Class responsibilities and behavior in LLD - System Design Exercise

Design: Class Responsibilities and Behavior Design

Focus on designing class responsibilities and behaviors in object-oriented design. Exclude implementation details like database or UI specifics.

Functional Requirements

FR1: Define clear responsibilities for each class in a system

FR2: Ensure classes have single, well-defined behaviors

FR3: Support interaction between classes through well-designed methods

FR4: Promote code reusability and maintainability

FR5: Allow easy extension or modification of class behaviors

Non-Functional Requirements

NFR1: Classes should follow the Single Responsibility Principle

NFR2: Interactions should minimize tight coupling

NFR3: Design should be understandable by developers with basic OOP knowledge

NFR4: Performance impact should be minimal for typical use cases

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

❓ Question 4

❓ Question 5

Key Components

Classes and their methods

Attributes representing class state

Interfaces or abstract classes for behavior contracts

Relationships like inheritance, composition, and aggregation

Design principles like SRP, encapsulation, and polymorphism

Design Patterns

Single Responsibility Principle (SRP)

Strategy pattern for interchangeable behaviors

Observer pattern for event-driven behavior

Command pattern for encapsulating requests

Decorator pattern for extending behavior dynamically

Reference Architecture

  +----------------+       +----------------+
  |   Class A      |       |   Class B      |
  |----------------|       |----------------|
  | - attribute1   |       | - attribute2   |
  |----------------|       |----------------|
  | + behavior1()  |<----->| + behavior2()  |
  +----------------+       +----------------+

  Classes have clear responsibilities and communicate via methods.

Components

Class

Object-Oriented Programming

Encapsulates data and behaviors related to a single responsibility.

Methods

OOP Methods

Define behaviors that the class can perform or expose.

Attributes

Class Fields

Store the state or data relevant to the class.

Interfaces/Abstract Classes

OOP Abstractions

Define contracts for behaviors that multiple classes can implement.

Relationships

Inheritance, Composition, Aggregation

Model how classes relate and collaborate to fulfill system behavior.

Request Flow

1. 1. Client calls a method on Class A to perform an action.

2. 2. Class A executes its behavior using its attributes.

3. 3. If needed, Class A calls a method on Class B to delegate part of the behavior.

4. 4. Class B performs its behavior and returns results to Class A.

5. 5. Class A completes the action and returns the result to the client.

Database Schema

Not applicable as this design focuses on class responsibilities and behavior rather than data persistence.

Scaling Discussion

Bottlenecks

Classes with too many responsibilities become hard to maintain.

Tight coupling between classes reduces flexibility and increases bugs.

Large inheritance hierarchies can be complex and fragile.

Behavior changes require modifying multiple classes if not well abstracted.

Solutions

Apply Single Responsibility Principle to split classes with multiple responsibilities.

Use interfaces and dependency injection to reduce coupling.

Favor composition over inheritance to build flexible behaviors.

Use design patterns like Strategy or Decorator to extend behavior without modifying existing classes.

Interview Tips

Time: Spend 10 minutes defining class responsibilities and behaviors, 15 minutes discussing interactions and design principles, and 20 minutes explaining patterns and scalability.

Explain how each class has a single clear responsibility.

Describe how behaviors are encapsulated in methods.

Discuss how classes communicate with minimal coupling.

Highlight use of design principles like SRP and encapsulation.

Show understanding of patterns to extend or modify behavior safely.