Class Diagram
A class diagram describes types, their operations, and the relationships between them. It is a structural view, not a timeline of method calls or a picture of every runtime object.
What you’ll understand
- A class diagram describes types, their operations, and the relationships between them. It is a structural view, not a timeline of method calls or a picture of every runtime object.
- Represent Booking and Customer as class boxes with operations, joined by a labeled association.
- Evaluate the tradeoff: A diagram is a selected view of the model, not a substitute for contracts or tests. Too many fields obscure responsibilities; include the details needed for the design discussion.
Useful first: Classes and Objects
The idea and when to recognize it
A class diagram describes types, their operations, and the relationships between them. It is a structural view, not a timeline of method calls or a picture of every runtime object.
- Requirement: A booking has a persistent link to a customer whose name can change.
- Change to anticipate: Show the classes and association direction that preserve the live customer relationship.
- Design goal: Represent Booking and Customer as class boxes with operations, joined by a labeled association.
Start with the concrete problem
A booking has a persistent link to a customer whose name can change.
Show the classes and association direction that preserve the live customer relationship.
| Direct implementation | Revised design |
|---|---|
| Drawing a copied name as an object association misrepresents the implementation and its update behavior. | Represent Booking and Customer as class boxes with operations, joined by a labeled association. |
| The baseline case can hide the problem. Compare the extension and boundary fixtures before accepting this design. | A diagram is a selected view of the model, not a substitute for contracts or tests. Too many fields obscure responsibilities; include the details needed for the design discussion. |
Responsibilities and contracts
Represent Booking and Customer as class boxes with operations, joined by a labeled association.
- Booking: Refers to one customer. Public operations: customer, label()
- Customer: Owns the current name. Public operations: rename(name)
Read the design
Select a participant to inspect its responsibility and public operations. Follow the labeled relationships; an arrow indicates direction, while the legend distinguishes ownership, substitution, and temporary use.
Preparing the design walkthrough…
Follow the interaction
Choose a supplied workload and compare the direct and revised implementations. Step through the same operation in both languages. Object values are recorded after each highlighted statement; previous steps remain unchanged.
Preparing the design walkthrough…
Implementation and expected output
Read the complete example alongside its input and expected output. The implementation selector changes the displayed program; the language selector preserves your current scenario and playback position.
Preparing the design walkthrough…
Try the idea in another setting
A library class diagram can show a Loan associated with a Member and a BookCopy while separating catalog metadata from copy identity.
- Name the new client contract before adapting the example.
- Identify which state belongs to each object and who controls its lifetime.
- Write one normal case and one boundary case before extending the design.
Tradeoffs and interview takeaways
A diagram is a selected view of the model, not a substitute for contracts or tests. Too many fields obscure responsibilities; include the details needed for the design discussion.
Common mistake: Drawing a copied name as an object association misrepresents the implementation and its update behavior.
- Explain the choice: Represent Booking and Customer as class boxes with operations, joined by a labeled association.
- Challenge your choice: A diagram is a selected view of the model, not a substitute for contracts or tests. Too many fields obscure responsibilities; include the details needed for the design discussion.
- Describe a requirement that would make the simpler implementation sufficient.
Design challenge and knowledge check
Preparing the design walkthrough…