Java递归应用：输出树形菜单

树节点类：

package cn.com.tree;

public class Node {
	private Integer id;
	private Integer parentId;
	private String name;
	private String link;
	
	public Integer getId() {
		return id;
	}
	public void setId(Integer id) {
		this.id = id;
	}
	public Integer getParentId() {
		return parentId;
	}
	public void setParentId(Integer parentId) {
		this.parentId = parentId;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public String getLink() {
		return link;
	}
	public void setLink(String link) {
		this.link = link;
	}
}

输出树形菜单类：

package cn.com.tree;

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

public class Tree {
	private StringBuffer html = new StringBuffer();
	private List<Node> nodes;
	
	public Tree(List<Node> nodes){
		this.nodes = nodes;
	}
	
	public String buildTree(){
		html.append("<ul>");
		for (Node node : nodes) {
			Integer id = node.getId();
			if (node.getParentId() == null) {
				html.append("\r\n<li id='" + id + "'>" + node.getName()+ "</li>");
				build(node);
			}
		}
		html.append("\r\n</ul>");
		return html.toString();
	}
	
	private void build(Node node){
		List<Node> children = getChildren(node);
		if (!children.isEmpty()) {
			html.append("\r\n<ul>");
			for (Node child : children) {
				Integer id = child.getId();
				html.append("\r\n<li id='" + id + "'>" + child.getName()+ "</li>");
				build(child);
			}
			html.append("\r\n</ul>");
		} 
	}
	
	private List<Node> getChildren(Node node){
		List<Node> children = new ArrayList<Node>();
		Integer id = node.getId();
		for (Node child : nodes) {
			if (id.equals(child.getParentId())) {
				children.add(child);
			}
		}
		return children;
	}
}

测试类：

package zzj.test;

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

import cn.com.tree.Node;
import cn.com.tree.Tree;


public class Test {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		List<Node> nodes = new ArrayList<Node>();
		
		Node node1 = new Node();
		node1.setId(1);
		node1.setName("node1");
		node1.setParentId(null);
		node1.setLink(null);
		nodes.add(node1);
		
		Node node11 = new Node();
		node11.setId(11);
		node11.setName("node11");
		node11.setParentId(1);
		node11.setLink(null);
		nodes.add(node11);
		
		Node node111 = new Node();
		node111.setId(111);
		node111.setName("node111");
		node111.setParentId(11);
		node111.setLink(null);
		nodes.add(node111);
		
		Node node12 = new Node();
		node12.setId(12);
		node12.setName("node12");
		node12.setParentId(1);
		node12.setLink(null);
		nodes.add(node12);
		
		Node node2 = new Node();
		node2.setId(2);
		node2.setName("node2");
		node2.setParentId(null);
		node2.setLink(null);
		nodes.add(node2);
		
		Node node21 = new Node();
		node21.setId(21);
		node21.setName("node21");
		node21.setParentId(2);
		node21.setLink(null);
		nodes.add(node21);
		
		Node node3 = new Node();
		node3.setId(3);
		node3.setName("node3");
		node3.setParentId(null);
		node3.setLink(null);
		nodes.add(node3);
		
		Tree tree = new Tree(nodes);
		System.out.println(tree.buildTree());
	}
}

输出：

<ul>
<li id='1'>node1</li>
<ul>
<li id='11'>node11</li>
<ul>
<li id='111'>node111</li>
</ul>
<li id='12'>node12</li>
</ul>
<li id='2'>node2</li>
<ul>
<li id='21'>node21</li>
</ul>
<li id='3'>node3</li>
</ul>

浏览器效果：