System Design ( Day - 55 )

Composite Design Pattern

Definition
Composite pattern composes object into tree like structure representing a part - whole hierarchy. It let client treats individual object and composition of object uniformly.

Design File System

When you have a tree-like structure—folders containing files and subfolders—the Composite Pattern lets you treat both “composite” (folders) and “leaf” (files) objects through a common interface. This makes your code simpler, more flexible, and easy to extend.

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🎯 The Problem

Imagine you need to:

• List all items in a folder

• Open every file under a directory (including nested subfolders)

• Compute the total size of a folder

You don’t want your client code to have special loops for files vs. folders. You’d rather call the same methods on both.

📂 File-System Example

📐 Standard UML Blueprint

• Component: declares the common interface (operation()).

• Leaf: implements the basic behavior.

• Composite: maintains a collection of Component children and implements operations by delegating to them.

🌟 Why Use Composite?

• Uniformity: Treat single objects and compositions the same way.

• Flexibility: Add new types of Component (e.g., ShortcutItem) without changing existing client code.

• Simplicity: Clients call one API (ls(), openAll(), getSize()) instead of branching on type.