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Design Patterns

Composite

Working with a composite touches every item once. So it takes O(n) time, where n is the total number of items, including nested ones.

The idea, in plain English

Composite works like folders on your computer. A folder can hold files, but it can also hold more folders, which hold more files and folders. Composite lets you treat one single file and a whole folder full of files the exact same way. Ask either one 'how big are you?' and each figures out its own answer.

How it works

  1. 1You give both the simple items (files) and the containers (folders) the same method name, like getSize().
  2. 2A simple item just returns its own answer directly.
  3. 3A container loops over everything inside it — files or more folders — and adds up their answers. This works no matter how deeply things are nested.

When you'd use it

Use this when you have tree-shaped data — file systems, UI components holding other components, or org charts — and want to treat a single item and a whole group the same way.

Common beginner mistakes

  • Don't give containers and simple items different method names. That forces you to check 'is this a file or a folder?' everywhere, instead of calling the same method on both.
  • Don't let a folder accidentally contain itself. That causes infinite loops when you try to add up its size.

Try it — edit and run

Click the code to edit · press ⌘/Ctrl+↵ to run

Editable code. Tab and Shift+Tab indent. Press Escape, then Tab, to move focus out of the editor.

Expected output — hit Run to try it
resume.pdf size: 200
Photos folder size: 500
Documents folder size: 700

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