我有一個(gè)接口IDto和FollowingResponseDto實(shí)現(xiàn)IDto。我有一個(gè)在名為的類中看起來像這樣的方法AbstractController：protected final List<? extends IDto> findPaginatedAndSortedInternal(page, size, sortBy, sortOrder, uriBuilder, response){ final Page<T> resultPage = getService().findAllPaginatedAndSortedRaw(page, size, sortBy, sortOrder); if (page > resultPage.getTotalPages()) { throw new ResourceNotFoundException(); } return DtoEntityConverter.convertToResponseDtoList(Lists.newArrayList(resultPage.getContent()));}這是代碼DtoEntityConverter.convertToResponseDtoList：public static List<? extends IDto> convertToResponseDtoList(List<? extends IEntity> entities) { List<? extends IDto> dtoList = new ArrayList<>(); entities.forEach(e -> e.convertToResponseDto()); return dtoList;}最后，我有一個(gè)擴(kuò)展類AbstractController：public class FollowingController extends AbstractReadOnlyController<Following> { @Override@RequestMapping(method = RequestMethod.GET)@ResponseBodypublic List<FollowingResponseDto> findAllPaginatedAndSorted( final int page, final int size, final String sortBy, final String sortOrder, final UriComponentsBuilder uriBuilder, final HttpServletResponse response) { return findPaginatedAndSortedInternal(page, size, sortBy, sortOrder, uriBuilder, response); }}return findPaginatedAndSortedInternal(page, size, sortBy, sortOrder, uriBuilder, response);FollowingController 中的調(diào)用給了我編譯錯(cuò)誤：Type mismatch: cannot convert from List<capture#2-of ? extends IDto> to List<FollowingResponseDto>我哪里錯(cuò)了？
1 回答
元芳怎么了
TA貢獻(xiàn)1798條經(jīng)驗(yàn) 獲得超7個(gè)贊
請注意，問題仍然是為什么您的內(nèi)部方法使用接口，而公開公開的方法使用具體類。通常情況恰恰相反。如果可能的話，我建議將返回類型更改findAllPaginatedAndSorted為List<? extends IDto>。
您不能從List<? extends IDto>to進(jìn)行轉(zhuǎn)換List<FollowingResponseDto>，因?yàn)榍罢呖梢园瑢?shí)現(xiàn)的其他類型IDto，而不僅僅是FollowingResponseDto.
想象一下以下場景：
interface I {}
class A implements I {}
class B implements I {}
List<I> interfaceList = ...;
interfaceList.add(new A());
interfaceList.add(new B());
List<A> aList = interfaceList; // error! interfaceList contains a B, and a B is not allowed in a list of As
現(xiàn)在您可能會爭辯說您的場景中沒有B，并且List<? extends IDto>僅包含 的實(shí)例FollowingResponseDto。但你的編譯器不知道這一點(diǎn)，也不能保證這在未來的任何時(shí)候都不會改變。
要解決此問題，您需要自己進(jìn)行轉(zhuǎn)換。要么在中間做一個(gè)邪惡的轉(zhuǎn)換List<?>，要么創(chuàng)建一個(gè)新的List<FollowingResponseDto>元素并單獨(dú)添加每個(gè)元素List<? extends IDto>：
邪惡的：
return (List<FollowingResponseDto>)(List<?>)findPaginatedAndSortedInternal(...);
不邪惡：
var idtoList = findPaginatedAndSortedInternal(...);
var followingResponseDtoList = new ArrayList<FollowingResponseDto>();
for (var idto : idtoList) {
if (idto instanceof FollowingResponseDto)
followingResponseDtoList.add((FollowingResponseDto)idto);
}
return followingResponseDtoList;
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