.

● They form detailed charts and diagrams inclusive of the specific code to be programmed/implemented in that class’ structure.

● The class charts describe the system types and are used in the system, making it implementation-independent. These charts are later disseminated to the system components.

Class Diagram Components:

The shape of a class has three rows, with the top one having the class name, the center one the class attributes, and the lowest one the operations/methods the class may use. Subclasses and classes are grouped to explain the relationship between each object and the others. Thus, the class diagram has case diagrams, activity diagrams, component, state, deployment diagrams, etc., connected with connectors and modeling language shapes. 

The standard class diagram has three sections already mentioned in the definition. They are the:

● Upper section: This has the name of the class and is always a required field, especially where the system is of an object or classifier.

● Centre section: This section has the class attributes used to describe the class qualities and attribute nature. It is only required if you were describing a specific instance or event in a particular class.

● Lower section: This section has the class methods and operations displayed as a list where each operation has its line. Yet, this section does not show how data interacts with the class.

Member access modifiers: Classes have separate and distinct access levels in a class diagram which depend on the visibility or access modifier. The access modifier is represented by a symbol that corresponds to it. Here’s the list of modifiers:

● Private (-)

● Public ( )

● Package (~)

● Protected (#)

● Static (underlined)

● Derived (/)

Member scope: The scope of the class member has either instances or classifiers depending on its scope. The static members are called classifiers, while the specific instances are called instances and are outlined in the theory of OOP. 

Additional components: Depending on the class diagram’s context, classes may represent interactions in the application, the main objects, or classes to be programmed. The various illustration components in the structure of a class diagram are:

● Classes: The class template is typically used to create objects and implement a specific system behavior.

● Data types: These are also called Classifiers and are used to define data values. The types of the data model can be enumerations or primitive types of classifiers.

● Signals: These are symbols representing the communications that may be asynchronous and one-way between the active objects of a class diagram.

● Packages: Class diagrams use shapes of the modeling language to organize the diagram’s related classifiers, then illustrated as a tabbed, large rectangular shape.

● Enumerations: These are user-defined data type representations and include identifier groups that demonstrate the enumeration values.

● Interfaces: These are attribute definitions of a cohesive behavior set and a collection of operation signatures. Similar to classes, the interfaces will need at least one class to implement it, whereas classes may have instances of its class type.

● Artifacts: These are essential model elements that illustrate the software system’s concrete entities like databases, documents, software components, executable files, and more.

● Objects: These occur as instances in (a) class/classes, and objects may be added to a class diagram to portray the prototypical or concrete instances.

● Interactions: These show the relationships and links between the classes and objects. Commonly occurring interactions are:

○ Inheritance: This is the process of inheritance by a sub-class or child taking on the parent/superclass functionality. It is also called class generalization and is symbolized as a closed arrowhead on a straight line connecting the child and parent classes. 
For example, take a parent class (Vehicle) and sub-class (Car). The sub-class inherits the parent class’s attributes like fuel type, speed, number of passengers, etc., and its characteristics like the number of doors, model type, automaker, etc. Method attributes like change direction, go, stop, etc., are also inherited attributes.

○ Association that is bidirectional: This shows the relationship between two classes who are aware of their relationship and their attributes. Such association has a symbol of a straight line between classes. 
○ Unidirectional association: This is relatively uncommon as a relationship between 2 classes. Here, only 1 of the classes is aware of the interactions and attributes with the other class. The modeling symbol for such association is an open arrowhead on a straight line from the knowing class to the other.

Example of class diagrams:

One of the easy ways to ensure the software is well prepared is to use class diagrams in software engineering. Creating a process flow class diagram is easy. 

Hotel management system class diagram: 

Software development courses teach you how to draw and read these class diagram charts. A hotel management system classdiagram will show the relationships between its objects and include staff responsibilities, guest details, room occupancy, and more. Each of these distinct classes would be under a separate arm for the particular class. 

For example, the staff responsibilities and guest details classes are shown as different branches or classes of the management system. Under the class category room, occupancy attributes/traits of inheritance and association are displayed. Here, each room is treated as a sub-class, and one will see the traits. All rooms inherit particular attributes like room service, washrooms, beds, door locks, etc. The association attributes include availability, room-keeping completed or not, etc. 

The Bottom Line

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