Interacting with the GUI from Java

import de.lessvoid.nifty.screen.Screen;

...

public void bind(Nifty nifty, Screen screen) {
    throw new UnsupportedOperationException("Not supported yet.");
}

public void onStartScreen() {
    throw new UnsupportedOperationException("Not supported yet.");
}

public void onEndScreen() {
    throw new UnsupportedOperationException("Not supported yet.");
}

package mygame;

import com.jme3.app.Application;
import com.jme3.app.state.BaseAppState;
import de.lessvoid.nifty.Nifty;
import de.lessvoid.nifty.screen.Screen;
import de.lessvoid.nifty.screen.ScreenController;

public class MyStartScreen extends BaseAppState implements ScreenController {

  @Override
  protected void initialize(Application app) {
      //It is technically safe to do all initialization and cleanup in the
      //onEnable()/onDisable() methods. Choosing to use initialize() and
      //cleanup() for this is a matter of performance specifics for the
      //implementor.
      //TODO: initialize your AppState, e.g. attach spatials to rootNode
  }

  @Override
  protected void cleanup(Application app) {
      //TODO: clean up what you initialized in the initialize method,
      //e.g. remove all spatials from rootNode
  }

  //onEnable()/onDisable() can be used for managing things that should
  //only exist while the state is enabled. Prime examples would be scene
  //graph attachment or input listener attachment.
  @Override
  protected void onEnable() {
      //Called when the state is fully enabled, ie: is attached and
      //isEnabled() is true or when the setEnabled() status changes after the
      //state is attached.
  }

  @Override
  protected void onDisable() {
      //Called when the state was previously enabled but is now disabled
      //either because setEnabled(false) was called or the state is being
      //cleaned up.
  }

  @Override
  public void update(float tpf) {
      //TODO: implement behavior during runtime
  }

  /**
   * Bind this ScreenController to a screen. This happens right before the
   * onStartScreen STARTED and only exactly once for a screen!
   * @param nifty nifty
   * @param screen screen
   */
  @Override
  public void bind(Nifty nifty, Screen screen) {
      throw new UnsupportedOperationException("Not supported yet.");
  }

  /**
   * called right after the onStartScreen event ENDED.
   */
  @Override
  public void onStartScreen() {
      throw new UnsupportedOperationException("Not supported yet.");
  }

  /**
   * called right after the onEndScreen event ENDED.
   */
  @Override
  public void onEndScreen() {
      throw new UnsupportedOperationException("Not supported yet.");
  }
}

<nifty>
  <screen id="start" controller="mygame.MyStartScreen">
      <!-- layer and panel code ... -->
  </screen>
</nifty>

nifty.addScreen("start", new ScreenBuilder("start") {{
  controller(new mygame.MyStartScreen());
}}.build(nifty));