Object-oriented design principles in LLD - Architecture Diagram

System Overview - Object-oriented design principles

This system demonstrates the core object-oriented design principles that help build flexible, reusable, and maintainable software. It shows how components like classes and objects interact following principles such as encapsulation, inheritance, polymorphism, and abstraction.

Architecture Diagram

User
  |
  v
Client Code
  |
  v
+-------------------+
|   Abstract Class   |
+-------------------+
      /      \
     /        \
+-------+  +---------+
|Class A|  | Class B |
+-------+  +---------+
     |          |
     v          v
+-------------------+
|   Interface(s)    |
+-------------------+

Components

User

actor

Interacts with the system by using client code

Client Code

component

Uses objects and calls methods following design principles

Abstract Class

class

Defines common interface and shared behavior for subclasses

Class A

class

Concrete implementation inheriting from Abstract Class

Class B

class

Another concrete implementation inheriting from Abstract Class

Interface(s)

interface

Defines contracts that classes implement to ensure polymorphism

Request Flow - 6 Hops

User → Client Code

Client Code → Abstract Class

Abstract Class → Class A or Class B

Class A or Class B → Interface(s)

Class A or Class B → Client Code

Client Code → User

Failure Scenario

Component Fails:Class A or Class B

Impact:If a concrete class implementation fails, the client code cannot complete the requested operation correctly.

Mitigation:Use polymorphism to switch to another class implementation or handle errors gracefully in client code.

Architecture Quiz - 3 Questions

Which component defines the common behavior that subclasses share?

A Interface(s)

B Abstract Class

C Client Code

D User

Design Principle

This architecture shows how object-oriented design principles like abstraction, inheritance, and polymorphism help build flexible systems. Abstract classes define shared behavior, interfaces enforce contracts, and concrete classes provide specific implementations. This separation allows easy extension and error handling.

Practice

1. Which of the following best describes the Single Responsibility Principle (SRP) in object-oriented design?

easy

A. Classes should be tightly coupled to improve performance.

B. A class should inherit from multiple classes to reuse code.

C. Objects should be able to change their behavior at runtime.

D. A class should have only one reason to change, meaning it should have only one job.

Solution

1. Step 1: Understand SRP definition

   The Single Responsibility Principle states that a class should have only one reason to change, meaning it should focus on a single job or responsibility.

2. Step 2: Evaluate options against SRP

   A class should have only one reason to change, meaning it should have only one job. matches this definition exactly. Options A, B, and C describe other concepts or incorrect ideas.

3. Final Answer:

   A class should have only one reason to change, meaning it should have only one job. -> Option D

4. Quick Check:

   Single Responsibility Principle = One class, one job [OK]

Hint: SRP means one class, one responsibility [OK]

Common Mistakes:

• Confusing SRP with inheritance rules
• Thinking SRP allows multiple unrelated jobs in one class
• Believing tight coupling is good design

2. Which syntax correctly demonstrates the Open/Closed Principle in class design?

easy

A. class Shape { void draw() {} } class Circle { void draw() { /* draw circle */ } }

B. class Shape { void draw() {} } class Circle extends Shape { void draw() { /* draw circle */ } }

C. class Shape { void draw() {} } class Circle extends Shape { void draw() { super.draw(); } }

D. class Shape { void draw() {} } class Circle extends Shape { void draw() { /* draw circle */ } } class Square extends Circle {}

Solution

1. Step 1: Understand Open/Closed Principle (OCP)

   OCP means software entities like classes should be open for extension but closed for modification. This means you can add new behavior by extending classes without changing existing code.

2. Step 2: Analyze options for OCP compliance

   class Shape { void draw() {} } class Circle extends Shape { void draw() { /* draw circle */ } } shows a base class Shape and a subclass Circle that extends Shape and overrides draw(), adding new behavior without modifying Shape. This follows OCP. class Shape { void draw() {} } class Circle { void draw() { /* draw circle */ } } lacks inheritance, so no extension. class Shape { void draw() {} } class Circle extends Shape { void draw() { super.draw(); } } calls super.draw() but does not add new behavior. class Shape { void draw() {} } class Circle extends Shape { void draw() { /* draw circle */ } } class Square extends Circle {} extends Circle with Square, which is allowed but less clear and may break OCP if Square changes Circle behavior.

3. Final Answer:

   class Shape { void draw() {} } class Circle extends Shape { void draw() { /* draw circle */ } } -> Option B

4. Quick Check:

   Open/Closed = Extend by subclassing, no base change [OK]

Hint: OCP means extend classes, don't modify them [OK]

Common Mistakes:

• Ignoring inheritance for extension
• Modifying base class instead of extending
• Confusing method overriding with no added behavior

3. Consider the following Python code snippet applying the Liskov Substitution Principle (LSP):

class Bird:
    def fly(self):
        return "Flying"

class Penguin(Bird):
    def fly(self):
        return "Cannot fly"

bird = Bird()
penguin = Penguin()
print(bird.fly())
print(penguin.fly())

What is the output of this code?

medium

A. "Flying" followed by "Cannot fly"

B. Both print "Flying"

C. Runtime error because Penguin cannot fly

D. Both print "Cannot fly"

Solution

1. Step 1: Understand code behavior

   Bird class has fly() returning "Flying". Penguin inherits Bird but overrides fly() to return "Cannot fly". bird.fly() prints "Flying". penguin.fly() prints "Cannot fly".

2. Step 2: Predict output lines

   First print outputs "Flying". Second print outputs "Cannot fly" as per Penguin's override.

3. Final Answer:

   "Flying" followed by "Cannot fly" -> Option A

4. Quick Check:

   Subclass override changes output correctly [OK]

Hint: Subclass method overrides parent method output [OK]

Common Mistakes:

• Assuming subclass must behave exactly like parent
• Expecting runtime error for overridden method
• Confusing method override with inheritance

4. Identify the problem in the following Java code violating the Dependency Inversion Principle (DIP):

class LightBulb {
    void turnOn() { System.out.println("Light on"); }
    void turnOff() { System.out.println("Light off"); }
}

class Switch {
    private LightBulb bulb = new LightBulb();
    void operate(boolean on) {
        if(on) bulb.turnOn();
        else bulb.turnOff();
    }
}

medium

A. Switch class should create multiple LightBulb instances.

B. LightBulb class should not have turnOn and turnOff methods.

C. Switch class directly depends on concrete LightBulb class instead of an abstraction.

D. Switch class should inherit from LightBulb.

Solution

1. Step 1: Understand Dependency Inversion Principle (DIP)

   DIP states that high-level modules should not depend on low-level modules directly; both should depend on abstractions (interfaces or abstract classes).

2. Step 2: Analyze code for DIP violation

   Switch class creates and uses a concrete LightBulb instance directly, tightly coupling Switch to LightBulb. This violates DIP because Switch depends on a concrete class, not an abstraction.

3. Final Answer:

   Switch class directly depends on concrete LightBulb class instead of an abstraction. -> Option C

4. Quick Check:

   DIP = Depend on abstractions, not concrete classes [OK]

Hint: High-level classes should depend on interfaces, not concrete classes [OK]

Common Mistakes:

• Thinking methods in LightBulb are wrong
• Believing multiple instances are needed
• Confusing inheritance with dependency

5. You are designing a payment system that must support multiple payment methods (credit card, PayPal, crypto). Which object-oriented design principle helps you add new payment methods without changing existing code, and how should you apply it?

hard

A. Use the Open/Closed Principle by defining a Payment interface and implementing new payment classes for each method.

B. Use the Single Responsibility Principle by putting all payment logic in one PaymentProcessor class.

C. Use the Liskov Substitution Principle by making all payment classes inherit from a base Payment class but override methods differently.

D. Use the Dependency Inversion Principle by making payment classes depend on concrete implementations.

Solution

1. Step 1: Identify the need for extensibility

   The system must allow adding new payment methods without modifying existing code, which is a classic use case for the Open/Closed Principle (OCP).

2. Step 2: Apply OCP with interface and implementations

   Define a Payment interface with common methods. Implement each payment method (credit card, PayPal, crypto) as separate classes implementing this interface. This allows adding new methods by creating new classes without changing existing ones.

3. Final Answer:

   Use the Open/Closed Principle by defining a Payment interface and implementing new payment classes for each method. -> Option A

4. Quick Check:

   OCP = Add new classes, no existing code change [OK]

Hint: OCP means add new classes, don't modify old ones [OK]

Common Mistakes:

• Putting all logic in one class violating SRP
• Confusing inheritance with extensibility
• Depending on concrete classes instead of abstractions