notes / solid / ocp.md · 2026-03 · 1 min

# Open Closed Principle

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### Code should be open for extension but closed for modification.

This simply means that you should be able to add new behavior without changing the existing code.

## 🍽️ Imagine a restaurant menu:

### ❌ Bad Design

Every time you add a new dish:

• You rewrite the whole menu
• Reprint everything
• Risk breaking old items

### ✅ Good Design with Open Closed Principle

Menu is structured like:

• Base menu stays the same
• You can add new dishes as extensions

##### You don’t touch old items!

##### You just add new ones!

## 🧑‍🍳 In software:

### ❌ Bad Design

class Menu {
    void showMenu() {
        System.out.println("1. Pasta");
        System.out.println("2. Pizza");
    }
}

Every time you want to add a new dish, you have to modify the showMenu method, which can lead to bugs and maintenance issues.

### ✅ Good Design with Open Closed Principle

abstract class MenuItem {
    abstract void show();
}   

class Pasta extends MenuItem {
    void show() {
        System.out.println("1. Pasta");
    }
}

class Pizza extends MenuItem {
    void show() {
        System.out.println("2. Pizza");
    }
}

Now, to add a new dish, you simply create a new class that extends MenuItem without modifying existing code. This way, the existing menu remains unchanged and you can easily add new items without risking bugs in the old code.