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What is a Use Case Diagram?

Posted by Chris Adams

Article Rating // 23110 Views // 9 Additional Answers & Comments

Categories: Business Analysis, Systems Analysis, Unified Modeling Language (UML), Use Cases


At the most basic level, the Use Case Diagram is a UML (Unified Modeling Language) diagram which shows in visual form (aka diagram) the interactions between a system’s users and the functions supported by the system.

Before we expand the definition, let’s focus on the key visual elements of a use case diagram:

  • Use Cases - A use case, depicted by an oval shape, represents a given system function or action which the user (actor) can initiate within the system.  

    • The use case diagram is a functional diagram (not a technical diagram therefore, a use case represents a piece of functionality (aka “the what”) and not system design (aka “the how”).  

    • Use cases can also be thought of as goals aka what a user may want to do with the system.  Example use case names: “Add New Customer”, “Generate Monthly Report”, “Withdraw Money”.

    • Did you notice how use cases are named? They are verb phrases.  Very important!

  • Actors - These are entities which interact with the system.  While the use case actors are visually represented as a human stick figure, the use case actors do not merely represent a human user but anything which interacts with the system and is outside the boundary of the system.

    • Actors could be entities which invoke use cases or trigger use cases.  Example: a “Bank Customer” actor may initiate the “Withdraw Money” use case.

    • Actors can also be other systems with which the system being represented by the boundary interacts with.  Example: a “Purchase Baseball Ticket” use case for a third party seller may interact with the “MLB System” actor to record the ticket sale.

  • Associations - The association, visually depicted as a solid line with an arrow at one end, represents the interaction between an actor and a use case.  The arrow points away from the object which initiates the interactions.

    • Example: the association between the “Bank Customer” actor and the “Withdraw Money” use case points to the use case because it is initiated by the “Bank Customer”.

    • Example: the association between the “Purchase Baseball Ticket” use case and the “MLB System” actor points to the “MLB System” because the interaction is initiated by the “Purchase Baseball Ticket” use case.

  • Relationships - Well… relationships are really a type of association - they are standard UML relationships and are used to visually display a relationship between two use cases.  There are three types of relationships in a use case diagram:

    • Generalization Relationship - shows that one use case (or actor)  is based on another aka it indicates that the child receives all the attributes and characteristics of the parent.  Some like to refer to the generalization relationship as inheritance.

    • Include Relationship - is a relationship between two use cases in which one use case uses or includes the functionality of another.  The include relationship is handy when the same piece of functionality is common among two or more use cases.  In this case, a common use case can be depicted and “included” or shown as being used by other use cases.

    • Extend Relationship - is also a relationship between two use cases where the functionality of the extension use case is not essential to and is optional for the use case being extended.

  • System Boundary - The system boundary is a rectangle which contains inside the use cases which are part of the system or subsystem being depicted with all the actors residing outside of the system boundary.  A good practice is to place the name of the system, or the area being studied, at the top of the system boundary - just inside the boundary.

So… What is a Use Case Diagram?   Let’s try to refine the definition first given:

A Use Case Diagram is a UML (Unified Modeling Language) diagram which shows in visual form (aka diagram):

  • the interactions between a system’s functions and external actors (human or not) which trigger functionality or are needed to fulfill a system functionality,

  • the relationships between various actors where there is a need to show that some actors are similar to others, and

  • the relationship between various system capabilities depicting when a given functionality is related to another in some way: inherited, re-used, optional, etc.

Use Case Diagram

What do you think a Use Case Diagram is?



ranjitnair posted on Monday, October 22, 2007 4:44 AM
Use case diagram is a modelling diagram which depicts how the client and other users of the system (like developer, administrator etc) will interact with the system.There are actors which are the users which interact with the system and the system contains of different use cases which does specefic process.
nithyawarrier posted on Thursday, November 22, 2007 11:03 PM
Interactions between system and actors is represented graphically using usecase diagram
Asha Paul posted on Wednesday, November 28, 2007 6:56 AM
A use case diagram is a type of behavioral diagram defined by the Unified Modeling Language (UML). Its purpose is to present a graphical overview of the functionality provided by a system in terms of actors, their goals—represented as use cases—and any dependencies between those use cases
Asha Paul
Roohi Agarwal posted on Sunday, February 3, 2008 3:58 PM
Use Case is a textual description of interaction between the Actor and the system. It is used during the analysis phase of the project to identify and partition the system functionality.

Use case describes the behaviour of the system when the actor sends an input.

Use case usually emphasizes on on WHAT a system should not on HOW it shoul ddo it.
Roohi Agarwal
kr_BA posted on Tuesday, March 11, 2008 5:45 AM
A use case diagram is a modeling technique in UML, which depicts the system level functionality via
user-system dilogue exchange in graphical manner.

Also it is a first stage where requirement model is born
and will iterate towards the discovery of static and dynamic behaviour of the candidate system.
Shilpa posted on Thursday, March 12, 2009 9:29 AM
Use Case diagram is a modelling technique in UML.
It has four components:

1.System boundary
2.Use case
3. Actor
rachel posted on Sunday, May 10, 2009 10:12 AM
A use case diagram aids in determining the scope of the system being designed via system boundary, the role or the system i.e Actor which interacts with the System being modeled,The Use cases which depict the functionality and the relationships between the use cases if any
LAMPEJO posted on Tuesday, October 25, 2011 7:26 AM
A Use case is the interaction between the initiating actor and the benefiting actor via the system. it also uses includes and extends for simplicity, clarity and unambiguity.
Neha posted on Thursday, August 25, 2016 5:20 AM
UML is an graphical representation of the users actions and system response. Through UML diagrams its always easy to understand the users action on the system. In spite of documenting use cases in 200 pages its always good to represent it with the diagrams which would be easy for the team to understand the use case well.
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