Composite Pattern Explained with Code and Real‑World Scenarios

What is the Composite Pattern?

The pattern unifies single leaf nodes and containers that hold multiple child nodes under a common interface, allowing client code to operate on both without distinguishing between an individual element and a collection of elements.

Core Goal

To treat leaves and branches in a tree structure uniformly, simplifying code for operations such as copy, delete, or display.

Typical Real‑World Scenarios

File systems: files (leaves) and folders (containers) both support copy and delete.

Backend side‑menu trees, organizational charts, product category hierarchies.

Permission menus, multi‑level comments, geographic region trees.

Java Implementation (File System Example)

1. Abstract Component (common contract for file and folder)

public abstract class FileComponent {
    protected String name;
    public FileComponent(String name){
        this.name = name;
    }
    // common print method
    public abstract void show();
    // folder‑only operations; leaf provides empty implementation
    public void add(FileComponent component){}
    public void remove(FileComponent component){}
}

2. Leaf Node – File

public class FileLeaf extends FileComponent{
    public FileLeaf(String name) {
        super(name);
    }
    @Override
    public void show() {
        System.out.println("文件：" + name);
    }
}

3. Composite Node – Folder

import java.util.ArrayList;
import java.util.List;
public class FolderComposite extends FileComponent{
    private List<FileComponent> childList = new ArrayList<>();
    public FolderComposite(String name) {
        super(name);
    }
    @Override
    public void add(FileComponent component) {
        childList.add(component);
    }
    @Override
    public void remove(FileComponent component) {
        childList.remove(component);
    }
    @Override
    public void show() {
        System.out.println("【文件夹】" + name);
        for(FileComponent child : childList){
            child.show();
        }
    }
}

4. Test Invocation

public class Test {
    public static void main(String[] args) {
        // leaf files
        FileComponent txt = new FileLeaf("笔记.txt");
        FileComponent img = new FileLeaf("图片.png");
        // sub‑folder
        FileComponent subFolder = new FolderComposite("素材文件夹");
        subFolder.add(img);
        // root folder
        FileComponent root = new FolderComposite("桌面");
        root.add(txt);
        root.add(subFolder);
        // unified printing without distinguishing file/folder
        root.show();
    }
}

Common Business Scenarios Summary

Hierarchical data such as system menus, department structures, multi‑level product categories.

File systems, resource directories, permission trees.

Multi‑level comments, e‑commerce category trees, geographic region trees.

Any structure requiring recursive traversal with uniform add, remove, and display operations.

Tip

Leaf nodes do not need to add child elements; providing empty add/remove methods is sufficient, making the pattern ideal for trees with a fixed depth.