遇到構建選單，構建樹形結構，資料庫一般就使用父id來表示，為了降低資料庫的查詢壓力，我們可以使用Java8中的Stream流一次性把資料查出來，然後通過流式處理

實體類

@Data
@Builder
public class Menu {
    /**
     * id
     */
    public Integer id;
    /**
     * 名稱
     */
    public String name;
    /**
     * 父id ，根節點為0
     */
    public Integer parentId;
    /**
     * 子節點資訊
     
 
*/
    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;
    }
    
}

遞迴組裝樹形結構：

@Test
    public void testtree(){
        //模擬從資料庫查詢出來
        List<Menu> menus = Arrays.asList(
                new Menu(1,"根節點",0),
                new Menu(2,"子節點1",1),
                
 
new Menu(3,"子節點1.1",2),
                new Menu(4,"子節點1.2",2),
                new Menu(5,"根節點1.3",2),
                new Menu(6,"根節點2",1),
                new Menu(7,"根節點2.1",6),
                new Menu(8,"根節點2.2",6),
                new Menu(9,"根節點2.2.1",7),
                new Menu(10,"根節點2.2.2",7),
                new Menu(11,"根節點3",1),
                new Menu(12,"根節點3.1",11)
        );

        //獲取父節點
        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("-------轉json輸出結果-------");
        System.out.println(JSON.toJSON(collect));
    }

    /**
     * 遞迴查詢子節點
     * @param root  根節點
     * @param all   所有節點
     * @return 根節點資訊
     */
    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;
    }