[java] response return does not generate properly

Description

Response return with a string parameter should ideally generate the code with String. But in certain scenario it does not generate. This is consistent behaviour , easily reproducible.

Swagger-codegen version

any latest version

Swagger declaration file content or url

If the below response code is written for "CreateUser"

       '201':
          description: The request has been fulfilled, and a new resource is created .
          schema:
            type: "string"
            description: ID of the Object created

then as expected

public String createUser(User body)

but where as if below response code is written for "CreateUser"

      '200':
          description: OK. Request is OK.
       '201':
          description: The request has been fulfilled, and a new resource is created .
          schema:
            type: "string"
            description: ID of the Object created

notice the subtle change in the above that the first response code 200 does not return anything but second response code 201 returns String
now when we generate the code
public void createUser(User body)

Command line used for generation

java -jar /usr/share/swagger/swagger-codegen.jar generate -l java -c config.json -i index.yaml

Steps to reproduce

Very easy , just replace the above in pet store example and generate the code

Related issues/PRs

Suggest a fix/enhancement

It is not clear what is expected behavior , should we mention only one return response code ? or will the generator always take the return type of first response code ?
What is the appropriate behavior ?