UML class diagrams basics in LLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a UML class diagram?

A UML class diagram is a visual representation that shows the structure of a system by displaying its classes, their attributes, methods, and the relationships between them.

beginner

What are the three main parts of a UML class diagram box?

The three main parts are: 1) Class name at the top, 2) Attributes (properties) in the middle, 3) Methods (operations) at the bottom.

beginner

What does an association relationship represent in a UML class diagram?

An association shows a connection between two classes, meaning objects of one class use or are linked to objects of another class.

intermediate

What is the difference between inheritance and composition in UML class diagrams?

Inheritance means one class is a specialized version of another (is-a relationship). Composition means one class contains another class as a part (has-a relationship) and controls its lifecycle.

intermediate

How is multiplicity shown in UML class diagrams and what does it mean?

Multiplicity is shown near the ends of association lines as numbers or ranges (e.g., 1, 0..*, *). It indicates how many instances of a class can be linked to instances of another class.

Which part of a UML class diagram shows the class's functions?

A The top compartment

B The bottom compartment

C The middle compartment

D Outside the box

The bottom compartment lists the methods or operations of the class.

What does a solid line with a hollow triangle arrow represent in UML?

A Inheritance

B Association

C Composition

D Dependency

A solid line with a hollow triangle arrow shows inheritance (generalization).

If a class 'Car' has a composition relationship with 'Engine', what does it mean?

A Car uses Engine temporarily

B Car is a type of Engine

C Car contains Engine and controls its lifecycle

D Car and Engine are unrelated

Composition means Car has an Engine as a part and controls its lifecycle.

What does multiplicity '0..*' mean in a UML association?

A Exactly zero instances

B One instance only

C At least one instance

D Zero or many instances

'0..*' means zero or many instances can be linked.

Which UML diagram focuses on showing classes and their relationships?

A Class diagram

B Use case diagram

C Sequence diagram

D Activity diagram

Class diagrams show classes and their relationships.

Describe the main components of a UML class diagram and their purpose.

Think about the box parts and how classes connect.

Explain the difference between inheritance and composition with simple examples.

Use real-life objects like animals or cars.

Practice

1. What does a UML class diagram primarily represent in system design?

easy

A. The structure of classes and their relationships

B. The sequence of operations in a process

C. The deployment of hardware components

D. The flow of data between systems

Solution

1. Step 1: Understand UML class diagram purpose

   A UML class diagram shows how classes are organized and connected in a system.

2. Step 2: Compare options to definition

   Only The structure of classes and their relationships describes structure of classes and relationships, which matches UML class diagrams.

3. Final Answer:

   The structure of classes and their relationships -> Option A

4. Quick Check:

   UML class diagram = class structure [OK]

Hint: Class diagrams show classes and links, not processes or hardware [OK]

Common Mistakes:

• Confusing class diagrams with sequence diagrams
• Thinking class diagrams show hardware layout
• Mixing data flow diagrams with class diagrams

2. Which of the following is the correct way to show a class attribute in a UML class diagram?

easy

A. * attributeName -> Type

B. - attributeName = Type

C. # attributeName - Type

D. + attributeName : Type

Solution

1. Step 1: Recall UML attribute notation

   Attributes use visibility (+ public, - private, # protected), name, colon, and type.

2. Step 2: Match options to notation

   + attributeName : Type matches correct format: '+ attributeName : Type'. Others use wrong symbols or separators.

3. Final Answer:

   + attributeName : Type -> Option D

4. Quick Check:

   Attribute syntax = + name : Type [OK]

Hint: Attributes use visibility + name : Type format [OK]

Common Mistakes:

• Using '=' instead of ':' to separate name and type
• Confusing visibility symbols
• Using arrows or dashes incorrectly

3. Given this UML class snippet:
+ name : String
- age : int
Which statement is true about the attributes?

medium

A. 'name' is private and 'age' is public

B. 'name' is public and 'age' is private

C. Both 'name' and 'age' are public

D. Both 'name' and 'age' are private

Solution

1. Step 1: Interpret visibility symbols

   '+' means public, '-' means private in UML.

2. Step 2: Apply to attributes

   'name' has '+', so public; 'age' has '-', so private.

3. Final Answer:

   'name' is public and 'age' is private -> Option B

4. Quick Check:

   + = public, - = private [OK]

Hint: Plus means public, minus means private [OK]

Common Mistakes:

• Mixing up '+' and '-' meanings
• Assuming all attributes are public by default
• Ignoring visibility symbols

4. Identify the error in this UML class attribute declaration:
- salary = float

medium

A. Visibility symbol '-' is incorrect

B. Type 'float' should be capitalized

C. The '=' sign should be ':' to separate name and type

D. Attribute name 'salary' is invalid

Solution

1. Step 1: Check UML attribute syntax

   Attributes use visibility, name, colon ':', then type. '=' is not used.

2. Step 2: Identify incorrect symbol

   The '=' sign is wrong; it should be ':' to separate attribute name and type.

3. Final Answer:

   The '=' sign should be ':' to separate name and type -> Option C

4. Quick Check:

   Use ':' not '=' in attributes [OK]

Hint: Use ':' not '=' between name and type [OK]

Common Mistakes:

• Using '=' instead of ':'
• Thinking visibility symbols are wrong
• Assuming type names must be capitalized

5. You want to model a simple library system with classes Book and Author. Each book has one author, and an author can write many books. Which UML relationship best represents this?

hard

A. Aggregation from Author to Book (one-to-many)

B. Composition from Book to Author (one-to-one)

C. Inheritance from Book to Author

D. Association from Book to Author (many-to-many)

Solution

1. Step 1: Understand relationship types

   Aggregation means a whole-part with shared lifecycle; composition means strong ownership; inheritance means 'is-a' relation; association is a general link.

2. Step 2: Apply to library example

   Author writes many books (one-to-many). Books can exist without author object lifecycle tightly bound, so aggregation fits best.

3. Final Answer:

   Aggregation from Author to Book (one-to-many) -> Option A

4. Quick Check:

   One author, many books = aggregation [OK]

Hint: One-to-many ownership without strong lifecycle = aggregation [OK]

Common Mistakes:

• Confusing composition with aggregation
• Using inheritance for unrelated classes
• Choosing many-to-many when one-to-many fits