REST call returning blank XML

I am pretty new to REST programming.

Below is my class which is intended to return XML/JSON but I am having some difficulties to make it return proper values. I tried returning Response, JsonArray and Object of my POJO class but its not working. I looked several threads but not able to figure out what exactly is the problem.

The resource class:

public class UserService {

    UserDBHandler userDBHandler; 
    Friend f;

    @GET
    @Path("users/{userId}/friends")
//  @Produces(MediaType.TEXT_PLAIN)
    @Produces({ MediaType.APPLICATION_XML,MediaType.APPLICATION_JSON})
    public Friend  getFriends(@PathParam("userId") String userId) throws SQLException, ClassNotFoundException
    {
        System.out.println("userId : " +userId);
        userDBHandler =  new UserDBHandler();
        f = new Friend();



        ArrayList<String> userList = userDBHandler.fetchUsers(userId);

        System.out.println("Array size: "+userList.size());
        Iterator<String> iterator = userList.iterator();

        while(iterator.hasNext())
        {
            f.setUri(iterator.next());
            System.out.println(f.getUri());
        }

        //JsonObject  object = Json.createObjectBuilder().add("frienduri",f.getUri()).build();
        //ResponseBuilder response = Response.ok(f);

        //return Json.createArrayBuilder().add(object).build()
        //return response.build();
        return f;
    }
}

The POJO class:

import javax.xml.bind.annotation.XmlRootElement;
import javax.xml.bind.annotation.XmlSeeAlso;

@XmlRootElement
public class Friend{

    private String friendURI;
    private String event;
    private String uri;

    String getUri() {
        return uri;
    }
    void setUri(String uri) {
        this.uri = uri;
    }
    String getFriendURI() {
        return friendURI;
    }
    void setFriendURI(String friendURI) {
        this.friendURI = friendURI;
    }
    String getEvent() {
        return event;
    }
    void setEvent(String event) {
        this.event = event;
    }
}

This is what I get when I return the Response or the Friend object:

**<?xml version="1.0" encoding="UTF-8" standalone="yes"?><f/>**

And when returning JsonArray this is what I get: [{"frienduri":{"string":"b@b.com","chars":"b@b.com","valueType":"STRING"}}]

Another problem I am facing is: if I create a constructor I get below error:

A MultiException has 1 exceptions. They are: 1. java.lang.NoSuchMethodException: Could not find a suitable constructor in repository.resources.UserService class.

My dev environment is Tomcat 8 , JDK 1.8, Eclipse Luna.

I am not using Maven or web.xml instead I have a Application class.

Thanks