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How to call a rest API which returns object of two different types

I am calling an external API (on which I do not have a control to make any change) using Spring's RestTemplate. The API returns response with an array as response body if the results are found and if not it returns response with String as response body stating "No records found". I am getting an exception when I am getting the no company found message because I typecasted the RestTemplate call to the custom object
ResponseEntity<Student[]> studentDetails = restTemplate.getForEntity(studentUrl, Student[].class);
The above code throws exception when the API returns String message "No records found".
What is the best way to deal with such a scenario?

In that case, you probably may use it like that
ResponseEntity<Object> studentDetails = restTemplate.getForEntity(studentUrl, Object.class);
And then check for the response type and cast the result.

Send string as json with GET request with POSTMAN

I have a Java Spring API which expects 2 params, a simple String and a object:
#RequestMapping(value = "list", method = RequestMethod.GET)
public ResponseEntity<ListResource> getList(#RequestParam("agentName") String agentName,
#RequestParam("paginationInfo") PaginationInfoList paginationInfo {
//After http request i expect to have here my java Object PaginationInfoList ready to use
}
I'm, trying to send http GET request with Postman but i get this error, then i suppose that i'm not sending the data object "paginationInfo" in the correct way.
"Failed to convert value of type 'java.lang.String' to required type 'com.pippo.prova.test.model.in.PaginationInfoList'; nested exception is java.lang.IllegalStateException: Cannot convert value of type 'java.lang.String' to required type 'com.pippo.prova.test.model.in.PaginationInfoList': no matching editors or conversion strategy found"
Since i can't change the way of sending, infact must be GET and i have to use #ReuqestParam, i don't know how to send json data in postman parameters. I'm trying this and also other options but i always get error.

You can bind The request params to an object. In postman you will have 3 params ("agentName", "pageSize" and "pageNumber") and your controller will receive 2 objects
public ResponseEntity<ListResource> getList(#RequestParam("agentName") String agentName,
#Valid PaginationInfoList paginationInfo)
http://dolszewski.com/spring/how-to-bind-requestparam-to-object/

only Response entity shown in Postman

I have a response which produces #Produces(MediaType.APPLICATION_JSON) like this:
return Response.status(200).entity(product).entity("some message").build();
why postman shows me unexpected s in json view result and some message in xml view

For the Response.ResponseBuilder.entity(Object)` you can see
public abstract Response.ResponseBuilder entity(Object entity)
Set the response entity in the builder.
Any Java type instance for a response entity, that is supported by the runtime can be passed. It is the callers responsibility to wrap the actual entity with GenericEntity if preservation of its generic type is required. Note that the entity can be also set as an input stream.
A specific entity media type can be set using one of the type(...) methods.
Since you call this method twice, only the last value is saved.
Since "some message" is not at all a JSON syntax, it is saying that the s (from some) is unexpected, it is expecting a {

JAX-RS Request specific media type

Lets say I have the following REST method:
#GET
#Path("get/{id}")
#Produces({"application/json", "application/xml"})
public Entity getEntity(#PathParam("id") int id) {
//do stuff
Entity entity = find(id);
return entity;
}
When I hit the rest endpoint with any browser by default I get back XML. Is there a way I can specify at the request which media type I want returned? or must I somehow include that information in the path?

You have to specify the Accept header with the media type you want in addition to the Content-Type header that states what is the content type of your request.
So use the Accept header instead of Content-Type header:
Accept: application/xml

You should manually prepare your response. For example if you want JSON answer, you should convert your Entity class to Json and return String.
You can use https://code.google.com/p/google-gson/ for conversion.
In fact it is your decision and your work to prepare the answer. The only thing to take in consideration that Rest works on top of HTTP, so some sort of text(JSON, XML, Plain text) is the most "friendly" format of information.

May be this example can help
RestClientWithAcceptHeader
RestClient client = new RestClient();
ClientResponse response = client.resource("http://example.com/resources/resource1").header("Accept", "application/json;q=1.0, application/xml;q=0.8").get();
// find what format was sent back
String contentType = response.getHeaders().getFirst("Content-Type");
if (contentType.contains("application/json")) {
JSONObject entity = response.getEntity(JSONObject.class); /* or use a String, InputStream, or other provider that supports the entity media type */
} else if (contentType.contains("application/xml") {
String entity = response.getEntity(String.class); /* or use a JAXB class, InputStream, etc. */
More here and here

how to mime type of the url

I am using cxf as a webservice.It supports xml and json format output of the requested data.I want to know that if some exception has occured in my code then i want to return him back the error code either in xml or json format.But i dont know when to give json and xml ,it depends on the requested url that user has asked.
example
#Path("/reports/ad-view/loginId/{loginId}/publisher/")
PublisherReports getPublisherReportsAdView(
#PathParam("loginId") String loginId,
#QueryParam("fromDate") String fromDate,
#QueryParam("toDate") String toDate,
#QueryParam("filterValue") String filterValue);

If you mean you want to detect the mime type used to make the request, then you can use the #Consumes annotation to dictate which method handles which type of request. So you could write:
// Called when an XML request is made
#Path("/reports/ad-view/loginId/{loginId}/publisher/")
#Consumes("application/xml")
PublisherReports getPublisherReportsAdViewXml(...
and:
// Called when a JSON request is made
#Path("/reports/ad-view/loginId/{loginId}/publisher/")
#Consumes("application/json")
PublisherReports getPublisherReportsAdViewJson(...
Then have each variant of the getPublisherReportsAdView() method call a common method to perform the actual processing logic but still handle exceptions differently depending on the method that gets called.

An alternative approach, which doesn't require additional methods, would be to add a parameter which is annotated with the #HeaderParam annotation and use that to hold the value of the 'Content-Type' request header.
e.g.:
PublisherReports getPublisherReportsAdView(
#PathParam("loginId") String loginId,
#QueryParam("fromDate") String fromDate,
#QueryParam("toDate") String toDate,
#QueryParam("filterValue") String filterValue,
#HeaderParam("Content-Type") String contentType)
{
...
The value of contentType will likely also include charset information, for example: application/json; charset=UTF-8 so you'll need to ignore that when working out if the request contained JSON or XML.