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2.3.1
 
Use Case Diagrams 
A use case diagram depicts the important scenarios for a system. It shows actors and use 
cases. An actor is the user in a use case. To be more specific an actor is a particular role 
of a user, so there is no one-to-one relationship between users and actors. A system may 
have several actors but only one user. Actors do not have to be a human being either, but 
anything which interacts with the system. For example can an actor be an external system 
which interacts with the system. 
A use case is a task which an actor wants the system to do. It should be a end-to-end 
task, with defined states of beginning and ending. Figure 2-3 below shows a use case of a 
customer who wants to withdraw money in a banking system. 
Withdraw money
Bank 
Customer
Figure 2-3: Use case diagram example 
2.3.2
 
Static Structure Diagrams 
Static structure diagrams can be used to describe a structural model of a system. For 
example a class model, relationship between processes and threads, etc. An example of 
the static structure showing the relationship between a Customer and an Account class is 
given in Figure 2-4 below. 
Account
balance
deposit(amount : Real) : Boolean
withdraw(amount : Real) : Boolean
0..n
Customer
Figure 2-4: Static structure diagram example 
There are several types of relationships available: Generalization, realization, dependency 
and assignment. 
2.3.3
 
Interaction Diagrams 
There are two forms of interaction diagrams which show communication of model 
elements. These two types are sequence diagrams and collaboration diagrams. They are 

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2.3 Background information: : Unified Modeling Language 
Page 7 of 93 
based on the same information, except that sequence diagrams are more useful to 
describe complex scenarios and when timing (such as in real-time systems) is important. 
Figure 2-5 shows a collaboration diagram of a customer depositing money to its 
account. 
bank customer : 
Customer
customer account : 
Account
1: deposit(Real)