OOP - lesson 6 | HackIndia Skills

Key point

One idea

SOLID is not a rulebook to impress seniors. It is a set of checks that helps your feature stay easy to change without breaking everything else.

Use it after you have working code. First ship the feature. Then clean the design where change is likely.

The five checks

S means Single Responsibility. A class should have one main reason to change. For example, Order should not also send WhatsApp messages and write invoices.

O means Open/Closed. Add a new behaviour by adding a new class when possible, not by editing one big if-else block every time.

The next three

L means Liskov Substitution. If code expects a PaymentMethod, any UpiPayment or CardPayment should work without surprise errors.

I means Interface Segregation. Do not force a class to implement methods it does not need. D means Dependency Inversion. High-level code should depend on interfaces, not hard-coded concrete classes.

Example

Small payment example

Here is a simple Dependency Inversion and Open/Closed example. Checkout uses the PaymentMethod interface. Adding NetBankingPayment later will not require changing Checkout.

interface PaymentMethod {
  boolean pay(int amountInRupees);
}

class UpiPayment implements PaymentMethod {
  public boolean pay(int amountInRupees) {
    System.out.println("Paid ₹" + amountInRupees + " using UPI");
    return true;
  }
}

class Checkout {
  private final PaymentMethod paymentMethod;

  Checkout(PaymentMethod paymentMethod) {
    this.paymentMethod = paymentMethod;
  }

  boolean placeOrder(int amountInRupees) {
    return paymentMethod.pay(amountInRupees);
  }
}

class Main {
  public static void main(String[] args) {
    Checkout checkout = new Checkout(new UpiPayment());
    checkout.placeOrder(499);
  }
}

Common mistake

Do not over-SOLID

Beginners sometimes create ten interfaces for a two-class feature. That makes the repo harder to read, not better.

Use SOLID to remove pain. If there is only one implementation and no likely change, a direct class may be fine.

Tip

Use AI carefully

Ask an AI tool to review your object model using SOLID. Then check the advice yourself. AI often suggests extra abstractions you do not need.

A useful prompt: Review these classes for SOLID issues. Suggest only changes that make the feature easier to test or extend. Avoid adding patterns unless needed.

Your to-do

SOLID review pass

Do each one yourself, then tap it to tick it off. The ticks are only a checklist for you: they are not marked or scored.

List your main classes and write each class’s job in one line.
Mark any class that has more than one reason to change.
Find any long if-else that chooses behaviour by type.
Check if a class directly creates a dependency that may change later.
Refactor only one problem that affects readability, testing, or extension.

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