Use recursion to form a multi-level directory tree structure, with possibly the most detailed notes of the whole network.

Use scenarios
In the development and implementation of the user's position 、 Job list display , Display the menu directory of users on the background management page .

One 、 Effect display

1. database structure ：
for example ： Electronic products, / laptop / Lenovo notebook Generate a three-level Directory

2. Data processing returns ：

{
    
    "code": 200,
    "msg": " Successful operation ",
    "data": [
        {
    
            "id": "1",
            "name": " Electronic products, ",
            "parentId": "0",
            "type": 1,
            "childList": [
                {
    
                    "id": "5",
                    "name": " laptop ",
                    "parentId": "1",
                    "type": 2,
                    "childList": [
                        {
    
                            "id": "21",
                            "name": " Lenovo notebook ",
                            "parentId": "5",
                            "type": 3,
                            "childList": []
                        },
                        {
    
                            "id": "22",
                            "name": " Alien notebook ",
                            "parentId": "5",
                            "type": 3,
                            "childList": []
                        },
                        {
    
                            "id": "23",
                            "name": " Dell notebook ",
                            "parentId": "5",
                            "type": 3,
                            "childList": []
                        }
                    ]
                },
                {
    
                    "id": "6",
                    "name": " mobile phone ",
                    "parentId": "1",
                    "type": 2,
                    "childList": [
                        {
    
                            "id": "24",
                            "name": " Apple mobile phone ",
                            "parentId": "6",
                            "type": 3,
                            "childList": []
                        },
                        {
    
                            "id": "25",
                            "name": " Pineapple mobile phone ",
                            "parentId": "6",
                            "type": 3,
                            "childList": []
                        }
                    ]
                },
                {
    
                    "id": "7",
                    "name": " The headset ",
                    "parentId": "1",
                    "type": 2,
                    "childList": []
                },
                {
    
                    "id": "8",
                    "name": " Electronic cigarettes ",
                    "parentId": "1",
                    "type": 2,
                    "childList": []
                }
            ]
        },
        {
    
            "id": "2",
            "name": " Articles for daily use ",
            "parentId": "0",
            "type": 1,
            "childList": [
                {
    
                    "id": "10",
                    "name": " The chair ",
                    "parentId": "2",
                    "type": 2,
                    "childList": []
                },
                {
    
                    "id": "11",
                    "name": " The bed ",
                    "parentId": "2",
                    "type": 2,
                    "childList": []
                },
                {
    
                    "id": "19",
                    "name": " toothpaste ",
                    "parentId": "2",
                    "type": 2,
                    "childList": []
                },
                {
    
                    "id": "20",
                    "name": " toothbrush ",
                    "parentId": "2",
                    "type": 2,
                    "childList": []
                },
                {
    
                    "id": "9",
                    "name": " Table ",
                    "parentId": "2",
                    "type": 2,
                    "childList": []
                }
            ]
        },
        {
    
            "id": "3",
            "name": " Hygienic Products ",
            "parentId": "0",
            "type": 1,
            "childList": [
                {
    
                    "id": "12",
                    "name": " toilet paper ",
                    "parentId": "3",
                    "type": 2,
                    "childList": []
                },
                {
    
                    "id": "13",
                    "name": " Wipes ",
                    "parentId": "3",
                    "type": 2,
                    "childList": []
                }
            ]
        },
        {
    
            "id": "4",
            "name": " School Supplies ",
            "parentId": "0",
            "type": 1,
            "childList": [
                {
    
                    "id": "14",
                    "name": " e-book ",
                    "parentId": "4",
                    "type": 2,
                    "childList": []
                },
                {
    
                    "id": "15",
                    "name": " Listening CD ",
                    "parentId": "4",
                    "type": 2,
                    "childList": []
                },
                {
    
                    "id": "16",
                    "name": " Physical book ",
                    "parentId": "4",
                    "type": 2,
                    "childList": []
                },
                {
    
                    "id": "17",
                    "name": " Pen ",
                    "parentId": "4",
                    "type": 2,
                    "childList": []
                },
                {
    
                    "id": "18",
                    "name": " Notebook ",
                    "parentId": "4",
                    "type": 2,
                    "childList": []
                }
            ]
        }
    ]
}

Two 、 Realize the idea

1. Get all categories .
2. Get all categories id aggregate . Use stream() Realization ,stream Use the tutorial
3. Get level 1 Classification Information . Also use stream() Realization .
4. Circular primary classification , Add a subcategory to the first level classification in the loop , And add the first level classification to the returned tree structure （ remarks ： Do not join the returned tree structure , Return directly to step 3 The classification information of ）.
5. The focus is on the steps 4 Add a subcategory to the first level classification , And subcategories are adding subcategories , Sub sub sub classification and add sub sub sub classification ·········· wait , The implementation process can use recursion .
6. step 5 Implementation process ： Write a recursive method , Add child nodes to the current node , First, get the byte point set of the current node , Then put this set into the current node sub node attribute , Then call the current recursive method again , Treat the newly obtained child node as the new current node , Get the new child node of the new current node , Be careful Call the current recursive method again , Treat the newly obtained child node as the new current node First, judge whether the new current node has child nodes （ Judgment method ： Get the word node array of the current node , According to the size()>0? Determine whether there are child nodes ）, If not, there is no need for recursion .
7. summary ：1-4 It's data preparation ,5-6 Is to implement recursion （ Recursion of adding child nodes to the current node ）.

3、 ... and 、 Code display

1. Main idea code

 @GetMapping("/list")
    public Result list() {
    
        // All categories 
        List<Category> categoryList = categoryService.list();
        // All categories id aggregate 
        List<String> idList = categoryList.stream().map(Category::getId).collect(Collectors.toList());
        // Returned tree classification results 
        List<Category> treeCategory = new ArrayList<>();
        // First level classification catalogue 
        List<Category> categories = categoryList.stream().filter(category -> !idList.contains(category.getParentId())).collect(Collectors.toList());
        // Cycle through the current level-1 catalog 
        for (Category category : categories) {
    
            // To the current classification node   add to   Sub classification node 
            addChild(categoryList,category);
            // After adding child node classifications to the current classification , Add to the returned tree structure 
            treeCategory.add(category);
        }
        // Return the returned tree structure 
        return Result.success(categories);
    }

    /** *  To the current classification node   add to   Sub classification node  * @param categoryList  All categories  * @param category  Current classification node  */
    public void addChild( List<Category> categoryList,Category category){
    
        // Cycle through all categories , Get all the children of the current node 
        List<Category> categoryListChild = categoryList.stream().filter(category1 -> category1.getParentId().equals(category.getId())).collect(Collectors.toList());
        // Add the child nodes of the current category to the current category 
        category.setChildList(categoryListChild);
        // Call this method again , Treat the child nodes of the current node as the current node , Continue adding child nodes , remarks ： This will cause a constant cycle 
        categoryListChild.forEach(category1 -> {
    
            // Add next step , Judge whether the current node has child nodes , No loop recursion 
            //if ()
            addChild(categoryList,category1);
        });
    }

    /** *  Judge the current node   Whether there is   Child node  * @param categoryList  All categories  * @param category  Current node  */
    public boolean haveChild( List<Category> categoryList,Category category){
    
        // Get the children of the current node 
        List<Category> categoryListChild = categoryList.stream().filter(category1 -> category1.getParentId().equals(category.getId())).collect(Collectors.toList());
        // The child node is greater than 0 There is , Otherwise, it doesn't exist 
        return categoryListChild.size()>0?true:false;
    }

2. Entity class code presentation
remarks ： There must be an array of child nodes in the entity class code . Entity class pairs use the top entity class image

@Data
@EqualsAndHashCode(callSuper = false)
@Accessors(chain = true)
public class Category implements Serializable {
    

    private static final long serialVersionUID = 1L;
    @TableId(value = "id", type = IdType.ID_WORKER_STR)
    private String id;

    /** *  Category name  */
    private String name;

    /** *  The superior id */
    private String parentId;

    /** *  Classification level  */
    private Integer type;
    /** *  Array of child nodes  */
    @TableField(exist = false)
    private List<Category> childList;
}