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What is a Class Diagram?

What is a Class Diagram?

By Bhargav Dhameliya
Release Date : March 7, 2024
5 min read

Class diagrams, which are an necessary a part of the Unified Modeling Language (UML), show relationships and dependencies among instructions in a hierarchy of objects. It shows the structure of the instructions the usage of boxes representing magnificence names, attributes, and techniques. These diagrams assist to recognize the shape and shape of the system, and become aware of non-desk bound capabilities. By developing elegance connections, software improvement is simplified and a framework for design and implementation is advanced.

Quick Summary :

Class diagrams, an integral part of UML, define class relationships in a hierarchy of objects. It provides a static view of the system architecture, showing the class structure through boxes representing class names, attributes, and methods. These diagrams aid in system understanding, design, and software development, and serve as prototypes for implementation.

What is a class Diagram?

Class diagrams are used to show relationships and source code based relationships between classes in Unified Modeling Language (UML). In this case, a class defines methods and variables on an object, which is either a specific object in the program or a single piece of code that represents that object. Class diagrams are useful in all forms of object-oriented programming (OOP). The concept is several years old but has been refined through the development of OOP modeling paradigms.

In a class diagram, classes are organized into groups with common attributes. A rectangular diagram resembles a flowchart in which the rectangles are plotted as boxes, with three rectangles in each box. The upper square contains the class name; The middle square contains the class objects; The lower square contains the methods of the class, also called functions. Lines, with arrows at one or both ends, connect the boxes. This notation describes relationships between classes, also known as associations.

Relation between classes

In class diagrams, relationships between instructions describe how instructions are linked or interact with each different inside a gadget. There are numerous forms of relationships in object-orientated modeling, each serving a selected cause like Association, Directed Association, Aggregation, Composition, Generalization(Inheritance), Realization (Interface Implementation)Dependency Relationship, Usage(Dependency) Relationship and many others.

Purpose of Class Diagrams

The primary reason of class diagrams is to construct a static view of an application. It is the most effective diagram this is extensively used for production, and it is able to be mapped with object-oriented languages. It is one of the maximum popular UML diagrams. Following are the motive of sophistication diagrams given underneath:

  • It analyses and designs a static view of an software.
  • It describes the important duties of a gadget.
  • It is a base for aspect and deployment diagrams.
  • It includes forward and opposite engineering.

What is Class?

In the actual world, you may often locate many person items all the identical kind. There may be thousands of different bicycles in lifestyles, all the identical make and model. Each bicycle turned into constructed from the equal set of blueprints and consequently incorporates the same additives. In object-oriented terms, we say that your bicycle is an example of the elegance of gadgets called bicycles. A elegance is the blueprint from which individual gadgets are created.

Class Notion

A class notation is composed of three parts:

Class Name

  • The class name occurs in the first partition.

Class Attributes

  • Attributes are proven in the 2d partition.
  • The characteristic type is proven after the colon.
  • Attributes map onto member variables (information participants) in code.

Class Operations (Methods)

  • Operations are shown within the 0.33 partition. They are offerings the elegance provides.
  • The go back kind of a method is proven after the colon on the give up of the approach signature.
  • The return kind of technique parameters is proven after the colon following the parameter name.
  • Operations map onto magnificence strategies in code

The graphical representation of the magnificence - MyClass as shown above:

  • MyClass has three attributes and 3 operations
  • Parameter p3 of op2 is of type int
  • op2 returns a waft
  • op3 returns a pointer (denoted with the aid of a *) to Class6

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Class Relationship

A elegance may be concerned in a single or greater relationships with other classes. A dating may be one of the following types :

Class Relationship

Relationship Names

  • Names of relationships are written inside the middle of the affiliation line.
  • Good relation names make experience when you read them out loud:
  • "Every spreadsheet carries a few variety of cells",
  • "an expression evaluates to a price"
  • They frequently have a small arrowhead to reveal the course wherein course to study the connection, e.G., expressions examine to values, but values do not examine to expressions.

Relationship Roles

  • A position is a directional cause of an association.
  • Roles are written at the ends of an affiliation line and describe the purpose played by means of that elegance in the courting.
  • E.G., A cellular is associated with an expression. The nature of the connection is that the expression is the formulation of the cell.


  • The arrows imply whether or not, given one instance collaborating in a courting, it's miles feasible to decide the times of the other class that are related to it.

The diagram above suggests that,

  • Given a spreadsheet, we will discover all of the cells that it includes, however that
  • we can not determine from a cellular in what spreadsheet it's miles contained.
  • Given a mobile, we can acquire the associated expression and fee, but
  • given a cost (or expression) we can not discover the mobile of which the ones are attributes.

Visibility of Attributes & Operations

In object-oriented layout, there is a notation of visibility for attributes and operations. UML identifies four forms of visibility: public, protected, non-public, and package.

The , -, # and ~ symbols earlier than an attribute and operation call in a class denote the visibility of the attribute and operation.

  • Denotes public attributes or operations
  • -denotes non-public attributes or operations
  • #denotes protected attributes or operations
  • ~ denotes package attributes or operations

Class Visibility Example

  • Attribute1 and op1 of MyClassName are public
  • attribute3 and op3 are blanketed.
  • Attribute2 and op2 are private.


How many objects of every class take part in the relationships and multiplicity can be expressed as:

  • Exactly one - 1
  • Zero or one - 0..1
  • Many - 0..* or *
  • One or greater - 1..*
  • Exact Number - e.G. 3..Four or 6
  • Or a complex courting - e.G. Zero..1, 3..Four, 6.* might imply any number of gadgets aside from 2 or 5

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Multiplicity example

Requirement: A Student can take many Courses and lots of Students may be enrolled in one Course.

In the instance under, the class diagram (on the left), describes the announcement of the requirement above for the static model even as the item diagram (on the right) suggests the snapshot (an instance of the class diagram) of the direction enrollment for the publications Software Engineering and Database Management respectively)

Aggregation example

  • An aggregation is a unique case of affiliation denoting a "consists-of" hierarchy
  • The combination is the determine class, the components are the kids instructions

Inheritance Example

  • Inheritance is another unique case of an affiliation denoting a "kind-of" hierarchy
  • Inheritance simplifies the analysis version via introducing a taxonomy
  • The child training inherit the attributes and operations of the discern elegance.

Class Diagram Example

Class Diagram Exampl

  • We can interpret the which means of the above magnificence diagram with the aid of studying through the factors as following.
  • Shape is an abstract elegance. It is proven in Italics.
  • Shape is a superclass. Circle, Rectangle and Polygon are derived from Shape. In other words, a Circle is-a Shape. This is a generalization / inheritance dating.
  • There is an affiliation between DialogBox and DataController.
  • Shape is a part-of Window. This is an aggregation relationship. Shape can exist with out Window.
  • Point is a component-of Circle. This is a composition dating. Point can't exist without a Circle.
  • Window is dependent on Event. However, Event is not dependent on Window.
  • The attributes of Circle are radius and middle. This is an entity elegance.
  • The technique names of Circle are place(), circum(), setCenter() and setRadius().
  • The parameter radius in Circle is an in parameter of kind float.
  • The approach place() of sophistication Circle returns a fee of kind double.
  • The attributes and approach names of Rectangle are hidden. Some different classes inside the diagram additionally have their attributes and method names hidden.

Multiple or single Diagram?

Inevitably, in case you are modeling a large system or a huge commercial enterprise region, there will be numerous entities you need to take into account. Should we use a couple of or a unmarried magnificence diagram for modeling the trouble? The solution is :

Multiple or single diagram

  • Instead of modeling every entity and its relationships on a unmarried magnificence diagram, it's far better to apply a couple of class diagrams.
  • Dividing a device into multiple class diagrams makes the gadget less complicated to apprehend, specially if each diagram is a graphical representation of a specific part of the system.

Class Diagram inSDLC

A Class Diagram within the Software Development Life Cycle (SDLC) is a form of static structure diagram in the Unified Modeling Language (UML) that illustrates the structure of a machine through showing training, their attributes, operations, and relationships amongst gadgets. It serves as a blueprint of a device or subsystem, modeling the items within the device and their relationships. Key additives of a category diagram encompass the name of the magnificence, attributes, and operations (techniques). Classes in a category diagram constitute gadgets or units of gadgets sharing not unusual structure and behavior, helping to visualize, describe, and document diverse aspects of a machine.

Benefits of class Diagrams

  • It can constitute the object model for complex systems.
  • It reduces the preservation time by way of offering an overview of how an utility is established earlier than coding.
  • It offers a general schematic of an utility for higher understanding.
  • It represents a detailed chart through highlighting the desired code, which is to be programmed.
  • It is beneficial for the stakeholders and the builders.

How to Draw class Diagram

The UML elegance diagram is used maximum widely to construct software program programs. It no longer most effective represents a static view of the system however additionally all of the most important aspects of an software. A series of sophistication diagrams as an entire represents a device.

Some key points which are had to preserve in thoughts whilst drawing a class diagram are given below:

  • To describe a whole component of the gadget, it is cautioned to give a meaningful name to the magnificence diagram.
  • The gadgets and their relationships ought to be recounted earlier.
  • The attributes and strategies (responsibilities) of each magnificence need to be known.
  • A minimal number of preferred homes need to be special as greater number of the unwanted assets will cause a complicated diagram.
  • Notes can be used as and while required by way of the developer to describe the components of a diagram.
  • The diagrams need to be redrawn and remodeled as many times to make it accurate earlier than producing its final version.


Class diagrams play an important role in understanding system design and its design in software development. By mapping class relations and dependencies, a static view of systems is provided, acting as a model for application. Used correctly, developers can streamline their development processes, reduce maintenance time, and increase collaboration between stakeholders.


  1. How do magnificence diagrams assist those involved in software improvement?

Learning diagrams offer members with a clear image of the machine shape, assisting them understand how additives interact and make contributions to decision-making processes.

  1. Can class diagrams be used in speedy software program improvement?

Yes, class diagrams can be integrated into agile approaches with the aid of offering high-degree technique layout concerns and contributing to iterative development and innovation efforts

  1. Are there tools for creating and managing class diagrams?

Yes, there are many UML diagram tools that make it easy to create, edit, and manage class diagrams. Some popular examples are Enterprise Architect, Lucidchart, and Visual Paradigm.

  1. How often should class diagrams be updated during the software development life cycle?

The class diagram should be updated whenever there is a significant change in the structure or composition of the system. Keeping it in line with evolving needs and applications is essential to ensure accuracy and relevance throughout the development process.

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