In the previous post, we covered:

SRP (keep classes focused)

OCP (extend without modifying)

Those help you structure code well.

But even with those applied, systems can still break in subtle ways:

inheritance behaving unexpectedly

classes forced to implement irrelevant methods

tight coupling making changes painful

That’s where the remaining SOLID principles come in:

LSP (Liskov Substitution Principle)

ISP (Interface Segregation Principle)

DIP (Dependency Inversion Principle)

LSP — Liskov Substitution Principle

Subclasses should be replaceable for their base classes without breaking behavior.

The classic mistake

Bird: - fly()

Penguin extends Bird: - fly() → throws error

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This design assumes:

All birds can fly

But penguins cannot.

Now, anywhere Bird is used, substituting it with Penguin breaks the system.

What actually went wrong

The issue is not inheritance.

The issue is:

The base class was designed incorrectly.

Correct approach

Bird: -…