SOLID principles in Software Engineering - Step-by-Step Execution

Concept Flow - SOLID principles

Start: Design a Class

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S: Single Responsibility

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O: Open/Closed

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L: Liskov Substitution

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I: Interface Segregation

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D: Dependency Inversion

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Better Software Design

The SOLID principles guide step-by-step improvements to class design for better software structure.

Execution Sample

Software Engineering

class Bird {
  fly() {}
}

class Penguin extends Bird {
  fly() { throw new Error('Cannot fly'); }
}

Shows a violation of Liskov Substitution: Penguin cannot replace Bird because it cannot fly.

Analysis Table

Step	Principle	Check	Result	Action
1	Single Responsibility	Does class have one reason to change?	Yes	Keep class focused
2	Open/Closed	Can class be extended without modification?	No	Refactor to allow extension
3	Liskov Substitution	Can subclass replace superclass without errors?	No	Fix subclass behavior
4	Interface Segregation	Are interfaces client-specific and minimal?	No	Split interfaces
5	Dependency Inversion	Do high-level modules depend on abstractions?	No	Introduce abstractions
6	End	All principles checked	Improved design	Apply principles in code

💡 All SOLID principles checked to improve software design quality.

State Tracker

Principle	Initial Status	After Applying Principle	Final Status
Single Responsibility	Multiple reasons to change	One reason to change	Focused class
Open/Closed	Modifications needed for new features	Extendable without changes	Stable extension
Liskov Substitution	Subclass breaks behavior	Subclass compatible	Reliable inheritance
Interface Segregation	Fat interfaces	Small, client-specific interfaces	Clear contracts
Dependency Inversion	High-level depends on low-level	Both depend on abstractions	Flexible dependencies

Key Insights - 3 Insights

Why can't a Penguin class just inherit from Bird if it can't fly?

Because of Liskov Substitution (step 3 in execution_table), subclasses must behave like their parent. Penguin breaks this by throwing an error in fly(), so it violates the principle.

What does it mean for a class to have a single responsibility?

At step 1, Single Responsibility means a class should have only one reason to change. If it handles multiple tasks, changes in one task affect the whole class, making maintenance harder.

Why split interfaces instead of one big interface?

At step 4, Interface Segregation says clients should only depend on methods they use. Big interfaces force clients to implement unused methods, causing unnecessary complexity.

Visual Quiz - 3 Questions

Look at the execution_table at step 3. What problem does the Penguin class cause?

A It has too many responsibilities

B It cannot replace Bird without errors

C It depends on low-level modules

D It uses a fat interface

💡 Hint

Check the 'Result' column at step 3 about subclass replacement.

At which step does the design ensure classes can be extended without modifying existing code?

A Step 2

B Step 4

C Step 1

D Step 5

💡 Hint

Look for 'Open/Closed' principle in the execution_table.

If a class depends directly on concrete classes instead of abstractions, which principle is violated?

A Single Responsibility

B Liskov Substitution

C Dependency Inversion

D Interface Segregation

💡 Hint

Check step 5 about dependencies on abstractions.

Concept Snapshot

SOLID principles guide good class design:
S: Single Responsibility - one reason to change
O: Open/Closed - extend without modifying
L: Liskov Substitution - subclasses replace parents safely
I: Interface Segregation - small, client-specific interfaces
D: Dependency Inversion - depend on abstractions, not details

Full Transcript

The SOLID principles are five key rules to design software classes better. First, Single Responsibility means each class should have only one job. Next, Open/Closed says classes should be extendable without changing existing code. Liskov Substitution requires subclasses to behave like their parent classes so they can be used interchangeably. Interface Segregation advises splitting large interfaces into smaller ones so clients only use what they need. Finally, Dependency Inversion means high-level modules should depend on abstractions, not concrete details. Following these steps improves code quality and maintainability.