Java. YouTube API. AuthenticationException.

Question

I'm facing 'authentication issue' in youtube api. to get the insight data csv file link. Here is my code.

package sample.youtube;

import java.io.IOException;
import java.net.MalformedURLException;
import java.net.URL;

import com.google.gdata.client.Query;
import com.google.gdata.client.youtube.YouTubeService;
import com.google.gdata.data.youtube.VideoFeed;
import com.google.gdata.util.ServiceException;



public class CSV_Download {

    public static final String UPLOADS_URL = "http://gdata.youtube.com/feeds/api/users/default/uploads"; 
    public static void main(String[] args) {

        YouTubeService service = new YouTubeService("insight api", "dev key");
         Query query = null; 
         try { 
             query = new Query(new URL(UPLOADS_URL));
         } catch (MalformedURLException e) { 
             // TODO Auto-generated catch block
             e.printStackTrace();
         }
         query.setFields("title,entry(title, media:group/media:player)");
         VideoFeed videoFeed = null;
         try { 
             videoFeed = service.query(query, VideoFeed.class); 
         } catch (IOException e) { 
             // TODO Auto-generated catch block 
             e.printStackTrace(); 
         } catch (ServiceException e) { 
             // TODO Auto-generated catch block 
             e.printStackTrace(); 
         }
         System.out.println(videoFeed.getTitle() + ":"); 
         System.out.println(" Link : "+videoFeed.getLink("http://gdata.youtube.com/schemas/2007#insight.views", "text/html") + ":");
    }

}

My error message:

com.google.gdata.util.AuthenticationException: User authentication required. User authentication required.

User authentication required.

Error 401

at com.google.gdata.client.http.HttpGDataRequest.handleErrorResponse(HttpGDataRequest.java:600) at com.google.gdata.client.http.GoogleGDataRequest.handleErrorResponse(GoogleGDataRequest.java:563) at com.google.gdata.client.http.HttpGDataRequest.checkResponse(HttpGDataRequest.java:552) at com.google.gdata.client.http.HttpGDataRequest.execute(HttpGDataRequest.java:530) at com.google.gdata.client.http.GoogleGDataRequest.execute(GoogleGDataRequest.java:535) at com.google.gdata.client.Service.getFeed(Service.java:1135) at com.google.gdata.client.Service.getFeed(Service.java:1077) at com.google.gdata.client.GoogleService.getFeed(GoogleService.java:662) at com.google.gdata.client.Service.query(Service.java:1237) at com.google.gdata.client.Service.query(Service.java:1178) at sample.youtube.CSV_Download.main(CSV_Download.java:30) Exception in thread "main" java.lang.NullPointerException at sample.youtube.CSV_Download.main(CSV_Download.java:38)

So, I inserted a line (given below) into the code just after declaring the YouTubeService client:

service.setUserCredentials("email_here", "password_here");

Now the authentication error is over and code runs successfully. But I'm getting Null as link. I didn't get any response from YouTube developer forum. What can be wrong?