Use Case Diagrams: Understanding Include and Extend Relationships

Use Case Diagrams: Understanding Include and Extend Relationships

Use case diagrams are powerful tools in software engineering for visualizing the functionality of a system from a user’s perspective. They help define what a system does, rather than how it does it. Within these diagrams, relationships between use cases are critical for modeling complex interactions. Two particularly important relationships are "include" and "extend". This tutorial will provide a comprehensive understanding of these relationships, illustrating their purpose and how to apply them correctly.

What are Include and Extend Relationships?

Both "include" and "extend" are relationships depicted in use case diagrams using dashed arrows. However, they signify different types of dependencies and have distinct applications. Understanding the difference is crucial for creating accurate and maintainable models.

Include Relationship:

The "include" relationship represents a mandatory dependency. It signifies that one use case always incorporates the behavior of another use case to complete its execution. Think of it as a subroutine or a reusable component. The base use case is incomplete without the included use case.

Extend Relationship:

The "extend" relationship signifies an optional dependency. It indicates that one use case may add behavior to another use case under specific conditions. The extended use case is a variation or an exception to the base use case. The base use case can function independently, even without the extending use case.

Diving Deeper: Include Relationship

Purpose:

• Code Reusability: Extract common behavior shared by multiple use cases into a single, reusable use case.
• Modularity: Break down complex use cases into smaller, more manageable units.
• Avoiding Redundancy: Eliminate duplicate steps in different use cases.

How it Works:

1. The base use case depends on the included use case to complete its functionality.
2. The included use case is always executed whenever the base use case is initiated.
3. The direction of the arrow points from the base use case to the included use case.

Example: Online Banking System

Consider an online banking system. Several use cases might require user authentication. Instead of repeating the authentication steps in each use case (e.g., "Transfer Funds," "View Account Balance," "Pay Bills"), you can create a separate use case called "Authenticate User" and include it in all these other use cases.

• "Transfer Funds" –> "Authenticate User"
• "View Account Balance" –> "Authenticate User"
• "Pay Bills" –> "Authenticate User"

This ensures that all these use cases consistently require user authentication before proceeding.

Diving Deeper: Extend Relationship

Purpose:

• Modeling Optional Behavior: Represent functionality that might occur under specific circumstances.
• Handling Exceptions: Capture exceptional scenarios or error conditions.
• Adding Flexibility: Allow the system to adapt to different user needs or situations.

How it Works:

1. The extending use case adds behavior to the base use case only when a certain condition is met.
2. The base use case can function independently without the extending use case.
3. The direction of the arrow points from the extending use case to the base use case.

Example: ATM System

Consider an ATM system. The base use case is "Withdraw Cash." An extending use case could be "Apply Fee," which is executed only if the user is withdrawing money from an ATM not owned by their bank.

"Apply Fee" –> "Withdraw Cash"

In this scenario, "Withdraw Cash" can function normally even if the user doesn’t incur a fee. The “Apply Fee” use case extends the functionality only when the condition (using an out-of-network ATM) is met.

Key Differences Summarized

| Feature | Include | Extend |
|—————–|—————————|—————————|
| Dependency | Mandatory | Optional |
| Execution | Always executed | Conditionally executed |
| Completeness | Base use case incomplete without included | Base use case complete without extending |
| Direction | Base -> Included | Extending -> Base |

Best Practices

• Keep it Simple: Avoid overly complex relationships.
• Use Sparingly: Don’t overuse include or extend. Focus on clear, concise models.
• Clearly Define Conditions: For extend relationships, precisely define the conditions under which the extending use case is executed.
• Consider Alternatives: Before using include or extend, consider if a more straightforward use case design is possible.

By understanding the nuances of include and extend relationships, you can create more accurate, maintainable, and effective use case diagrams. This will ultimately lead to better software design and development.