Structural

Composite

Composes objects into tree structures to represent part-whole hierarchies.

File systemsUI hierarchiesOrganization charts

Understanding Composite

The Composite pattern organizes objects into tree structures to represent part-whole hierarchies. Both individual objects (leaves) and groups of objects (composites) implement the same interface, so client code can treat a single file the same way it treats an entire folder. In Go, this is implemented via a shared interface with a recursive method — the composite's implementation iterates over its children and aggregates results. This pattern is the foundation of file systems, UI component trees, and organizational hierarchies.

Key Concepts

  • •Component interface — shared interface that both leaves and composites implement, enabling uniform treatment
  • •Leaf — end objects with no children that perform the actual work (e.g., a File)
  • •Composite — container that holds child components and delegates operations to them recursively (e.g., a Folder)
  • •Recursive composition — composites can contain other composites, enabling arbitrarily deep tree structures

When to Use

✅ Use when
  • • You need to represent tree-like hierarchies (files, UI, org charts)
  • • Client code should treat groups and individuals the same way
  • • You need recursive operations across the entire tree (size, count, render)
  • • The depth of nesting is unknown at compile time
⚠️ Avoid when
  • • Your data is flat — a simple slice or map is sufficient
  • • Leaf and composite behaviors are very different and hard to unify
  • • You need strict type checking between different node types

Structure

<<Component>> Operation()
↓ implements
Leaf
Composite
children: []Component

How It Works

📁 Root
📄 File1
📁 SubFolder
📄 File2
Leaf
Composite
1

Leaf Nodes

Individual objects that perform actual work.

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Basic Implementation

File system with files and folders:

main.go
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Real-World Example: Organization Hierarchy

Employee hierarchy with salary calculations:

main.go
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