PHASE 2Beginner ~6 min· topic 10 of 22Structural

Structural 5 — Composite

In one line

Treat individual objects and groups of objects uniformly — a tree where leaves and nodes share an interface.

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Think of it like this

A folder on your computer. A folder can hold files AND other folders, and 'get size' works the same on both: a file returns its size, a folder adds up everything inside it.

Key ideas

01
Solves: recursive structures — org chart, file system, UI component tree, menu.
02
Component interface → Leaf and Composite both implement it; composite holds children.
03
price() or size() recurses: a folder's size is the sum of its children.

Java / Spring map

→
java.awt.Component/Container; Spring's CompositePropertySource.

Code & diagrams

Composite.javajava

Pricing menu items or a folder of files — same method either way.

public interface MenuItem { double price(); }

public final class Dish implements MenuItem {          // leaf
  private final double price;
  public Dish(double price) { this.price = price; }
  public double price() { return price; }
}

public final class Combo implements MenuItem {          // composite
  private final List<MenuItem> items = new ArrayList<>();
  public Combo add(MenuItem m) { items.add(m); return this; }
  public double price() {                                  // recursion
    return items.stream().mapToDouble(MenuItem::price).sum();
  }
}

// Meal deal = new Combo().add(new Dish(120)).add(new Dish(80)); → 200
// And a Combo can contain another Combo — unbounded nesting, one client.

Explain without notes

Why does Composite give you the Open/Closed principle for free here?

Practice

Model a file/folder size tree with Composite.

Trade-offs

↔
Uniform interface can hide 'leaf-only' operations you end up guarding with instanceof.

Completion checklist

I can model any recursive part-whole structure with Composite and compute aggregates recursively.

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