@SuppressWarnings("unchecked")
    public Collection<String> getUserDataKeys() {
        if (userData != null) {
            return userData.keySet();
        }

        return Collections.EMPTY_SET;
    }
Called internally by com.jme3.util.clone.Cloner.  Do not call directly.
jme3-core/src/main/java/com/jme3/scene/Spatial.java