一、适用场景：表示对象的【整体–部分】树形层次结构，同时忽略单个对象和组合对象的差异。比如一颗树，是由分支节点和叶子结点组成的层次结构，同时，我们希望给用户提供统一的接口对结点进行操作，不论是分支结点还是叶子结点
二、示例。
1.定义抽象的结点类：

package cn.edu.tju.app;

public abstract class TreeNode {
    private String content;

    public String getContent() {
        return content;
    }

    public void setContent(String content) {
        this.content = content;
    }

    public abstract  void print();
    public abstract void addNode(TreeNode node);
}

2.定义两个类，分别继承上述抽象类，分别代表根结点和叶子结点

package cn.edu.tju.app;

import java.util.LinkedList;
import java.util.List;

public class TreeNodeBranch extends TreeNode {
    private List<TreeNode> nodeList=new LinkedList<>();
    @Override
    public void print() {
        System.out.println(this.getContent());
        for(TreeNode treeNode:nodeList){
            treeNode.print();
        }
    }

    @Override
    public void addNode(TreeNode node){
        nodeList.add(node);
    }
}
package cn.edu.tju.app;

public class TreeNodeLeaf extends TreeNode {
    @Override
    public void print() {
        System.out.println(this.getContent());
    }

    @Override
    public  void addNode(TreeNode node){

    }
}

3.定义测试类，构建树，所有的操作都统一使用抽象类TreeNode中定义的方法，这就屏蔽了分支结点和叶子结点的差异

package cn.edu.tju.app;

public class TreeNodeTest {
    public static void main(String[] args) {
        TreeNode root=new TreeNodeBranch();
        root.setContent("root");

        TreeNode firstLevelChild11=new TreeNodeBranch();
        firstLevelChild11.setContent("child11");
        TreeNode firstLevelChild12=new TreeNodeBranch();
        firstLevelChild12.setContent("child12");

        TreeNode secondLevelChild21=new TreeNodeLeaf();
        secondLevelChild21.setContent("child21");
        TreeNode secondLevelChild22=new TreeNodeLeaf();
        secondLevelChild22.setContent("child22");
        TreeNode secondLevelChild23=new TreeNodeLeaf();
        secondLevelChild23.setContent("child23");
        TreeNode secondLevelChild24=new TreeNodeLeaf();
        secondLevelChild24.setContent("child24");

        root.addNode(firstLevelChild11);
        root.addNode(firstLevelChild12);

        firstLevelChild11.addNode(secondLevelChild21);
        firstLevelChild11.addNode(secondLevelChild22);
        firstLevelChild12.addNode(secondLevelChild23);
        firstLevelChild12.addNode(secondLevelChild24);

        root.print();
    }
}

4.运行结果：

设计模式：21-组合模式_ide