Class Diagrams and Entity Relationship Diagrams (ERD) - Create UML and ERD Structure Models

Oct 25, 2026

Class diagrams and entity relationship diagrams (ERDs) model system structure in different ways. Class diagrams describe both structural and behavioural aspects, while ERDs focus only on structural features to provide a static view. The material covers core elements such as classes, associations (binary, n-ary, aggregation, composition, generalization, dependency, realization, and association classes), attributes, and operations. It also provides step-by-step guidance for creating class diagrams and ERDs, including identifying entities/classes, finding relationships, drawing rough diagrams, defining multiplicity or cardinality, and adding keys and attributes.

LanguageEnglish

CategoryTechnology

FormatPDF

Pages14

Reading Time35 min

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Class Diagrams and Entity Relationship Diagrams (ERD)

Class Diagrams

Creating a Class Diagram

Entity Relationship Diagrams

Creating an ERD

1 Class Diagrams and Entity Relationship Diagrams (ERD) Class diagrams and ERDs both model the structure of a system. Class diagrams represent the dynamic aspects of a system: both the structural and behavioural features. ERDs, depicting only structural features provide a static view of the system. 2 Class Diagrams 2.1 Elements ofa class diagram: 2.1.1 class | Class Name | | --- | | | | | A class is a general concept (represented as a square box) . A class defines the structural attributes and behavioural characteristics of that concept. Shown as a rectangle labeled with the class name. 2.1.2 association | Class 1 | Association | Class 2 | | --- | --- | --- | | | | | | | | | | | | | A (semantic) relationship between classes. A line that joins two classes. 2.1.2.1 binary | Person | Eats | Food | | --- | --- | --- | | | | | | | | | | | | | Simple association between two classes. A solid triangle with the association name indicates the direction in which the association is meant to be read. 2.1.2.2 n-ary | Class 1 | | --- | | | | | | Class 2 | | --- | | | | | n-ary Association expresses an association between multiple classes | Class 3 | | --- | | | | | 2.1.2.3 Aggregation | Team | | --- | | | | | | Member | | --- | | | | | “has-a” relationship 2.1.2.4 Composition | Car | | --- | | | | | | Engine | | --- | | | | | 2.1.2.5 Generalization | Car | | --- | | | | | | Volvo | | --- | | | | | 2.1.2.6 Dependency | Project Manager | | --- | | | | | | Project | | --- | | | | | | Team | | --- | | | | | “is-composed-of” relationship “is-a-kind-of” relationship The source class depends on (uses) the target class. (not used for requirements analysis) 2.1.2.7 Realization | «datatype»
Human Resources | | --- | | | | | | «datatype»
Project Manager | | --- | | | | | | | Worker | | --- | --- | | | | | | | | | | | | | | «datatype»
System Administrator | | --- | | | | | Class supports all operations of target class but not all attributes or associations.(not used for requirements analysis) 2.1.2.8 Association Classes | class 2 | | --- | | | | | | class 1 | | --- | | | | | Association Class Can be applied to binary or n-ary associations. Used to capture the attributes of an association. 2.1.3 attribute | class 1 | | --- | | -attribute 1
-attribute 2
-attribute 3 | | | 2.1.4 operation | class 1 | | --- | | -attribute 1
-attribute 2
-attribute 3 | | +operation 1()
+operation 2()
+operation 3() | What is known about each object of this class type. Typical characteristics. What objects of this class type do. Typical actions that are relevant to the current analysis. 3 Creating a Class Diagram Here are some steps to help create a class diagram: 3.1 Identify Classes These are the abstract or physical “things” in our system which we wish to describe. Find all the nouns and noun phrases in the domain descriptions you have obtained through your analysis. Consider these class candidates. 3.2 Find Associations Now find the verbs that join the nouns. e.g., The professor (noun) teaches (verb) students (noun) . The verb in this case, defines an association between the two nouns. Identify the type of association. Use a matrix to define the associations between classes. 3.3 Draw Rough Class Diagram Put classes in rectangles and draw the associations connecting the classes. 3.4 Fill in Multiplicity Determine the number of occurrences of one class for a single occurrence of the associated class. 3.5 Identify Attributes Name the information details (fields) which are relevant to the application domain for each class. 3.6 Identify Behaviours Specify the operations that are required for each class. (assume getter and setter methods for each attribute.) 3.7 Review your diagram and fine tune it. Look for inconsistencies and errors. Fix them. Make sure you have c

FAQs

How do class diagrams and ERDs differ in what they model?

Class diagrams capture both structural and behavioural features, while ERDs depict structural features only, giving a static view of the system.

What are the main elements of a class diagram?

A class diagram includes classes, associations, attributes, and operations, where classes define structural attributes and behavioural characteristics.

What steps are involved in creating a class diagram?

Identify classes, find associations, draw a rough diagram, fill in multiplicity, identify attributes, identify behaviours, then review and fine-tune for inconsistencies.

Oct 25, 2026

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