Use case diagrams in LLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a use case diagram?

A use case diagram is a simple visual tool that shows how users (actors) interact with a system to achieve goals (use cases). It helps understand system functions from the user's perspective.

beginner

Who are actors in a use case diagram?

Actors represent users or other systems that interact with the system. They can be people, external systems, or devices that perform actions with the system.

beginner

What is the purpose of use cases in a use case diagram?

Use cases describe specific actions or services the system provides to actors. They represent goals or tasks that actors want to accomplish using the system.

beginner

What does an association line represent in a use case diagram?

An association line connects an actor to a use case, showing that the actor participates in or initiates that use case.

intermediate

How do use case diagrams help in system design?

They help clarify system requirements by showing who uses the system and what they want to do. This makes it easier to plan features and communicate with stakeholders.

What does an actor represent in a use case diagram?

A A database inside the system

B A user or external system interacting with the system

C A programming function

D A hardware component

Actors are users or external systems that interact with the system.

What is the main purpose of a use case in a use case diagram?

A To show the system's internal code structure

B To list all system errors

C To represent the physical hardware

D To describe a goal or task an actor wants to achieve

Use cases describe goals or tasks actors want to accomplish using the system.

Which symbol typically represents an actor in a use case diagram?

A A stick figure

B A rectangle

C A circle

D A diamond

Actors are usually shown as stick figures to represent users or external entities.

What does a line connecting an actor to a use case indicate?

A The use case is a subtask of the actor

B The actor owns the use case

C The actor interacts with the use case

D The actor is a system component

The line shows the actor participates in or initiates the use case.

Why are use case diagrams useful in early system design?

A They help understand user needs and system functions

B They replace all other diagrams

C They show detailed code logic

D They show database schemas

Use case diagrams help clarify user needs and system functions early on.

Explain what a use case diagram is and describe its main components.

Think about who uses the system and what they want to do.

Describe how use case diagrams help in understanding system requirements.

Consider how diagrams help talk with users and developers.

Practice

1. What is the primary purpose of a use case diagram in system design?

easy

A. To show user roles and their interactions with the system

B. To display database table relationships

C. To illustrate the internal code structure

D. To list all system hardware components

Solution

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

   Use case diagrams focus on users (actors) and what they do with the system (use cases).

2. Step 2: Compare with other diagram types

   Database diagrams show tables, code diagrams show code structure, hardware diagrams show physical parts, none are use case diagrams.

3. Final Answer:

   To show user roles and their interactions with the system -> Option A

4. Quick Check:

   Use case diagram = user roles + actions [OK]

Hint: Use case = users + actions, not code or hardware [OK]

Common Mistakes:

• Confusing use case diagrams with class or sequence diagrams
• Thinking use case diagrams show database details
• Assuming use case diagrams show internal code logic

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

easy

A. A rectangle with the actor's name inside

B. An oval shape showing a system function

C. A stick figure representing a user

D. A diamond shape indicating a decision

Solution

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

   Actors are shown as stick figures to represent users or external systems.

2. Step 2: Differentiate other symbols

   Ovals represent use cases (functions), rectangles can be system boundaries, diamonds are for decisions in flowcharts, not use case diagrams.

3. Final Answer:

   A stick figure representing a user -> Option C

4. Quick Check:

   Actor symbol = stick figure [OK]

Hint: Actors are stick figures, use cases are ovals [OK]

Common Mistakes:

• Using rectangles for actors instead of stick figures
• Confusing use case ovals with actor symbols
• Mixing flowchart symbols with use case diagram symbols

3. Consider a use case diagram with actors Customer and Admin, and use cases Place Order, Cancel Order, and Update Inventory. Which use case should only be connected to the Admin actor?

medium

A. Place Order

B. Cancel Order

C. Place Order and Cancel Order

D. Update Inventory

Solution

1. Step 1: Analyze actor responsibilities

   Customers place and cancel orders; Admin manages inventory updates.

2. Step 2: Match use cases to actors

   Only Admin should connect to Update Inventory as it is a system management task.

3. Final Answer:

   Update Inventory -> Option D

4. Quick Check:

   Admin-only use case = Update Inventory [OK]

Hint: Admin manages system tasks, customers do orders [OK]

Common Mistakes:

• Connecting customers to inventory updates
• Assuming customers can update system data
• Linking all use cases to all actors

4. A use case diagram shows an actor connected to a use case with a dashed arrow labeled <<extend>>. What does this indicate?

medium

A. The extended use case adds optional behavior to the base use case

B. The actor is not allowed to perform the extended use case

C. The actor must always perform the extended use case

D. The use case is a system boundary

Solution

1. Step 1: Understand <<extend>> relationship

   The <<extend>> arrow shows optional or conditional behavior added to a base use case.

2. Step 2: Interpret actor connection meaning

   The actor can perform the base use case; the extended use case happens only sometimes.

3. Final Answer:

   The extended use case adds optional behavior to the base use case -> Option A

4. Quick Check:

   <<extend>> = optional behavior [OK]

Hint: <<extend>> means optional addition, not mandatory [OK]

Common Mistakes:

• Thinking <<extend>> means mandatory action
• Confusing <<extend>> with <<include>>
• Assuming <<extend>> defines system boundaries

5. You are designing a use case diagram for an online library system. Which of the following best represents a correct use case for the Member actor that includes an Admin actor managing the system?

hard

A. Member: Add Book, Remove Book; Admin: Borrow Book, Return Book

B. Member: Borrow Book, Return Book; Admin: Add Book, Remove Book

C. Member: Borrow Book, Add Book; Admin: Remove Book, Return Book

D. Member: Remove Book, Return Book; Admin: Borrow Book, Add Book

Solution

1. Step 1: Identify typical member actions

   Members usually borrow and return books, not manage inventory.

2. Step 2: Identify admin responsibilities

   Admins add or remove books to keep the library updated.

3. Step 3: Match actions to actors correctly

   Member: Borrow Book, Return Book; Admin: Add Book, Remove Book correctly assigns borrowing and returning to Member, and adding/removing to Admin.

4. Final Answer:

   Member: Borrow Book, Return Book; Admin: Add Book, Remove Book -> Option B

5. Quick Check:

   Member = borrow/return, Admin = add/remove [OK]

Hint: Members use books; Admins manage books [OK]

Common Mistakes:

• Swapping member and admin roles
• Assigning management tasks to members
• Mixing use cases between actors