Inheritance and interface notation in LLD - System Design Exercise

Design: Inheritance and Interface Notation System

Focus on notation and diagram representation of inheritance and interfaces. Implementation details of classes or methods are out of scope.

Functional Requirements

FR1: Represent class inheritance relationships clearly

FR2: Represent interface implementation relationships

FR3: Support multiple inheritance where allowed

FR4: Show abstract classes and interfaces distinctly

FR5: Allow notation for method overriding and interface methods

FR6: Be understandable by developers with basic OOP knowledge

Non-Functional Requirements

NFR1: Notation must be simple and unambiguous

NFR2: Must scale to diagrams with up to 50 classes/interfaces

NFR3: Should be usable in low-fidelity design tools (e.g., whiteboard, paper)

NFR4: Latency and availability not applicable

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

❓ Question 4

❓ Question 5

Key Components

Class boxes with name and stereotype

Arrows for inheritance (solid line with closed arrowhead)

Dashed arrows for interface implementation

Labels or stereotypes for abstract classes and interfaces

Method lists inside class/interface boxes

Design Patterns

UML class diagram notation

Use of stereotypes like <<interface>>, <<abstract>>

Arrow styles for inheritance vs implementation

Notation for method overriding

Separation of interface and implementation

Reference Architecture

  +----------------+          +----------------+
  | <<abstract>>   |          | <<interface>>  |
  |   Animal       |          |   Movable      |
  |----------------|          |----------------|
  | +eat()         |          | +move()        |
  +-------^--------+          +-------^--------+
          |                           |
          | inherits                  | implements
  +-------+--------+          +-------+--------+
  |   Dog          |          |   Car          |
  |----------------|          |----------------|
  | +bark()        |          | +drive()       |
  +----------------+          +----------------+

Components

Class Box

Diagram element

Represents a class or interface with name and methods

Inheritance Arrow

Solid line with closed arrowhead

Shows class inheritance relationship

Interface Implementation Arrow

Dashed line with closed arrowhead

Shows a class implements an interface

Stereotypes

Text labels like <<interface>>, <<abstract>>

Distinguish interfaces and abstract classes

Request Flow

1. 1. Identify all classes and interfaces to represent.

2. 2. Draw boxes for each class/interface with name and methods.

3. 3. Add stereotype labels to indicate abstract or interface.

4. 4. Connect classes with solid arrows to show inheritance.

5. 5. Connect classes to interfaces with dashed arrows for implementation.

6. 6. Use method lists to show overridden or interface methods.

Database Schema

Not applicable for notation system; focus is on diagram elements and relationships.

Scaling Discussion

Bottlenecks

Diagram clutter with many classes and interfaces

Difficulty distinguishing multiple inheritance arrows

Confusion between inheritance and implementation arrows

Limited space for method details in boxes

Solutions

Group related classes into packages or modules

Use color coding or different arrow thickness for clarity

Provide legend explaining arrow styles and stereotypes

Use expandable/collapsible sections for method lists

Interview Tips

Time: Spend 10 minutes clarifying requirements and constraints, 20 minutes drawing and explaining notation, 15 minutes discussing scaling and alternatives.

Explain difference between inheritance and interface implementation

Show how notation distinguishes abstract classes and interfaces

Discuss arrow styles and stereotypes for clarity

Mention how to handle complexity in large diagrams

Relate notation to common UML standards for familiarity

Practice

1. What does the notation <|-- represent in system design diagrams?

easy

A. Dependency, where one class depends on another

B. Interface implementation, where a class follows a set of rules

C. Inheritance, where one class is a specialized version of another

D. Aggregation, where one class contains another

Solution

1. Step 1: Understand notation meanings

   The notation <|-- is used to show inheritance in system design diagrams.

2. Step 2: Differentiate from interface notation

   Interface implementation uses <|.., not <|--.

3. Final Answer:

   Inheritance, where one class is a specialized version of another -> Option C

4. Quick Check:

   Inheritance = <|-- [OK]

Hint: Inheritance uses solid arrow <|-- [OK]

Common Mistakes:

• Confusing inheritance with interface implementation
• Mixing aggregation with inheritance notation
• Assuming dependency uses <|--

2. Which of the following is the correct notation for a class implementing an interface?

easy

A. Class <|-- Interface

B. Class <|.. Interface

C. Interface <|-- Class

D. Interface <|.. Class

Solution

1. Step 1: Recall interface implementation notation

   Interface implementation is shown by a dashed arrow <|.. from class to interface.

2. Step 2: Verify direction and notation

   The arrow points from the class to the interface, so Class <|.. Interface is correct.

3. Final Answer:

   Class <|.. Interface -> Option B

4. Quick Check:

   Interface implementation = <|.. from class to interface [OK]

Hint: Interface uses dashed arrow <|.. from class [OK]

Common Mistakes:

• Reversing arrow direction between class and interface
• Using solid arrow for interface implementation
• Confusing interface and inheritance notation

3. Given the following UML snippet:

interface IAnimal
class Dog <|.. IAnimal
class Labrador <|-- Dog

Which statement is true?

medium

A. IAnimal inherits from Dog and Labrador

B. Labrador implements IAnimal and inherits from Dog

C. Dog inherits from Labrador and implements IAnimal

D. Dog implements IAnimal and Labrador inherits from Dog

Solution

1. Step 1: Analyze interface implementation

   Dog implements IAnimal, shown by <|.. from Dog to IAnimal.

2. Step 2: Analyze inheritance

   Labrador inherits from Dog, shown by <|-- from Labrador to Dog.

3. Final Answer:

   Dog implements IAnimal and Labrador inherits from Dog -> Option D

4. Quick Check:

   Interface = <|.., Inheritance = <|-- [OK]

Hint: Dashed arrow = interface, solid arrow = inheritance [OK]

Common Mistakes:

• Mixing inheritance and interface directions
• Assuming interface inherits classes
• Confusing which class inherits which

4. Identify the error in this UML notation:

class Vehicle <|.. Car
interface Drivable <|-- Vehicle

medium

A. Vehicle cannot implement Car because Car is a class

B. Drivable cannot inherit Vehicle because interfaces use dashed arrows for inheritance

C. Arrow directions are reversed for both lines

D. No error, notation is correct

Solution

1. Step 1: Check first line meaning

   Vehicle <|.. Car means Vehicle implements Car, but Car is a class, not an interface, so this is invalid.

2. Step 2: Check second line meaning

   Drivable <|-- Vehicle means Drivable inherits Vehicle, but Drivable is an interface and Vehicle a class, so invalid.

3. Final Answer:

   Vehicle cannot implement Car because Car is a class -> Option A

4. Quick Check:

   Classes implement interfaces only [OK]

Hint: Classes implement interfaces, not other classes [OK]

Common Mistakes:

• Allowing classes to implement classes
• Reversing inheritance arrow direction
• Ignoring interface vs class roles

5. You are designing a system where Smartphone inherits from Device and implements two interfaces: Camera and GPS. Which UML notation correctly represents this?

hard

A. Smartphone <|-- Device
Smartphone <|.. Camera
Smartphone <|.. GPS

B. Device <|-- Smartphone
Smartphone <|.. Camera
Smartphone <|.. GPS

C. Device <|.. Smartphone
Camera <|-- Smartphone
GPS <|-- Smartphone

D. Smartphone <|.. Device
Camera <|.. Smartphone
GPS <|.. Smartphone

Solution

1. Step 1: Identify inheritance direction

   Smartphone inherits from Device, so arrow <|-- points from Smartphone to Device: Smartphone <|-- Device.

2. Step 2: Identify interface implementation

   Smartphone implements Camera and GPS interfaces, so dashed arrows <|.. point from Smartphone to each interface.

3. Step 3: Verify all notations

   Smartphone <|-- Device
   Smartphone <|.. Camera
   Smartphone <|.. GPS

   correctly shows inheritance and interface implementation directions.

4. Final Answer:

   Smartphone <|-- Device Smartphone <|.. Camera Smartphone <|.. GPS -> Option A

5. Quick Check:

   Inheritance arrow points to parent, interface arrow dashed from class [OK]

Hint: Inheritance arrow points to parent class, interface arrow dashed from class [OK]

Common Mistakes:

• Reversing inheritance arrow direction
• Using solid arrow for interface implementation
• Pointing interface arrows from interface to class