How to send YouTube videos from your Android app to a Chromecast device: https://medium.com/@soffritti.pierfrancesco/how-to-send-youtube-videos-from-your-android-app-to-a-chromecast-device-541d59ea1260

SimpleChromecastConnectionListener.java

private class SimpleChromecastConnectionListener implements ChromecastConnectionListener {
  
  @Override
  public void onChromecastConnecting() {
    Log.d(getClass().getSimpleName(), "onChromecastConnecting");
  }
  
  @Override
  public void onChromecastConnected(ChromecastYouTubePlayerContext chromecastYouTubePlayerContext) {
    Log.d(getClass().getSimpleName(), "onChromecastConnected");
    initializeCastPlayer(chromecastYouTubePlayerContext);
  }
  
  @Override
  public void onChromecastDisconnected() {
    Log.d(getClass().getSimpleName(), "onChromecastDisconnected");
  }
  
  private void initializeCastPlayer(ChromecastYouTubePlayerContext chromecastYouTubePlayerContext) {
    chromecastYouTubePlayerContext.initialize(new AbstractYouTubePlayerListener() {
      @Override
      public void onReady(YouTubePlayer youTubePlayer) {
        youtubePlayer.loadVideo("6JYIGclVQdw", 0f);
      }
    });
  }
}