gdata api - Youtube thumbnail api -

i want retrieve thumbnail videos youtube calling youtube api (gdata) using java? can guide me?

youtube-videos have 4 thumbnails, accessible specific url:

http://i.ytimg.com/vi/video_id/default.jpg   http://i.ytimg.com/vi/video_id/1.jpg   http://i.ytimg.com/vi/video_id/2.jpg   http://i.ytimg.com/vi/video_id/3.jpg 

so knowing video id can thumbnails that.