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The Composite Pattern (Structural)

Purpose

The Composite pattern allows you to treat individual objects and groups of objects in the same way. It’s useful for building tree-like structures—such as folders, menus, or graphical elements—where each part, whether simple or complex, supports the same operations.

Problem It Solves

Many applications deal with structures that mix simple and complex elements. Without this pattern, you'd need different logic to handle each type, leading to repetitive and error-prone code. The Composite pattern unifies them under one interface, so they can be processed in the same way.

When to Use It

Use the Composite pattern when:

  • You need to represent part-whole hierarchies, like file systems or organizational charts.
  • You want to treat individual objects and groups consistently.
  • Your structure is recursive, such as a tree or nested container.

When Not to Use It

Avoid this pattern if:

  • Your data structure is flat and doesn’t need hierarchy.
  • A simpler class design would be easier to understand and maintain.
  • The abstraction adds more complexity than benefit.

How It Works

The Composite pattern defines three main components:

  1. Component – An interface or abstract class for all objects in the structure.
  2. Leaf – A basic object that performs work and doesn’t have children.
  3. Composite – A container that can hold other components (leaves or other composites) and delegate operations to them.

Both Leaf and Composite follow the same interface, making them interchangeable in the structure.

Real-World Analogy

Think of a company org chart. An employee is a leaf. A department manager (composite) oversees a group of employees and possibly other departments. Whether you’re asking for a report from a person or a department, you use the same method.

Simplified Example

Here’s a basic implementation in Python:

from typing import List

class Component:
    def operation(self) -> str:
        pass

class Leaf(Component):
    def __init__(self, name: str):
        self.name = name

    def operation(self) -> str:
        return f"Leaf({self.name})"

class Composite(Component):
    def __init__(self, name: str):
        self.name = name
        self._children: List[Component] = []

    def add(self, component: Component) -> None:
        self._children.append(component)

    def remove(self, component: Component) -> None:
        self._children.remove(component)

    def operation(self) -> str:
        results = [child.operation() for child in self._children]
        return f"Composite({self.name})[{' + '.join(results)}]"

Usage Example

root = Composite("root")
leaf1 = Leaf("A")
leaf2 = Leaf("B")
subtree = Composite("branch")
subtree.add(Leaf("C"))

root.add(leaf1)
root.add(subtree)
root.add(leaf2)

print(root.operation())  # Output: Composite(root)[Leaf(A) + Composite(branch)[Leaf(C)] + Leaf(B)]

Learn More

See the full implementation here: Composite Pattern on GitHub