/**
 * Menu
 *
 * @author lcry
 * @date 2020/06/01 20:36
 */
    @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;
}