notes / design-patterns / structural / composite.md · 2026-03 · 2 min

# Composite Pattern

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### Treat individual objects and groups of objects in the same way.

This means that you can work with a single object and a collection of objects using the same interface.

## 🍽️ Imagine a restaurant:

### ❌ Bad Design

You treat items differently:

• A single dish → handled one way
• A combo meal → handled another way

👉 Different logic for each case 👉 Code becomes complex and messy

### ✅ Good Design with Composite Pattern

You treat everything as a menu item:

• 🍕 Single dish → MenuItem
• 🍱 Combo meal (multiple dishes) → MenuItem

Now both can be handled the same way

##### No special cases needed!

##### Uniform behavior everywhere!

## 🧑‍💻 In software:

### ❌ Bad Design

class Dish {
    void showPrice() {
        System.out.println("Dish price");
    }
}

class ComboMeal {
    List<Dish> dishes;

    void showComboPrice() {
        // different logic
    }
}

You need separate logic for Dish and ComboMeal.

### ✅ Good Design with Composite Pattern

interface MenuItem {
    void showPrice();
}

class Dish implements MenuItem {
    public void showPrice() {
        System.out.println("Dish price");
    }
}

import java.util.List;

class ComboMeal implements MenuItem {
    private List<MenuItem> items;

    ComboMeal(List<MenuItem> items) {
        this.items = items;
    }

    public void showPrice() {
        for (MenuItem item : items) {
            item.showPrice();
        }
    }
}

Now you can treat both single dishes and combo meals the same way:

MenuItem dish = new Dish();
MenuItem combo = new ComboMeal(List.of(dish, new Dish()));

dish.showPrice();
combo.showPrice();

##### Same interface for single and grouped objects!

##### Simpler and more flexible design!