Inheritance and interface notation in LLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is inheritance in system design?

Inheritance allows a new component or class to reuse properties and behaviors of an existing one, promoting code reuse and easier maintenance.

beginner

What does an interface represent in system design?

An interface defines a contract of methods or behaviors that a component must implement, without specifying how these methods work internally.

intermediate

How is inheritance typically shown in diagrams?

Inheritance is shown with a solid line and a hollow arrow pointing from the child (subclass) to the parent (superclass).

intermediate

How is interface implementation shown in diagrams?

Interface implementation is shown with a dashed line and a hollow arrow pointing from the implementing class to the interface.

advanced

Why use interfaces instead of inheritance?

Interfaces allow different components to share common behaviors without forcing a class hierarchy, enabling more flexible and decoupled designs.

Which notation represents inheritance in UML diagrams?

A Solid line with hollow arrow

B Dashed line with hollow arrow

C Solid line with filled arrow

D Dashed line with filled arrow

Inheritance is shown with a solid line and a hollow arrow pointing to the parent class.

What does an interface specify?

A A contract of methods to implement

B How methods work internally

C Data storage details

D Inheritance hierarchy

Interfaces specify a contract of methods that must be implemented, without defining internal workings.

Which line style shows interface implementation?

A Solid line

B Dotted line

C Double line

D Dashed line

Interface implementation is shown with a dashed line and a hollow arrow.

Inheritance promotes which of the following?

A Loose coupling

B Code reuse

C Interface segregation

D Data hiding

Inheritance allows reuse of code by sharing properties and behaviors from parent to child.

Why might you prefer interfaces over inheritance?

A To force a strict class hierarchy

B To avoid defining any methods

C To allow flexible behavior sharing without hierarchy

D To increase coupling

Interfaces allow different classes to share behaviors without being forced into a class hierarchy.

Explain how inheritance and interface notation differ in system design diagrams.

Think about the line style and arrow type used in UML.

Describe why interfaces are important in designing flexible systems compared to inheritance.

Consider how interfaces help avoid rigid class structures.

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