1、背景
??项目开发中，比如一本书，具有多级目录；一个一级分类下面还有二级分类，二级下面有三级…。此时需要后端返回一颗完整的无限层级的菜单树。
2、思路
??构建树的一般思想都是使用Map之类的映射处理或者使用递归之类的处理，Map有个好处是直观明了，具体实现可见本人的另外一篇（Java使用Map构建无限层级树）。但是递归对数据保护的比较好。本处介绍一下使用递归来构建树：
- 找出所有的顶级父级，组装成List。
- 递归找出每一个顶级父级的孩子节点。
3、实现
import com.alibaba.fastjson.JSON;
import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;
import java.util.Objects;
import java.util.stream.Collectors;
/**
* 说明：构建无限层级树
*
* @author： Fisher.Hao
* @date： 2021-07-24 19:20:12
*/
public class Test {
public static void main(String[] args) {
//模仿数据库中查询出来的数据
List<Person> dataIsSort = getData(true);
List<Person> dataNoSort = getData(false);
//获取顶级父级节点
List<Person> parentListIsSort = getParentList(dataIsSort);
List<Person> parentListNoSort = getParentList(dataNoSort);
//排序
for (Person parent : parentListIsSort) {
parent.setChildren(getChildrenList(dataIsSort, parent));
}
//不排
for (Person parent : parentListNoSort) {
parent.setChildren(getChildrenList(dataNoSort, parent));
}
System.out.println("排序后的JSON:************************************************************************");
System.out.println(JSON.toJSONString(parentListIsSort));
System.out.println("不排序后的JSON:************************************************************************");
System.out.println(JSON.toJSONString(parentListNoSort));
}
/**
* 获取所有的顶级父级节点
*
* @param allPersonList
* 所有节点
* @return 返回所有的顶级父节点
*/
private static List<Person> getParentList(final List<Person> allPersonList) {
//顶级父级节点的parent为0
return allPersonList.stream().filter(p -> Objects.equals(p.getParentId(), 0)).collect(Collectors.toList());
}
/**
* 获取顶级父节点的孩子节点
*
* @param allPersonList
* 所有的数据
* @param parent
* 顶级父
* @return 孩子节点
*/
private static List<Person> getChildrenList(final List<Person> allPersonList, final Person parent) {
//遍历所有的节点，获取当前父节点的所以的孩子节点
List<Person> childrenList = allPersonList.stream().filter(p -> Objects.equals(p.getParentId(), parent.getId())).collect(Collectors.toList());
//递归处理：找到孩子节点的孩子节点，直至其没有孩子节点
for (Person child : childrenList) {
//当前孩子节点child作为父节点继续遍历
child.setChildren(getChildrenList(allPersonList, child));
}
//返回孩子节点，此时孩子节点的孩子节点都已经填充满了
return childrenList;
}
/**
* 构造带有父子关系的数据
*
* @param isSort
* 是否排序，模仿需要指定顺序的情况，此处指定为从小到大
* @return 初始化数据
*/
private static List<Person> getData(boolean isSort) {
Person p1 = new Person(1, 0, "顶级父1");
Person p3 = new Person(3, 1, "子1_3");
Person p4 = new Person(4, 1, "子1_4");
Person p21 = new Person(21, 3, "子1_3_21");
Person p22 = new Person(22, 4, "子1_4_22");
Person p23 = new Person(23, 4, "子1_4_23");
Person p24 = new Person(24, 23, "子1_4_23_24");
Person p5 = new Person(5, 1, "子1_5");
Person p6 = new Person(6, 1, "子1_6");
Person p7 = new Person(7, 1, "子1_7");
Person p2 = new Person(2, 0, "顶级父2");
Person p8 = new Person(8, 2, "子2_8");
Person p14 = new Person(14, 8, "子2_8_14");
Person p15 = new Person(15, 8, "子2_8_15");
Person p17 = new Person(17, 8, "子2_8_17");
Person p18 = new Person(18, 14, "子2_8_14_18");
Person p19 = new Person(19, 14, "子2_8_14_19");
Person p20 = new Person(20, 15, "子2_8_15_20");
Person p9 = new Person(9, 2, "子2_9");
Person p10 = new Person(10, 2, "子2_10");
Person p11 = new Person(11, 2, "子2_11");
Person p12 = new Person(12, 2, "子2_12");
Person p13 = new Person(13, 2, "子2_13");
Person p16 = new Person(16, 0, "顶级父16");
List<Person> list = new ArrayList<Person>();
//打乱排序
list.add(p15);
list.add(p14);
list.add(p8);
list.add(p23);
list.add(p11);
list.add(p12);
list.add(p5);
list.add(p13);
list.add(p18);
list.add(p19);
list.add(p24);
list.add(p10);
list.add(p17);
list.add(p7);
list.add(p9);
list.add(p21);
list.add(p4);
list.add(p16);
list.add(p6);
list.add(p20);
list.add(p22);
list.add(p1);
list.add(p2);
list.add(p3);
if (isSort) {
return list.stream().distinct().sorted(Comparator.comparingInt(Person::getId)).collect(Collectors.toList());
}
return list.stream().distinct().collect(Collectors.toList());
}
}
class Person {
/**
* 主id，不可能为空
*/
private Integer id;
/**
* 父id，当其为0时表示为顶级父
*/
private Integer parentId;
private String name;
/**
* 孩子节点
*/
private List<Person> children;
public Person(Integer id, Integer parentId, String name) {
this.id = id;
this.parentId = parentId;
this.name = name;
}
public Integer getParentId() {
return parentId;
}
public void setParentId(Integer parentId) {
this.parentId = parentId;
}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public List<Person> getChildren() {
return children;
}
public void setChildren(List<Person> children) {
this.children = children;
}
}
4、运行结果
排序后的JSON:************************************************************************
[{"children":[{"children":[{"children":[],"id":21,"name":"子1_3_21","parentId":3}],"id":3,"name":"子1_3","parentId":1},{"children":[{"children":[],"id":22,"name":"子1_4_22","parentId":4},{"children":[{"children":[],"id":24,"name":"子1_4_23_24","parentId":23}],"id":23,"name":"子1_4_23","parentId":4}],"id":4,"name":"子1_4","parentId":1},{"children":[],"id":5,"name":"子1_5","parentId":1},{"children":[],"id":6,"name":"子1_6","parentId":1},{"children":[],"id":7,"name":"子1_7","parentId":1}],"id":1,"name":"顶级父1","parentId":0},{"children":[{"children":[{"children":[{"children":[],"id":18,"name":"子2_8_14_18","parentId":14},{"children":[],"id":19,"name":"子2_8_14_19","parentId":14}],"id":14,"name":"子2_8_14","parentId":8},{"children":[{"children":[],"id":20,"name":"子2_8_15_20","parentId":15}],"id":15,"name":"子2_8_15","parentId":8},{"children":[],"id":17,"name":"子2_8_17","parentId":8}],"id":8,"name":"子2_8","parentId":2},{"children":[],"id":9,"name":"子2_9","parentId":2},{"children":[],"id":10,"name":"子2_10","parentId":2},{"children":[],"id":11,"name":"子2_11","parentId":2},{"children":[],"id":12,"name":"子2_12","parentId":2},{"children":[],"id":13,"name":"子2_13","parentId":2}],"id":2,"name":"顶级父2","parentId":0},{"children":[],"id":16,"name":"顶级父16","parentId":0}]
不排序后的JSON:************************************************************************
[{"children":[],"id":16,"name":"顶级父16","parentId":0},{"children":[{"children":[],"id":5,"name":"子1_5","parentId":1},{"children":[],"id":7,"name":"子1_7","parentId":1},{"children":[{"children":[{"children":[],"id":24,"name":"子1_4_23_24","parentId":23}],"id":23,"name":"子1_4_23","parentId":4},{"children":[],"id":22,"name":"子1_4_22","parentId":4}],"id":4,"name":"子1_4","parentId":1},{"children":[],"id":6,"name":"子1_6","parentId":1},{"children":[{"children":[],"id":21,"name":"子1_3_21","parentId":3}],"id":3,"name":"子1_3","parentId":1}],"id":1,"name":"顶级父1","parentId":0},{"children":[{"children":[{"children":[{"children":[],"id":20,"name":"子2_8_15_20","parentId":15}],"id":15,"name":"子2_8_15","parentId":8},{"children":[{"children":[],"id":18,"name":"子2_8_14_18","parentId":14},{"children":[],"id":19,"name":"子2_8_14_19","parentId":14}],"id":14,"name":"子2_8_14","parentId":8},{"children":[],"id":17,"name":"子2_8_17","parentId":8}],"id":8,"name":"子2_8","parentId":2},{"children":[],"id":11,"name":"子2_11","parentId":2},{"children":[],"id":12,"name":"子2_12","parentId":2},{"children":[],"id":13,"name":"子2_13","parentId":2},{"children":[],"id":10,"name":"子2_10","parentId":2},{"children":[],"id":9,"name":"子2_9","parentId":2}],"id":2,"name":"顶级父2","parentId":0}]
5、JSON图示
6、格式化展示
[
{
"children":[
{
"children":[
{
"children":[
],
"id":21,
"name":"子1_3_21",
"parentId":3
}
],
"id":3,
"name":"子1_3",
"parentId":1
},
{
"children":[
{
"children":[
],
"id":22,
"name":"子1_4_22",
"parentId":4
},
{
"children":[
{
"children":[
],
"id":24,
"name":"子1_4_23_24",
"parentId":23
}
],
"id":23,
"name":"子1_4_23",
"parentId":4
}
],
"id":4,
"name":"子1_4",
"parentId":1
},
{
"children":[
],
"id":5,
"name":"子1_5",
"parentId":1
},
{
"children":[
],
"id":6,
"name":"子1_6",
"parentId":1
},
{
"children":[
],
"id":7,
"name":"子1_7",
"parentId":1
}
],
"id":1,
"name":"顶级父1",
"parentId":0
},
{
"children":[
{
"children":[
{
"children":[
{
"children":[
],
"id":18,
"name":"子2_8_14_18",
"parentId":14
},
{
"children":[
],
"id":19,
"name":"子2_8_14_19",
"parentId":14
}
],
"id":14,
"name":"子2_8_14",
"parentId":8
},
{
"children":[
{
"children":[
],
"id":20,
"name":"子2_8_15_20",
"parentId":15
}
],
"id":15,
"name":"子2_8_15",
"parentId":8
},
{
"children":[
],
"id":17,
"name":"子2_8_17",
"parentId":8
}
],
"id":8,
"name":"子2_8",
"parentId":2
},
{
"children":[
],
"id":9,
"name":"子2_9",
"parentId":2
},
{
"children":[
],
"id":10,
"name":"子2_10",
"parentId":2
},
{
"children":[
],
"id":11,
"name":"子2_11",
"parentId":2
},
{
"children":[
],
"id":12,
"name":"子2_12",
"parentId":2
},
{
"children":[
],
"id":13,
"name":"子2_13",
"parentId":2
}
],
"id":2,
"name":"顶级父2",
"parentId":0
},
{
"children":[
],
"id":16,
"name":"顶级父16",
"parentId":0
}
]