java 不存在类型变量 U 的实例，因此 Optional<U> 符合 Response
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No instance(s) of type variable(s) U exist so that Optional<U> conforms to Response
提问by user5325596
I have the following:
我有以下几点：
Optional<Resource> updatedResource = update(resourceID, data);
if (updatedResource.isPresent()) {
return Response.status(Response.Status.OK).entity(updatedResource.get()).build();
}
I would like to avoid the isPresentand getcalls if possible, so I tried
如果可能的话，我想避免调用isPresent和get调用，所以我尝试了
return update(resourceID, data).map(updatedResource -> Response.status(Response.Status.OK).entity(updatedResource).build();
but IntelliJ shows me the following error:
但是 IntelliJ 向我显示了以下错误：
No instance(s) of type variable(s) U exist so that Optional<U> conforms to Response
No instance(s) of type variable(s) U exist so that Optional<U> conforms to Response
Why do I get this error, and is there a way to avoid it and also avoid isPresentand get?
为什么我会收到这个错误，有没有办法避免它并避免isPresent和get？
采纳答案by Eran
Based on the error, the return type of your method is Response. However, update(resourceID, data).map(updatedResource -> Response.status(Response.Status.OK).entity(updatedResource).build())returns an Optional<U>, so you have to change the return type to Optional<Response>.
根据错误，您的方法的返回类型是Response. 但是，update(resourceID, data).map(updatedResource -> Response.status(Response.Status.OK).entity(updatedResource).build())返回Optional<U>，因此您必须将返回类型更改为Optional<Response>。
So the method would look like this:
所以这个方法看起来像这样：
public Optional<Response> yourMethod (...) {
return update(resourceID, data).map(updatedResource -> Response.status(Response.Status.OK).entity(updatedResource).build());
}
Or, if you don't want to change the return type, add an orElsecall, to specify a default value:
或者，如果您不想更改返回类型，请添加orElse调用以指定默认值：
public Response yourMethod (...) {
return update(resourceID, data).map(updatedResource -> Response.status(Response.Status.OK).entity(updatedResource).build()).orElse(defaultValue);
}
