Composite (Structural)

Intent / problem it solves

Compose objects into tree structures to represent part-whole hierarchies. Clients treat individual objects and compositions uniformly through a common interface.

When to use / when NOT

Use for nested menus, file systems, UI trees, org charts, or AST-like structures.

Avoid when you only have shallow lists without recursion, or when a flat structure plus IDs is simpler.

Structure

Component declares children operations; leaf has no children; composite stores child components.

Loading diagram…

Go example

package main

import (
	"fmt"
	"strings"
)

type Graphic interface {
	Draw(indent int) string
}

type Circle struct{ label string }

func (circle Circle) Draw(indent int) string {
	return strings.Repeat("  ", indent) + "circle:" + circle.label
}

type Group struct {
	label    string
	children []Graphic
}

func (group *Group) Add(child Graphic) { group.children = append(group.children, child) }

func (group Group) Draw(indent int) string {
	lines := []string{strings.Repeat("  ", indent) + "group:" + group.label}
	for _, child := range group.children {
		lines = append(lines, child.Draw(indent+1))
	}
	return strings.Join(lines, "\n")
}

func main() {
	root := Group{label: "scene"}
	root.Add(Circle{label: "sun"})
	fmt.Println(root.Draw(0))
}

JavaScript example

class Node {
  render(depth) {
    throw new Error('override');
  }
}

class TextNode extends Node {
  constructor(value) {
    super();
    this.value = value;
  }

  render(depth) {
    return `${'  '.repeat(depth)}text:${this.value}`;
  }
}

class ContainerNode extends Node {
  constructor(name) {
    super();
    this.name = name;
    this.children = [];
  }

  add(child) {
    this.children.push(child);
  }

  render(depth) {
    const header = `${'  '.repeat(depth)}group:${this.name}`;
    const body = this.children.map((child) => child.render(depth + 1));
    return [header, ...body].join('\n');
  }
}

const root = new ContainerNode('layout');
root.add(new TextNode('hello'));
console.log(root.render(0));

Interview phrase

“Composite gives one interface for leaves and containers so callers recurse without instanceof chains; I use it for menus and document models.”

Map to LLD for folder trees, nested UI, or expression evaluation in LLD case studies.

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