组合模式实际上是一种树形数据结构。以windows目录系统举例，怎么样用java语言描述一个文件夹？

定义一个文件夹类，文件夹类中包含若干个子文件类和若干个文件类。

进一步抽象，把文件夹和文件都看做节点，于是一个文件夹就可以描述为一个节点类，包含若干个子节点。

我们看看组合模式的代码

// 抽象节点
public abstract class Node 
{
    protected String name;
    abstract void add(Node node);
    abstract void removed(Node node);
    abstract void foreach();

    public String getName() 
    {
        return name;
    }

    public void setName(String name) 
    {
        this.name = name;
    }
}

// 文件类
public class File extends Node
{
    @Override
    void add(Node node) 
    {
    }

    @Override
    void removed(Node node) 
    {
    }

    @Override
    void foreach() 
    {
        System.out.println("File name is " + name);
    }
    
    public File(String name)
    {
        this.name = name;
    }
}

// 文件夹类
import java.util.ArrayList;
import java.util.List;

public class Folder extends Node
{
    List<Node> nodes = new ArrayList<Node>();
    
    @Override
    void add(Node node) 
    {
        nodes.add(node);
    }

    @Override
    void removed(Node node) 
    {
        nodes.remove(node);
    }

    @Override
    void foreach() 
    {
        System.out.println("Folder name is " + name);
        for(Node node : nodes)
        {
            node.foreach();
        }
    }
    
    public Folder(String name)
    {
        this.name = name;
    }
}

 测试类

package com.coshaho.learn.component;

public class Test 
{
    public static void main(String[] args)
    {
        Folder root = new Folder("Root");
        Folder music = new Folder("Music");
        Folder software = new Folder("Software");
        root.add(music);
        root.add(software);
        File explain = new File("explain.txt");
        File moon = new File("moon.mp3");
        File word = new File("word.exe");
        root.add(explain);
        music.add(moon);
        software.add(word);
        root.foreach();
    }
}

在实际应用中，我们希望使用json传递树形结构数据，我们希望每层节点定义都一样，不通过类来区分叶子节点和非叶子节点。这个时候我们可以简单的在Node对象中增加一个isLeaf属性，用来表示该节点是否是叶子节点。这种数据结构是一种递归结构，可以使用递归算法解析。

package com.coshaho.learn.component;

import java.util.ArrayList;
import java.util.List;

import org.springframework.util.CollectionUtils;

public class TreeNode 
{
    private String name;
    
    private boolean isLeaf;
    
    private List<TreeNode> nodes = new ArrayList<TreeNode>();
    
    private TreeNode(String name, boolean isLeaf)
    {
        this.name = name;
        this.isLeaf = isLeaf;
    }
    
    void add(TreeNode node) 
    {
        nodes.add(node);
    }

    void removed(TreeNode node) 
    {
        nodes.remove(node);
    }
    
    public String toString()
    {
        String nameInfo = "name: " + name;
        String typeInfo = "isLeaf: " + isLeaf;
        String value ="";
        if(isLeaf || CollectionUtils.isEmpty(nodes))
        {
            value = "{" + nameInfo + "," + typeInfo + "}";
        }
        else
        {
            String childrenInfo = "children: [";
            for(TreeNode node : nodes)
            {
                childrenInfo = childrenInfo + node.toString() + ",";
            }
            childrenInfo = childrenInfo.substring(0, childrenInfo.length() - 1) + "]";
            value = "{" + nameInfo + "," + typeInfo + "," + childrenInfo + "}";
        }
        return value;
    }
    
    public static void main(String[] args)
    {
        TreeNode all = new TreeNode("All", false);
        TreeNode guangdong = new TreeNode("GuangdDong", true);
        TreeNode sichuan = new TreeNode("SiChuan", false);
        TreeNode chengdu = new TreeNode("ChengDu", true);
        all.add(guangdong);
        all.add(sichuan);
        sichuan.add(chengdu);
        System.out.println(all);
    }
}