close fullscreen of Youtube Player on back button inside fragment

Just to add to Andre's reply.

In your parent Activity add:

public YouTubePlayer youTubePlayer;
public boolean isYouTubePlayerFullScreen;

In your fragment add:

MainActivity mainActivity = (MainActivity) getActivity();

Then, on YouTubePlayerFragment initialization:

    @Override
    public void onInitializationSuccess(YouTubePlayer.Provider provider, YouTubePlayer player, boolean b) {
        mainActivity.youTubePlayer = player;
        player.loadVideo(videoId);

        player.setOnFullscreenListener(new YouTubePlayer.OnFullscreenListener() {
            @Override
            public void onFullscreen(boolean b) {
                mainActivity.isYouTubePlayerFullScreen = b;
            }
        });

    }

Now your Activity has access to YouTubePlayer and information if player is full screen or not.

Next, inside your Activity, override onBackPressed like this:

@Override
public void onBackPressed() {
    if (youTubePlayer != null && isYouTubePlayerFullScreen){
            youTubePlayer.setFullscreen(false);
    } else {
        super.onBackPressed();
    }
}

You can override onBackPressed() in your activity and in case your player is in fullscreen, you can handle it there as follows:

@Override
public void onBackPressed() {

    if (youtubePlayer != null && isFullscreen) {
        // if fullscreen, set fullscreen false
        youtubePlayer.setFullscreen(false);

    } else {
        // if NOT fullscreen, perform default call on back press
        super.onBackPressed();
    }
}

P.S. In above code fragment, isFullscreen is instance variable of our activity and is synced with youtubePlayer state from within OnFullscreenListener set on youtubePlayer.setOnFullScreenListener().

Hope that makes sense :)