Traverse tree structure using Stream in Java 8

1 Introduction

You may usually encounter some needs, such as building menus and building tree structures. Databases are generally represented by parent ids. In order to reduce the query pressure on the database, we can use the Stream stream in Java8 to check out the data at once, and then use Let's take a look at streaming processing. In order to simplify the code implementation, we simulate viewing all the data in the database into the List.

2 Actual combat

2.1 Entity class: Menu.java

/**
 * Menu
 * @author wuKeFan
 */
@Data
@Builder
public class Menu {
    
    
    /**
     * id
     */
    public Integer id;
    /**
     * 名称
     */
    public String name;
    /**
     * 父id ，根节点为0
     */
    public Integer parentId;
    /**
     * 子节点信息
     */
    public List<Menu> childList;


    public Menu(Integer id, String name, Integer parentId) {
    
    
        this.id = id;
        this.name = name;
        this.parentId = parentId;
    }

    public Menu(Integer id, String name, Integer parentId, List<Menu> childList) {
    
    
        this.id = id;
        this.name = name;
        this.parentId = parentId;
        this.childList = childList;
    }
    
}

2.2 Recursively assemble tree structure

	@Test
    public void testtree(){
    
    
        //模拟从数据库查询出来
        List<Menu> menus = Arrays.asList(
                new Menu(1,"根节点",0),
                new Menu(2,"子节点1",1),
                new Menu(3,"子节点1.1",2),
                new Menu(4,"子节点1.2",2),
                new Menu(5,"根节点1.3",2),
                new Menu(6,"根节点2",1),
                new Menu(7,"根节点2.1",6),
                new Menu(8,"根节点2.2",6),
                new Menu(9,"根节点2.2.1",7),
                new Menu(10,"根节点2.2.2",7),
                new Menu(11,"根节点3",1),
                new Menu(12,"根节点3.1",11)
        );

        //获取父节点
        List<Menu> collect = menus.stream().filter(m -> m.getParentId() == 0).map(
                (m) -> {
    
    
                    m.setChildList(getChildrens(m, menus));
                    return m;
                }
        ).collect(Collectors.toList());
        System.out.println("-------转json输出结果-------");
        System.out.println(JSON.toJSON(collect));
    }

    /**
     * 递归查询子节点
     * @param root  根节点
     * @param all   所有节点
     * @return 根节点信息
     */
    private List<Menu> getChildrens(Menu root, List<Menu> all) {
    
    
        List<Menu> children = all.stream().filter(m -> {
    
    
            return Objects.equals(m.getParentId(), root.getId());
        }).map(
                (m) -> {
    
    
                    m.setChildList(getChildrens(m, all));
                    return m;
                }
        ).collect(Collectors.toList());
        return children;
    }

2.3 Format print results