带Void的Java模板形式参数

Question

I have two entities extending ResponseEntity:

public class VoidResponseEntity<Void> extends ResponseEntity<Void> {
    ... }

public class InfoResponseEntity<Info> extends ResponseEntity<Info> {
    ... }

public class Info {
    long id
}

In my another method I should return one of it:

public <T extends ?????> ResponseEntity<T> foo(...) {
     if (condition1) {
            return new InfoResponseEntity<Info>(new Info());
        }
        return new VoidResponseEntity<Void>();
}

What should I write instead of "?????" in method signature, wildcard? Or just T?

Answer 1

If your method is deciding the response entity type, I suspect your method shouldn't be generic in the first place:

public ResponseEntity<?> foo() {
    if (condition1) {
        return new InfoResponseEntity<Info>(new Info());
    }
    return new VoidResponseEntity<Void>();
}

In other words, your foo method is saying "I return some kind of response entity, but I can't tell you at compile time what the type argument it will be."

Additionally, it sounds like your concrete classes shouldn't be generic - they should be:

public class VoidResponseEntity extends ResponseEntity<Void> {
    ...
}

public class InfoResponseEntity extends ResponseEntity<Info> {
    ... 
}

Currently the Void and Info in your VoidResponseEntity and InfoResponseEntity classes are type parameters - not the Void and Info classes that I suspect you wanted them to be.

Answer 2

According to JavaDoc if you are using the method "foo" as controller you should be passing the ResponseEntity not the Type parameter.

Example for source ResponseEntity.

@RequestMapping("/handle")
 public ResponseEntity<String> handle() {
   HttpHeaders responseHeaders = new HttpHeaders();
   responseHeaders.set("MyResponseHeader", "MyValue");
   return new ResponseEntity<String>("Hello World", responseHeaders, HttpStatus.CREATED);
 }

So in your case the method should look like this (If my earlier assumption is correct about the question)

public ResponseEntity<?> foo(...) {
     if (condition1) {
            return new InfoResponseEntity<Info>(new Info());
        }
        return new VoidResponseEntity<Void>();
}