java list转树状结构

地区表

CREATE TABLE `zone`  (
  `id` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL,
  `name` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL,
  `code` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL,
  `parent_id` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL
) ENGINE = InnoDB CHARACTER SET = utf8 COLLATE = utf8_general_ci ROW_FORMAT = Dynamic;

实体类

public class Zone {
    private String id;
    private String name;
    private String parentId;
    private List children;
    }

工具类

public class ZoneUtils {

    
    public static List buildTree2(List zoneList) {
        List result = new ArrayList<>();
        for (Zone zone : zoneList) {
            if (zone.parentId.equals("0")) {
                result.add(zone);
            }
            for (Zone child : zoneList) {
                if (child.parentId.equals(zone.id)) {
                    zone.addChildren(child);
                }
            }
        }
        return result;
    }

    
    public static List buildTree3(List zoneList) {
        Map> zoneByParentIdMap = new HashMap<>();
        zoneList.forEach(zone -> {
            List children = zoneByParentIdMap.getOrDefault(zone.parentId, new ArrayList<>());
            children.add(zone);
            zoneByParentIdMap.put(zone.parentId, children);
        });
        zoneList.forEach(zone -> zone.children = zoneByParentIdMap.get(zone.id));
        return zoneList.stream()
                .filter(v -> v.parentId.equals("0"))
                .collect(Collectors.toList());
    }

    
    public static List buildTree3_01(List zoneList) {
        Map> zoneByParentIdMap = zoneList.stream().collect(Collectors.groupingBy(Zone::getParentId));
        zoneList.forEach(zone -> zone.children = zoneByParentIdMap.get(zone.id));
        return zoneList.stream().filter(v -> v.parentId.equals("0")).collect(Collectors.toList());
    }
}