飘乙己：List转Tree有4种写法！

/**
 * 区域平台
 * @author:Jam
 */
@Data
public class Platform {
    private String id;
    private String parentId;
    private String name;
    private String platformCode;
    private List<Platform> children;
    public Platform(String id, String platformCode,String parentId, String name) {
        this.id = id;
        this.parentId = parentId;
        this.name = name;
        this.platformCode = platformCode;
    }
}

public class PlatformTest {
    private final List<Platform> platformList = Lists.newArrayList();
    private ObjectMapper objectMapper = new ObjectMapper();
    @BeforeEach
    private void init(){
        Platform platform0 = new Platform("1","001","0","集团");
        Platform platform1 = new Platform("2","QYPT001","1","销委会");
        Platform platform2 = new Platform("3","QYPT002","2","吉龙大区");
        Platform platform3 = new Platform("4","QYPT003","2","江苏大区");
        Platform platform4 = new Platform("5","QYPT004","4","南京分区");
        Platform platform5 = new Platform("6","QYPT005","1","教育BG");
        Platform platform6 = new Platform("7","QYPT006","6","华南大区");
        Platform platform7 = new Platform("8","QYPT007","6","华东大区");
        platformList.add(platform0);
        platformList.add(platform1);
        platformList.add(platform2);
        platformList.add(platform3);
        platformList.add(platform4);
        platformList.add(platform5);
        platformList.add(platform6);
        platformList.add(platform7);
    }
}

@SneakyThrows
@Test
public void test1(){
  System.out.println(platformList.size());
  List<Platform> result = Lists.newArrayList();
  for (Platform platform : platformList) {
    //获取根节点
    if(platform.getParentId().equals("0")){
      result.add(platform);
    }
    for(Platform child : platformList){
      if(child.getParentId().equals(platform.getId())){
        platform.addChild(child);
      }
    }
  }
  System.out.println(objectMapper.writeValueAsString(result));
}

public void addChild(Platform platform){
  if(children == null){
    children = new ArrayList<>();
  }
  children.add(platform);
}

@SneakyThrows
@Test
public void test2(){
  Map<String, List<Platform>> platformMap = new HashMap<>();
  platformList.forEach(platform -> {
    List<Platform> children = platformMap.getOrDefault(platform.getParentId(), new ArrayList<>());
    children.add(platform);
    platformMap.put(platform.getParentId(),children);
  });
  platformList.forEach(platform -> platform.setChildren(platformMap.get(platform.getId())));
  List<Platform> result = platformList.stream().filter(v -> v.getParentId().equals("0")).collect(Collectors.toList());
  System.out.println(objectMapper.writeValueAsString(result));
}

@SneakyThrows
@Test
public void test4(){
  Map<String, List<Platform>> groupMap = platformList.stream().collect(Collectors.groupingBy(Platform::getParentId));
  platformList.forEach(platform -> platform.setChildren(groupMap.get(platform.getId())));
  List<Platform> collect = platformList.stream()
    .filter(platform -> platform.getParentId().equals("0")).collect(Collectors.toList());
  System.out.println(objectMapper.writeValueAsString(collect));
}