【Java报错】java.lang.ClassCastException: xxxClass cannot be cast to java.lang.Comparable 问题重现+解决

// 获取的List对象集合
  List<SomeRes> someResList = service.getSomeResList();
  // 要放入的Set集合
  Set<SomeRes> someResSet = null;
  for (SomeRes res : someResList ) {
    someResSet = new TreeSet<>();
    someResSet.add(res);
  }

public class SomeRes implements Comparable<SomeRes> {
    @ApiModelProperty(value = "设备ID")
    @JsonProperty(value = "equip")
    private String equip;
    @ApiModelProperty(value = "车牌号")
    @JsonProperty(value = "car_number")
    private String carNumber;
    @Override
    public int compareTo(@NotNull SomeRes o) {
        return this.carNumber.compareTo(o.carNumber);
    }
}