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Generic method, equality constraint

Hello Im trying to implement a generic method as controller base method, but the problem which I cannot understand happens with the generic method signature.

<T> ResponseEntity<T> makeApiCall(String path, HttpMethod httpMethod, T body, boolean isAdmin){

String sender = isAdmin ? adminHash : userHash;
HttpHeaders headers = new HttpHeaders();
headers.add("Authorization", sender);
headers.add("Content-Type", "application/json");
HttpEntity<T> entity = new HttpEntity<>(body,headers);
ResponseEntity<T> responseEntity = restTemplate.exchange(path, HttpMethod.POST, entity, body.getClass());
return responseEntity;
}


The compile error I currently have is as follows:

Incompatible equality constraint: T and capture of ? extends Object


2 Answers
2

You don't say exactly where the problem occurs, but I think this will occur on the restTemplate.exchange( call, as a result of passing body.getClass() as a parameter. This is because the return type of body.getClass() is Class<? extends Object>, as in the Javadoc:

restTemplate.exchange(

body.getClass()

body.getClass()

Class<? extends Object>

The actual result type is Class<? extends |X|> where |X| is the erasure of the static type of the expression on which getClass is called

Class<? extends |X|>

|X|

getClass

The problem is that you can't guarantee that body is specifically a T - it could be a subclass of T. As such, the result of body.getClass() might not be a Class<T>.

body

T

T

body.getClass()

Class<T>

If you want to be type-safe, you would need to pass that in as an additional parameter to the method.

<T> ResponseEntity<T> makeApiCall(
String path, HttpMethod httpMethod, T body, Class<T> bodyClass,
boolean isAdmin){
// ...
ResponseEntity<T> responseEntity =
restTemplate.exchange(
path, HttpMethod.POST, entity, bodyClass);
// ...
}


Note that the only way to obtain a Class<T> is to use a class literal, e.g. String.class if T is String. This precludes the use of generic body types, since there are no generic class literals.

Class<T>

String.class

T

String

getClass()

Class<T>

T

T

Foo<Bar>

Class<Foo<Bar>>

You need to cast body.getClass() to Class<T>

cast body.getClass()

Class<T>

@SuppressWarnings("unchecked")
<T> ResponseEntity<T> makeApiCall(String path, HttpMethod httpMethod, T body, boolean isAdmin){

String sender = isAdmin ? adminHash : userHash;

HttpHeaders headers = new HttpHeaders();
headers.add("Authorization", sender);
headers.add("Content-Type", "application/json");
HttpEntity<T> entity = new HttpEntity<>(body,headers);
ResponseEntity<T> responseEntity = restTemplate.exchange(path, HttpMethod.POST, entity, (Class<T>) body.getClass());
return responseEntity;
}


To make it type-safe you need to pass the class object as a parameter explicitly:

<T> ResponseEntity<T> makeApiCall(String path, HttpMethod httpMethod, T body, Class<T> clazz, boolean isAdmin){
String sender = isAdmin ? adminHash : userHash;

HttpHeaders headers = new HttpHeaders();
headers.add("Authorization", sender);
headers.add("Content-Type", "application/json");
HttpEntity<T> entity = new HttpEntity<>(body,headers);
ResponseEntity<T> responseEntity = restTemplate.exchange(path, HttpMethod.POST, entity, clazz);
return responseEntity;
}


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