Why Use case diagrams in Software Engineering? - Purpose & Use Cases

The Big Idea

What if you could see how users interact with your system in one simple picture?

The Scenario

Imagine trying to explain how a new app works just by writing long paragraphs or talking without any visuals.

You have to describe every user action and system response in words, which can get confusing fast.

The Problem

Writing everything in text is slow and easy to misunderstand.

People might miss important steps or get lost in details.

It's hard to see the big picture or how users interact with the system.

The Solution

Use case diagrams show the system's functions and users visually.

They make it easy to understand who does what and how the system responds.

This clear picture helps everyone agree on how the system should work.

Before vs After

✗ Before

User logs in, then selects product, then places order, then receives confirmation.

✓ After

Actor: User -> Use Cases: Login, Select Product, Place Order, Receive Confirmation

What It Enables

Use case diagrams let teams quickly grasp system behavior and user goals, speeding up design and communication.

Real Life Example

A team designing an online store uses a use case diagram to show how customers browse items, add to cart, and checkout, making sure everyone understands the process.

Key Takeaways

Manual text descriptions are slow and confusing.

Use case diagrams provide a clear visual of user interactions.

They improve communication and speed up system design.

Practice

1. What is the main purpose of a use case diagram in software engineering?

easy

A. To test the software performance

B. To design the database schema

C. To write the program code

D. To show users and their interactions with system functions

Solution

1. Step 1: Understand the role of use case diagrams

   Use case diagrams visually represent how users (actors) interact with the system's functions (use cases).

2. Step 2: Compare with other software tasks

   Designing databases, coding, and testing are different activities not shown by use case diagrams.

3. Final Answer:

   To show users and their interactions with system functions -> Option D

4. Quick Check:

   Use case diagrams = show actors and interactions [OK]

Hint: Use case diagrams focus on users and their actions [OK]

Common Mistakes:

• Confusing use case diagrams with database design
• Thinking use case diagrams show code details
• Assuming use case diagrams test software

2. Which of the following is the correct symbol for an actor in a use case diagram?

easy

A. A stick figure

B. A rectangle

C. An oval

D. A diamond

Solution

1. Step 1: Identify actor symbol in use case diagrams

   Actors are represented by stick figures to symbolize users or external systems.

2. Step 2: Recognize other symbols

   Ovals represent use cases, rectangles and diamonds are not used for actors.

3. Final Answer:

   A stick figure -> Option A

4. Quick Check:

   Actor symbol = stick figure [OK]

Hint: Actors are stick figures in use case diagrams [OK]

Common Mistakes:

• Confusing actors with use cases (ovals)
• Choosing rectangles or diamonds for actors
• Not recognizing standard UML symbols

3. Consider a use case diagram with an actor "Customer" connected to two use cases: "Place Order" and "Track Order". What does this connection represent?

medium

A. Customer is a system component

B. Customer can perform both placing and tracking orders

C. Use cases depend on each other

D. Customer is a database entity

Solution

1. Step 1: Understand actor-use case connections

   Lines between an actor and use cases show the actor can perform those actions.

2. Step 2: Interpret the given connections

   Customer connected to "Place Order" and "Track Order" means the customer can do both activities.

3. Final Answer:

   Customer can perform both placing and tracking orders -> Option B

4. Quick Check:

   Actor connected to use cases = actor performs those actions [OK]

Hint: Lines show actor's possible actions [OK]

Common Mistakes:

• Thinking actors are system parts
• Assuming use cases depend on each other
• Confusing actors with database entities

4. A use case diagram shows an actor connected to a use case with a dashed arrow labeled "<<include>>". What is the likely issue if the arrow is drawn as a solid line instead?

medium

A. The diagram incorrectly shows a direct association instead of an include relationship

B. The actor symbol is wrong

C. The use case should be a rectangle

D. The diagram is missing actors

Solution

1. Step 1: Understand <<include>> notation

   The <<include>> relationship is shown with a dashed arrow from one use case to another, not from actor to use case.

2. Step 2: Identify the error in arrow style

   A solid line from actor to use case shows direct association, not an include relationship, so the diagram is incorrect.

3. Final Answer:

   The diagram incorrectly shows a direct association instead of an include relationship -> Option A

4. Quick Check:

   Include = dashed arrow between use cases, not solid actor line [OK]

Hint: <<include>> uses dashed arrows between use cases only [OK]

Common Mistakes:

• Drawing solid lines for <<include>>
• Confusing actor symbols with use case symbols
• Misplacing <<include>> arrows from actors

5. You want to model a system where a "User" can either "Login" or "Register" before accessing "Dashboard". Which use case diagram structure correctly shows that "Dashboard" use case requires either "Login" or "Register" first?

hard

A. "Login" and "Register" use cases extend "Dashboard" use case

B. "Dashboard" use case extends "Login" and "Register" use cases

C. "Dashboard" use case includes both "Login" and "Register" use cases

D. Login" and "Register" use cases are actors

Solution

1. Step 1: Understand include vs extend relationships

   Include means mandatory sub-function; extend means optional or conditional addition.

2. Step 2: Model login/register before dashboard

   Since "Dashboard" requires either "Login" or "Register" first, "Dashboard" use case includes both "Login" and "Register" use cases to show mandatory preconditions.

3. Final Answer:

   "Dashboard" use case includes both "Login" and "Register" use cases -> Option C

4. Quick Check:

   Include = mandatory use case inclusion [OK]

Hint: Use include for mandatory preconditions before main use case [OK]

Common Mistakes:

• Using extend instead of include for mandatory steps
• Confusing actors with use cases
• Reversing direction of include/extend arrows